4. Classification of proteins
Proteins and their main features
Proteins or proteins (which in Greek means "first" or "most important"), quantitatively prevail over all macromolecules present in a living cell, and account for more than half of the dry weight of most organisms. The concept of proteins as a class of compounds was formed in the 17th-19th centuries. During this period, substances with similar properties were isolated from various objects of the living world (seeds and juices of plants, muscles, blood, milk): they formed viscous solutions, coagulated when heated, when burning, the smell of burnt wool was felt and ammonia was released. Since all these properties were previously known for egg whites, the new class of compounds was called proteins. After the appearance at the beginning of the XIX century. More advanced methods of analysis of substances have determined the elemental composition of proteins. They found C, H, O, N, S. K late XIX centuries More than 10 amino acids have been isolated from proteins. Based on the results of studying the products of protein hydrolysis, the German chemist E. Fischer (1852-1919) suggested that proteins are built from amino acids.
As a result of Fischer's work, it became clear that proteins are linear polymers of a-amino acids linked to each other by an amide (peptide) bond, and the whole variety of representatives of this class of compounds could be explained by differences in the amino acid composition and the order of alternation of different amino acids in the polymer chain.
The first studies of proteins were carried out with complex protein mixtures, for example: with blood serum, egg white, extracts of plant and animal tissues. Later, methods for the isolation and purification of proteins were developed, such as precipitation, dialysis, chromatography on cellulose and other hydrophilic ion exchangers, gel filtration, and electrophoresis. Let's take a closer look at these methods at laboratory work and a seminar.
At the present stage, the main areas of study of proteins are the following:
¨ study of the spatial structure of individual proteins;
¨ study of the biological functions of various proteins;
¨ study of the mechanisms of functioning of individual proteins (at the level of individual atoms, atomic groups of a protein molecule).
All these stages are interrelated, because one of the main tasks of biochemistry is precisely to understand how the amino acid sequences of different proteins enable them to perform different functions.
Biological functions of proteins
Enzymes - these are biological catalysts, the most diverse, numerous class of proteins. Almost all chemical reactions involving organic biomolecules present in the cell are catalyzed by enzymes. More than 2000 different enzymes have been discovered so far.
Transport proteins - Plasma transport proteins bind and transfer specific molecules or ions from one organ to another. For example, hemoglobin, contained in erythrocytes, when passing through the lungs, binds oxygen and delivers it to peripheral tissues, where oxygen is released. Blood plasma contains lipoproteinscarrying lipids from the liver to other organs. Cell membranes contain another type of cellular transport proteins that are capable of binding certain molecules (eg glucose) and transporting them through the membrane into the cell.
Food and storage proteins.The most famous examples of such proteins are wheat, corn, and rice seed proteins. Food proteins include egg albumin - the main component of egg white, casein - the main protein of milk.
Contractile and motor proteins.Actin and myosin - proteins that function in the contractile system of skeletal muscle, as well as in many non-muscle tissues.
Structural proteins. Collagen - the main component of cartilage and tendons. This protein has a very high tensile strength. Bundles contain elastin is a structural protein that can stretch in two dimensions. Hair, nails are composed almost exclusively of strong insoluble protein - keratin... The main component of silk threads and spider webs is fibroin protein.
Protective proteins. Immunoglobulins or antibodies are specialized cells produced in lymphocytes. They have the ability to recognize viruses or foreign molecules that have entered the body, and then trigger a system to neutralize them. Fibrinogen and thrombin - proteins involved in the process of blood clotting, they protect the body from loss of blood in the event of damage to the vascular system.
Regulatory proteins. Some proteins are involved in the regulation of cellular activity. These include many hormonessuch as insulin (regulates glucose metabolism).
Protein classification
By solubility
Albumin. Soluble in water and salt solutions.
Globulins. Slightly soluble in water, but highly soluble in saline solutions.
Prolamins. Soluble in 70-80% ethanol, insoluble in water and absolute alcohol. Rich in arginine.
Histones. Soluble in saline solutions.
Scleroproteins. Insoluble in water and saline solutions. The content of glycine, alanine, proline is increased.
By the shape of the molecules
Based on the ratio of the axes (longitudinal and transverse), two large classes of proteins can be distinguished. Have globular proteins the ratio is less than 10 and in most cases does not exceed 3-4. They are characterized by compact packaging of polypeptide chains. Examples of globular proteins: many enzymes, insulin, globulin, blood plasma proteins, hemoglobin.
Fibrillar proteins, in which the ratio of the axes exceeds 10, consist of bundles of polypeptide chains, spirally wound on top of each other and interconnected by transverse covalent or hydrogen bonds (keratin, myosin, collagen, fibrin).
Physical properties of proteins
On the physical properties of proteins such as ionization, hydration, solubility various methods of protein isolation and purification are based.
Since proteins contain ionogenic, i.e. ionizable amino acid residues (arginine, lysine, glutamic acid, etc.), therefore, they are polyelectrolytes. With acidification, the degree of ionization of anionic groups decreases, and of cationic ones, it increases; with alkalization, the opposite pattern is observed. At a certain pH, the number of negatively and positively charged particles becomes the same, this state is called isoelectric (the total charge of the molecule is zero). The pH value at which the protein is in the isoelectric state is called isoelectric pointand denote pI... One of the methods of their separation is based on different ionization of proteins at a certain pH value - the method electrophoresis.
The polar groups of proteins (ionic and non-ionic) are able to interact with water and hydrate. The amount of water associated with protein reaches 30-50 g per 100 g of protein. There are more hydrophilic groups on the surface of the protein. Solubility depends on the number of hydrophilic groups in the protein, on the size and shape of molecules, and on the total charge. The combination of all these physical properties of protein allows the use of the method molecular sievesor gel filtrationto separate proteins. Method dialysis is used to purify proteins from low molecular weight impurities and is based on large sizes of protein molecules.
The solubility of proteins also depends on the presence of other solutes, for example, neutral salts. At high concentrations of neutral salts, proteins precipitate, and for precipitation ( salting out) different proteins require different salt concentrations. This is due to the fact that charged protein molecules adsorb ions of opposite charge. As a result, the particles lose their charges and electrostatic repulsion, resulting in protein precipitation. The salting-out method can be used to fractionate proteins.
Primary structure of proteins
Primary protein structure the composition and sequence of amino acid residues in a protein molecule is called. The amino acids in the protein are linked by peptide bonds.
All molecules of a given individual protein are identical in amino acid composition, sequence of amino acid residues, and polypeptide chain length. Sequencing the amino acid sequence of proteins is a laborious task. We will discuss this topic in more detail at the seminar. Insulin was the first protein for which an amino acid sequence was established. Bovine insulin has a molar mass of about 5700. Its molecule consists of two polypeptide chains: an A-chain containing 21 aa, and a B-chain containing 30 aa, these two chains are connected by two disulfide (-SS-) connections. Even small changes in the primary structure can significantly change the properties of the protein. Sickle cell disease is the result of a change in only 1 amino acid in the b-chain of hemoglobin (Glu ® Val).
Species specificity of the primary structure
When studying amino acid sequences homologous proteins isolated from different species, several important conclusions were drawn. Homologous proteins include those proteins that perform the same functions in different species. An example is hemoglobin: in all vertebrates, it performs the same function associated with oxygen transport. Homologous proteins of different species usually have polypeptide chains of the same or nearly the same length. In the amino acid sequences of homologous proteins, the same amino acids are always found in many positions - they are called invariant remainders. At the same time, significant differences are observed in other positions of proteins: in these positions the amino acids vary from species to species; such amino acid residues are called variable... The entire set of similarities in the amino acid sequences of homologous proteins is combined into the concept sequence homology. The presence of such homology suggests that the animals from which the homologous proteins were isolated have a common evolutionary origin. An interesting example is a complex protein - cytochrome c - a mitochondrial protein that participates as an electron carrier in biological oxidation processes. M "12500, contains" 100 a.k. Were installed a.k. sequences for 60 species. 27 a.k. - are the same, which indicates that all these residues play an important role in determining the biological activity of cytochrome c. The second important conclusion drawn from the analysis of amino acid sequences is that the number of residues by which cytochromes from any two species differ is proportional to the phylogenetic difference between these species. For example, the cytochrome c molecules from horse and yeast differ by 48 aa, in duck and chicken - by 2 aa, in chicken and turkey they do not differ. Information on the number of differences in amino acid sequences of homologous proteins from different species is used to construct evolutionary maps reflecting the successive stages of the emergence and development of various species of animals and plants in the process of evolution.
Secondary structure of proteins
- this is the folding of a protein molecule in space without taking into account the influence of side substituents. There are two types of secondary structure: a-helix and b-structure (folded layer). Let us dwell in more detail on the consideration of each type of secondary structure.
a-Spiral is a right helix with the same pitch, equal to 3.6 amino acid residues. The a-helix is \u200b\u200bstabilized by intramolecular hydrogen bonds that arise between the hydrogen atoms of one peptide bond and the oxygen atoms of the fourth peptide bond.
Side substituents are located perpendicular to the plane of the a-helix.
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So the properties of this protein are determined by the properties of the side groups of amino acid residues: included in the composition of one or another protein. If the side substituents are hydrophobic, then the a-helix protein is also hydrophobic. An example of such a protein is the protein keratin, which makes up hair.
As a result, it turns out that the a-helix is \u200b\u200bpermeated with hydrogen bonds and is a very stable structure. When such a spiral is formed, two tendencies work:
¨ the molecule tends to the minimum energy, i.e. to the formation of the largest number of hydrogen bonds;
¨ Due to the rigidity of the peptide bond, only the first and fourth peptide bonds can approach each other in space.
IN folded layer peptide chains are arranged parallel to each other, forming a figure like a folded leaf. Peptide chains interacting with each other by hydrogen bonds can be a large number of... The circuits are located antiparallel.
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The more peptide chains are included in the folded layer, the stronger the protein molecule.
Let's compare the properties of the protein materials of wool and silk and explain the difference in the properties of these materials in terms of the structure of the proteins of which they are composed.
Keratin, a wool protein, has a secondary a-helix structure. Woolen thread is not as strong as silk thread, it stretches easily when wet. This property is explained by the fact that when a load is applied, hydrogen bonds are broken and the spiral is stretched.
Fibroin, a silk protein, has a secondary b-structure. The silk thread does not stretch and is very tear-resistant. This property is explained by the fact that many peptide chains in the folded layer interact with each other by hydrogen bonds, which makes this structure very strong.
Amino acids differ in their ability to participate in the formation of a-helices and b-structures. Glycine, asparagine, and tyrosine are rarely found in a-helices. Proline destabilizes the a-helical structure. Explain why? The b-structures include glycine, almost no proline, glutamic acid, asparagine, histidine, lysine, serine are found.
The structure of one protein can contain sections of b-structures, a-helices and irregular sections. At irregular sites, the peptide chain can bend relatively easily and change conformation, while the helix and folded layer are rather rigid structures. The content of b-structures and a-helices in different proteins is not the same.
Tertiary structure of proteins
is determined by the interaction of side substituents of the peptide chain. For fibrillar proteins, it is difficult to identify general patterns in the formation of tertiary structures. As for globular proteins, such patterns exist, and we will consider them. The tertiary structure of globular proteins is formed by additional folding of the peptide chain containing b-structures, a-helices and irregular regions, so that hydrophilic side groups of amino acid residues appear on the surface of the globule, and hydrophobic side groups are hidden deep into the globule, sometimes forming a hydrophobic pocket.
Forces stabilizing the tertiary structure of a protein.
Electrostatic interaction between differently charged groups, the extreme case is ionic interactions.
Hydrogen bondsarising between the side groups of the polypeptide chain.
Hydrophobic interactions.
Covalent interactions (the formation of a disulfide bond between two cysteine \u200b\u200bresidues to form cystine). The formation of disulfide bonds leads to the fact that distant regions of the polypeptide molecule are brought closer together and fixed. Disulfide bonds are destroyed by the action of reducing agents. This property is used for perming hair, which is almost entirely a keratin protein permeated with disulfide bonds.
The nature of the spatial folding is determined by the amino acid composition and the alternation of amino acids in the polypeptide chain (primary structure). Consequently, each protein has only one spatial structure corresponding to its primary structure. Small changes in the conformation of protein molecules occur when interacting with other molecules. These changes sometimes play a huge role in the functioning of protein molecules. So, when an oxygen molecule is attached to hemoglobin, the conformation of the protein changes somewhat, which leads to the effect of cooperative interaction when the remaining three oxygen molecules are attached. This change in conformation in underlies the theory of inducing correspondence in explaining the group specificity of some enzymes.
In addition to the covalent disulfide bond, all other bonds stabilizing the tertiary structure are weak in nature and are easily destroyed. When a large number of bonds that stabilize the spatial structure of a protein molecule are broken, an ordered conformation unique for each protein is broken, and the biological activity of the protein is often lost. Such a change in spatial structure is called denaturation.
Protein function inhibitors
Taking into account that different ligands differ in Kb, it is always possible to select a substance similar in structure to the natural ligand, but having a greater Kbw with a given protein. For example, CO has Kb 100 times more than O 2 with hemoglobin, so 0.1% CO in the air is enough to block a large number of hemoglobin molecules. Many medicines work in the same way. For example, ditilin.
Acetylcholine is a mediator of the transmission of nerve impulses to the muscle. Ditilin blocks a receptor protein with which acetylcholine binds and creates a paralyzing effect.
9.Relation of the structure of proteins with their functions on the example of hemoglobin and myoglobin
Carbon dioxide transport
Hemoglobin not only transports oxygen from the lungs to the peripheral tissues, but also accelerates the transport of CO 2 from the tissues to the lungs. Hemoglobin binds CO 2 immediately after the release of oxygen (»15% of the total CO 2). In erythrocytes, there is an enzymatic process of formation of carbonic acid from CO 2 coming from the tissues: CO 2 + H 2 O \u003d H 2 CO 3. Carbonic acid rapidly dissociates into HCO 3 - and H +. To prevent a dangerous increase in acidity, there must be a buffer system capable of absorbing excess protons. Hemoglobin binds two protons for every four oxygen molecules released and determines the buffering capacity of the blood. The process is reversed in the lungs. The released protons bind with the bicarbonate ion to form carbonic acid, which, under the action of the enzyme, turns into CO 2 and water, CO 2 is exhaled. Thus, O 2 binding is closely associated with CO 2 exhalation. This reversible phenomenon is known as bohr effect. The Bohr effect is not found in myoglobin.
Isofunctional proteins
A protein that performs a specific function in a cell can be presented in several forms - isofunctional proteins, or isozymes. Although such proteins perform the same function, they differ in binding constant, which leads to some differences in functional terms. For example, several forms of hemoglobin were found in human erythrocytes: HbA (96%), HbF (2%), HbA 2 (2%). All hemoglobins are tetramers built from protomers a, b, g, d (HbA -a 2 b 2, HbF - a 2 g 2, HbA 2 - a 2 d 2). All protomers are similar in their primary structure, and very great similarity is observed in the secondary and tertiary structures. All forms of hemoglobins are designed to transport oxygen to tissue cells, but HbF, for example, has a greater affinity for oxygen than HbA. HbF is characteristic of the embryonic stage of human development. It is able to take oxygen away from HbA, which ensures a normal oxygen supply to the fetus.
Iso-proteins are the result of more than one structural gene in the gene pool of a species.
PROTEINS: STRUCTURE, PROPERTIES AND FUNCTIONS
1. Proteins and their main features
2. Biological functions of proteins
3. Amino acid composition of proteins
4. Classification of proteins
5. Physical properties of proteins
6. Structural organization of protein molecules (primary, secondary, tertiary structures)
The content of the article
PROTEINS (article 1)- a class of biological polymers present in every living organism. With the participation of proteins, the main processes that ensure the vital activity of the body take place: respiration, digestion, muscle contraction, transmission of nerve impulses. Bone tissue, skin, hair, horny formations of living beings are composed of proteins. For most mammals, the growth and development of the body occurs due to products containing proteins as a food component. The role of proteins in the body and, accordingly, their structure is very diverse.
Protein composition.
All proteins are polymers whose chains are assembled from amino acid fragments. Amino acids are organic compoundscontaining in their composition (in accordance with the name) amino group NH 2 and an organic acidic, i.e. carboxyl, COOH group. Of the entire variety of existing amino acids (theoretically, the number of possible amino acids is unlimited), only those with only one carbon atom between the amino group and the carboxyl group participate in the formation of proteins. In general, the amino acids involved in the formation of proteins can be represented by the formula: H 2 N – CH (R) –COOH. The R group attached to the carbon atom (the one between the amino and carboxyl group) determines the difference between the amino acids that make up proteins. This group can only consist of carbon and hydrogen atoms, but more often contains, in addition to C and H, various functional (capable of further transformations) groups, for example, HO-, H 2 N-, etc. There is also a variant when R \u003d H.
The organisms of living beings contain more than 100 different amino acids, however, not all are used in the construction of proteins, but only 20, the so-called "fundamental" ones. Table 1 shows their names (most of the names have developed historically), the structural formula, as well as the widely used abbreviation. All structural formulas are arranged in the table so that the main amino acid fragment is on the right.
| Name | Structure | Designation |
| GLYCINE | GLI | |
| ALANIN | ALA | |
| VALIN | SHAFT | |
| Leucine | LEY | |
| Isoleucine | ILE | |
| SERIN | CEP | |
| THREONINE | TRE | |
| CYSTEINE | CIS | |
| METIONIN | MET | |
| LYSINE | LIZ | |
| ARGININE | ARG | |
| ASPARAGINIC ACID | ASN | |
| ASPARAGIN | ASN | |
| GLUTAMIC ACID | GLU | |
| GLUTAMINE | GLN | |
| Phenylalanine | Hair dryer | |
| Tyrosine | TIR | |
| TRIPTOFAN | THREE | |
| HISTIDINE | GIS | |
| Proline | Missile defense | |
| In international practice, the abbreviated designation of the listed amino acids is accepted using the Latin three-letter or one-letter abbreviations, for example, glycine - Gly or G, alanine - Ala or A. | ||
Among these twenty amino acids (Table 1), only proline contains an NH group next to the carboxyl group COOH (instead of NH 2), since it is part of the cyclic fragment.
Eight amino acids (valine, leucine, isoleucine, threonine, methionine, lysine, phenylalanine and tryptophan), placed in the table against a gray background, are called indispensable, since the body must constantly receive them from protein foods for normal growth and development.
A protein molecule is formed as a result of the sequential connection of amino acids, while the carboxyl group of one acid interacts with the amino group of the neighboring molecule, as a result, a peptide bond –CO – NH– is formed and a water molecule is released. In fig. 1 shows the serial connection of alanine, valine and glycine.
Figure: one SERIAL AMINO ACIDS CONNECTION during the formation of a protein molecule. The path from the terminal amino group H 2 N to the terminal carboxyl group COOH was chosen as the main direction of the polymer chain.
To describe the structure of a protein molecule in a compact manner, abbreviated designations of amino acids (Table 1, third column) involved in the formation of the polymer chain are used. A fragment of the molecule shown in Fig. 1 is written as follows: H 2 N-ALA-VAL-GLI-COOH.
Protein molecules contain from 50 to 1500 amino acid residues (shorter chains are called polypeptides). The individuality of a protein is determined by the set of amino acids that make up the polymer chain and, no less important, by the order of their alternation along the chain. For example, an insulin molecule consists of 51 amino acid residues (this is one of the shortest-chain proteins) and consists of two parallel chains of unequal length connected to each other. The sequence of amino acid fragments is shown in Fig. 2.
Figure: 2 INSULIN MOLECULEcomposed of 51 amino acid residues, fragments of identical amino acids are marked with the corresponding background color. Cysteine \u200b\u200bamino acid residues contained in the chain (abbreviated designation CIS) form disulfide bridges -S-S-, which bind two polymer molecules, or form bridges within one chain.
Cysteine \u200b\u200bamino acid molecules (Table 1) contain reactive sulfhydride groups –SH, which interact with each other, forming –S – S– disulfide bridges. The role of cysteine \u200b\u200bin the world of proteins is special, with its participation cross-links are formed between polymer protein molecules.
The unification of amino acids into a polymer chain occurs in a living organism under the control of nucleic acids, it is they who provide a strict assembly order and regulate the fixed length of the polymer molecule ( cm... NUCLEIC ACIDS).
Protein structure.
The composition of a protein molecule, presented in the form of alternating amino acid residues (Fig. 2), is called the primary structure of the protein. Hydrogen bonds appear between the imino groups HN and carbonyl groups CO present in the polymer chain ( cm... HYDROGEN BOND), as a result, the protein molecule acquires a certain spatial shape, called the secondary structure. The most common are two types of secondary structure of proteins.
The first option, called the α-helix, is realized using hydrogen bonds within one polymer molecule. The geometric parameters of the molecule, determined by the bond lengths and bond angles, are such that the formation of hydrogen bonds is possible for the H-N and C \u003d O groups, between which there are two peptide fragments H-N-C \u003d O (Fig. 3).
The composition of the polypeptide chain shown in Fig. 3 is written in abbreviated form as follows:
H 2 N-ALA VAL-ALA-LEI-ALA-ALA-ALA-ALA-VAL-ALA-ALA-ALA-COOH.
As a result of the contracting action of hydrogen bonds, the molecule acquires the shape of a spiral - the so-called α-helix; it is depicted as a curved spiral-like ribbon passing through the atoms forming a polymer chain (Fig. 4)
Figure: 4 VOLUME MODEL OF A PROTEIN MOLECULE in the form of an α-helix. Hydrogen bonds are shown with green dashed lines. The cylindrical shape of the spiral is visible at a certain angle of rotation (hydrogen atoms are not shown in the figure). The color of individual atoms is given in accordance with international rules that recommend black for carbon atoms, blue for nitrogen, red for oxygen, yellow for sulfur (white is recommended for hydrogen atoms not shown in the figure, in this case the entire structure depicted on a dark background).
Another variant of the secondary structure, called the β-structure, is also formed with the participation of hydrogen bonds, the difference is that the H-N and C \u003d O groups of two or more polymer chains arranged in parallel interact. Since the polypeptide chain has a direction (Fig. 1), variants are possible when the direction of the chains coincides (parallel β-structure, Fig. 5), or they are opposite (antiparallel β-structure, Fig. 6).
Polymer chains of various compositions can participate in the formation of the β-structure, while the organic groups framing the polymer chain (Ph, CH 2 OH, etc.), in most cases, play a secondary role, the interposition of the H-N and C \u003d O groups is of decisive importance. Since the H-N and C \u003d O groups are directed in different directions relative to the polymer chain (up and down in the figure), it becomes possible to simultaneously interact with three or more chains.
The composition of the first polypeptide chain in Fig. five:
H 2 N-LEY-ALA-FEN-GLI-ALA-ALA-COOH
The composition of the second and third chain:
H 2 N-GLI-ALA-SER-GLI-TRE-ALA-COOH
The composition of the polypeptide chains shown in Fig. 6, the same as in Fig. 5, the difference is that the second chain has the opposite (in comparison with Fig. 5) direction.
The formation of a β-structure within one molecule is possible when a chain fragment in a certain region turns out to be rotated by 180 °, in this case two branches of one molecule have the opposite direction, as a result of which an antiparallel β-structure is formed (Fig. 7).
The structure shown in Fig. 7 in a flat image is shown in Fig. 8 as a volumetric model. Sections of the β-structure are conventionally denoted in a simplified manner by a flat wavy ribbon that passes through the atoms forming the polymer chain.
In the structure of many proteins, sections of the α-helix and ribbon-like β-structures, as well as single polypeptide chains, alternate. Their interposition and alternation in the polymer chain is called the tertiary structure of the protein.
Methods for depicting the structure of proteins are shown below using the plant protein cambin as an example. The structural formulas of proteins, which often contain up to hundreds of amino acid fragments, are complex, cumbersome and difficult to understand; therefore, sometimes simplified structural formulas are used - without symbols of chemical elements (Fig. 9, option A), but at the same time they retain the color of the valence lines in accordance with international rules (fig. 4). In this case, the formula is presented not in a flat, but in a spatial image, which corresponds to the real structure of the molecule. This method makes it possible, for example, to distinguish between disulfide bridges (similar to those in insulin, Fig. 2), phenyl groups in the lateral framing of the chain, etc. The image of molecules in the form of volumetric models (balls connected by rods) is somewhat more clear (Fig. 9, option B). However, both methods do not allow one to show the tertiary structure, so the American biophysicist Jane Richardson suggested depicting α-structures in the form of spirally twisted ribbons (see Fig. 4), β-structures in the form of flat wavy ribbons (Fig. 8), and the connecting them single chains - in the form of thin bundles, each type of structure has its own color. This method is now widely used for imaging the tertiary structure of a protein (Fig. 9, variant B). Sometimes, for more informational content, they show together the tertiary structure and a simplified structural formula (Fig. 9, option D). There are also modifications of the method proposed by Richardson: α-helices are depicted in the form of cylinders, and β-structures in the form of flat arrows indicating the direction of the chain (Fig. 9, variant E). Less common is the method in which the entire molecule is depicted as a bundle, where unequal structures are distinguished by different colors, and disulfide bridges are shown in the form of yellow bridges (Fig. 9, option E).
Variant B is most convenient for perception, when, when depicting the tertiary structure, the structural features of the protein (amino acid fragments, the order of their alternation, hydrogen bonds) are not indicated, while proceeding from the fact that all proteins contain "details" taken from a standard set of twenty amino acids ( Table 1). The main task when imaging a tertiary structure is to show the spatial arrangement and alternation of secondary structures.
Figure: nine DIFFERENT IMAGE OPTIONS OF THE STRUCTURE OF CRAMBIN PROTEIN.
A - structural formula in the spatial image.
B - structure in the form of a volumetric model.
B - tertiary structure of the molecule.
D - a combination of options A and B.
D is a simplified representation of the tertiary structure.
E - tertiary structure with disulfide bridges.
The most convenient for perception is the volumetric tertiary structure (variant B), freed from the details of the structural formula.
A protein molecule with a tertiary structure, as a rule, takes on a certain configuration, which is formed by polar (electrostatic) interactions and hydrogen bonds. As a result, the molecule takes the form of a compact coil - globular proteins (globules, lat... ball), or threadlike - fibrillar proteins (fibra, lat... fiber).
An example of a globular structure is the albumin protein; the albumin class includes chicken egg protein. The albumin polymer chain is assembled mainly from alanine, aspartic acid, glycine, and cysteine, alternating in a specific order. The tertiary structure contains α-helices connected by single chains (Fig. 10).
Figure: ten GLOBULAR STRUCTURE OF ALBUMIN
An example of a fibrillar structure is the fibroin protein. They contain a large amount of glycine, alanine and serine residues (every second amino acid residue is glycine); residues of cysteine \u200b\u200bcontaining sulfhydride groups are absent. Fibroin, the main component of natural silk and spider webs, contains β-structures connected by single chains (Fig. 11).
Figure: eleven FIBRILLARY PROTEIN FIBROIN
The possibility of a certain type of tertiary structure formation is inherent in the primary protein structure, i.e. predetermined by the order of alternation of amino acid residues. From certain sets of such residues, α-helices mainly arise (there are quite a few such sets), another set leads to the appearance of β-structures, and single chains are characterized by their composition.
Some protein molecules, while retaining a tertiary structure, are able to combine into large supramolecular aggregates, while they are held together by polar interactions, as well as hydrogen bonds. Such formations are called the quaternary structure of the protein. For example, the protein ferritin, consisting mainly of leucine, glutamic acid, aspartic acid, and histidine (all 20 amino acid residues in ferricin are in varying amounts) forms a tertiary structure of four parallel-folded α-helices. When molecules are combined into a single ensemble (Fig. 12), a quaternary structure is formed, which can include up to 24 ferritin molecules.
Fig. 12 FORMATION OF THE QUATERARY STRUCTURE OF THE GLOBULAR PROTEIN FERRITIN
Another example of supramolecular formations is the structure of collagen. It is a fibrillar protein whose chains are built mainly from glycine, alternating with proline and lysine. The structure contains single chains, triple α-helices, alternating with ribbon-like β-structures, stacked in the form of parallel bundles (Fig. 13).
Fig. 13 SUPERMOLECULAR STRUCTURE OF COLLAGEN FIBRILLARY PROTEIN
Chemical properties of proteins.
Under the action of organic solvents, waste products of some bacteria (lactic acid fermentation) or with an increase in temperature, the destruction of secondary and tertiary structures occurs without damage to its primary structure, as a result, the protein loses its solubility and loses its biological activity, this process is called denaturation, that is, the loss of natural properties. for example, curdling sour milk, curdled protein of a boiled chicken egg. At elevated temperatures, proteins of living organisms (in particular, microorganisms) quickly denature. Such proteins are not able to participate in biological processes, as a result, microorganisms die, so boiled (or pasteurized) milk can last longer.
The peptide bonds H-N-C \u003d O, which form the polymer chain of the protein molecule, are hydrolyzed in the presence of acids or alkalis, and the polymer chain breaks, which, ultimately, can lead to the original amino acids. Peptide bonds that make up α-helices or β-structures are more resistant to hydrolysis and various chemical influences (as compared to the same bonds in single chains). A more delicate disassembly of the protein molecule into its constituent amino acids is carried out in an anhydrous medium using hydrazine H 2 N – NH 2, while all amino acid fragments, except for the last one, form the so-called hydrazides of carboxylic acids containing the C (O) –HN – NH 2 ( fig. 14).
Figure: 14. DECOMPOSITION OF POLYPEPTIDE
Such an analysis can provide information about the amino acid composition of a particular protein, but it is more important to know their sequence in a protein molecule. One of the methods widely used for this purpose is the action on the polypeptide chain of phenyl isothiocyanate (FITC), which in an alkaline medium is attached to the polypeptide (from the end that contains the amino group), and when the reaction of the medium changes to acidic, it detaches from the chain, taking with it a fragment of one amino acid (Fig. 15).
Figure: fifteen SEQUENTIAL DECOMPOSITION OF POLYPEPTIDE
Many special techniques have been developed for such an analysis, including those that begin to "disassemble" a protein molecule into its constituent components, starting from the carboxyl end.
The transverse disulfide S-S bridges (formed during the interaction of cysteine \u200b\u200bresidues, Figs. 2 and 9) cleave, converting them into HS-groups by the action of various reducing agents. The action of oxidizing agents (oxygen or hydrogen peroxide) leads again to the formation of disulfide bridges (Fig. 16).
Figure: 16. SPLITTING OF DISULPHIDE BRIDGES
To create additional cross-links in proteins, the reactivity of amino and carboxyl groups is used. More accessible for various interactions are amino groups that are in the side framing of the chain - fragments of lysine, asparagine, lysine, proline (Table 1). When such amino groups interact with formaldehyde, the condensation process takes place and cross bridges –NH – CH2 – NH– appear (Fig. 17).
Figure: 17 CREATION OF ADDITIONAL CROSS-Bridges BETWEEN PROTEIN MOLECULES.
The terminal carboxyl groups of a protein are capable of reacting with complex compounds of some polyvalent metals (chromium compounds are more often used), while cross-linking also occurs. Both processes are used in leather tanning.
The role of proteins in the body.
The role of proteins in the body is varied.
Enzymes(fermentatio lat... - fermentation), their other name is enzymes (en zumh greek... - in yeast) are proteins with catalytic activity, they can increase the speed of biochemical processes thousands of times. Under the action of enzymes, the constituent components of food - proteins, fats and carbohydrates - are broken down to simpler compounds, from which new macromolecules are then synthesized, which are necessary for the body of a certain type. Enzymes are also involved in many biochemical synthesis processes, for example, in the synthesis of proteins (some proteins help synthesize others). Cm... ENZYMES
Enzymes are not only highly efficient catalysts, but also selective ones (they direct the reaction strictly in a given direction). In their presence, the reaction proceeds with almost 100% yield without the formation of by-products, and at the same time the conditions of the course are mild: normal atmospheric pressure and temperature of a living organism. For comparison, the synthesis of ammonia from hydrogen and nitrogen in the presence of a catalyst - activated iron - is carried out at 400–500 ° C and a pressure of 30 MPa, the ammonia yield is 15–25% per cycle. Enzymes are considered unsurpassed catalysts.
Intensive research on enzymes began in the middle of the 19th century, now more than 2000 different enzymes have been studied, this is the most diverse class of proteins.
The names of the enzymes are as follows: to the name of the reagent with which the enzyme interacts, or to the name of the catalyzed reaction, add the ending -ase, for example, arginase decomposes arginine (Table 1), decarboxylase catalyzes decarboxylation, ie. elimination of CO 2 from the carboxyl group:
- COOH → - CH + CO 2
Often, for a more accurate designation of the role of the enzyme, both the object and the type of reaction are indicated in its name, for example, alcohol dehydrogenase - an enzyme that dehydrates alcohols.
For some enzymes, discovered a long time ago, the historical name (without the ending -ase) has been preserved, for example, pepsin (pepsis, greek... digestion) and trypsin (thrypsis greek... liquefaction), these enzymes break down proteins.
For systematization, enzymes are combined into large classes, the classification is based on the type of reaction, the classes are named according to the general principle - the name of the reaction and the ending - aza. Some of these classes are listed below.
Oxidoreductase- enzymes that catalyze redox reactions. Dehydrogenases belonging to this class carry out proton transfer, for example, alcohol dehydrogenase (ADH) oxidizes alcohols to aldehydes, the subsequent oxidation of aldehydes to carboxylic acids catalyze aldehyde dehydrogenases (ALDH). Both processes occur in the body during the conversion of ethanol into acetic acid (Fig. 18).
Figure: 18 TWO-STAGE OXIDATION OF ETHANOL to acetic acid
It is not ethanol that has a narcotic effect, but the intermediate product acetaldehyde, the lower the activity of the ALDH enzyme, the slower the second stage - the oxidation of acetaldehyde to acetic acid, and the longer and stronger the intoxicating effect of ethanol ingestion. The analysis showed that more than 80% of the representatives of the yellow race have a relatively low ALDH activity and therefore a significantly more severe alcohol tolerance. The reason for this innate decreased ALDH activity is that some of the glutamic acid residues in the "weakened" ALDH molecule are replaced by lysine fragments (Table 1).
Transferases- enzymes that catalyze the transfer of functional groups, for example, transiminase catalyzes the movement of the amino group.
Hydrolases- enzymes that catalyze hydrolysis. The previously mentioned trypsin and pepsin hydrolyze peptide bonds, and lipases cleave the ester bond in fats:
–RС (О) ОR 1 + Н 2 О → –RС (О) ОН + HOR 1
Lyases- enzymes that catalyze reactions that are not hydrolytic, as a result of such reactions, the C-C, C-O, C-N bonds are broken and new bonds are formed. The enzyme decarboxylase belongs to this class
Isomerase- enzymes that catalyze isomerization, for example, the conversion of maleic acid into fumaric acid (Fig. 19), this is an example of cis - trans isomerization (see ISOMERIA).
Figure: 19. ISOMERIZATION OF MALEIC ACID into fumaric acid in the presence of an enzyme.
In the work of enzymes, the general principle is observed, according to which there is always a structural correspondence between the enzyme and the reagent of the accelerated reaction. According to the figurative expression of one of the founders of the doctrine of enzymes E. Fischer, the reagent approaches the enzyme like a key to a lock. In this regard, each enzyme catalyzes a specific chemical reaction or a group of reactions of the same type. Sometimes an enzyme can act on one single compound, for example, urease (uron greek... - urine) catalyzes only the hydrolysis of urea:
(H 2 N) 2 C \u003d O + H 2 O \u003d CO 2 + 2NH 3
The finest selectivity is displayed by enzymes that distinguish between optically active antipodes - left and right-handed isomers. L-arginase acts only on levogyrate arginine and does not affect the dextrorotatory isomer. L-lactate dehydrogenase acts only on levorotatory lactic acid esters, the so-called lactates (lactis lat... milk), while D-lactate dehydrogenase only breaks down D-lactates.
Most of the enzymes act not on one, but on a group of related compounds, for example, trypsin "prefers" to cleave peptide bonds formed by lysine and arginine (Table 1.)
The catalytic properties of some enzymes, such as hydrolases, are determined solely by the structure of the protein molecule itself, another class of enzymes, oxidoreductases (for example, alcohol dehydrogenase), can be active only in the presence of non-protein molecules associated with them - vitamins that activate Mg, Ca, Zn, Mn ions and fragments of nucleic acids (Fig. 20).
Figure: 20 ALCOHOLDEHYDROGENASE MOLECULE
Transport proteins bind and transfer various molecules or ions across cell membranes (both inside and outside the cell), as well as from one organ to another.
For example, hemoglobin binds oxygen as blood passes through the lungs and delivers it to various tissues of the body, where oxygen is released and then used to oxidize food components, this process serves as a source of energy (sometimes the term "burning" of food in the body is used).
In addition to the protein part, hemoglobin contains a complex compound of iron with a cyclic porphyrin molecule (porphyros greek... - purple), which causes the red color of the blood. It is this complex (Fig. 21, left) that plays the role of an oxygen carrier. In hemoglobin, the iron porphyrin complex is located inside the protein molecule and is retained by polar interactions, as well as coordination bonds with nitrogen in histidine (Table 1), which is part of the protein. The O2 molecule, which is carried by hemoglobin, joins by means of a coordination bond to the iron atom from the side opposite to that to which histidine is attached (Fig. 21, right).
Figure: 21 STRUCTURE OF THE IRON COMPLEX
The structure of the complex in the form of a volumetric model is shown on the right. The complex is retained in the protein molecule by a coordination bond (blue dotted line) between the Fe atom and the N atom in histidine, which is part of the protein. The O 2 molecule, which is carried by hemoglobin, is coordinated (red dotted line) attached to the Fe atom from the opposite country of the flat complex.
Hemoglobin is one of the most thoroughly studied proteins, it consists of a-helices connected by single chains, and contains four iron complexes. Thus, hemoglobin is like a bulky package for the transfer of four oxygen molecules at once. In shape, hemoglobin corresponds to globular proteins (Fig. 22).
Figure: 22 GLOBULAR FORM OF HEMOGLOBIN
The main "advantage" of hemoglobin is that the addition of oxygen and its subsequent elimination during transmission to various tissues and organs is quick. Carbon monoxide, CO (carbon monoxide), binds to Fe in hemoglobin even faster, but, unlike O 2, forms a complex that is difficult to decompose. As a result, such hemoglobin is not able to bind O 2, which leads (when inhaling large amounts of carbon monoxide) to the death of the body from suffocation.
The second function of hemoglobin is the transfer of exhaled CO 2, but in the process of temporary binding of carbon dioxide, it is not the iron atom that is involved, but the H 2 N group of the protein.
The "performance" of proteins depends on their structure, for example, the replacement of a single amino acid residue of glutamic acid in the hemoglobin polypeptide chain with a valine residue (a rarely observed congenital anomaly) leads to a disease called sickle cell anemia.
There are also transport proteins that can bind fats, glucose, amino acids and transport them both inside and outside cells.
Transport proteins of a special type do not carry the substances themselves, but perform the functions of a "transport regulator", passing certain substances through the membrane (the outer wall of the cell). Such proteins are more often called membrane proteins. They have the shape of a hollow cylinder and, being built into the membrane wall, provide the movement of some polar molecules or ions into the cell. An example of a membrane protein is porin (Fig. 23).
Figure: 23 PORIN PROTEIN
Food and storage proteins, as the name suggests, serve as sources of internal nutrition, often for the embryos of plants and animals, as well as in the early stages of development of young organisms. Food proteins include albumin (Fig. 10), the main component of egg white, and casein, the main protein in milk. Under the action of the enzyme pepsin, casein is curdled in the stomach, this ensures its retention in the digestive tract and effective assimilation. Casein contains fragments of all the amino acids the body needs.
In ferritin (Fig. 12), which is contained in animal tissues, iron ions are stored.
Storage proteins also include myoglobin, which resembles hemoglobin in composition and structure. Myoglobin is concentrated mainly in muscles, its main role is to store oxygen, which hemoglobin gives it. It is quickly saturated with oxygen (much faster than hemoglobin), and then gradually transfers it to various tissues.
Structural proteins perform a protective function (skin) or support - they hold the body together and give it strength (cartilage and tendons). Their main component is the fibrillar protein collagen (Fig. 11), the most abundant protein in the animal world in mammals, accounting for almost 30% of the total mass of proteins. Collagen has a high tensile strength (the strength of the skin is known), but due to the low content of cross-links in the collagen of the skin, animal skins are not very suitable in their raw form for the manufacture of various products. To reduce the swelling of the skin in water, shrinkage during drying, as well as to increase the strength in the watered state and increase the elasticity in collagen, additional cross-links are created (Fig.15a), this is the so-called leather tanning process.
In living organisms, collagen molecules that have arisen in the process of growth and development of the body are not renewed or replaced by newly synthesized ones. As the body ages, the number of cross-links in collagen increases, which leads to a decrease in its elasticity, and since there is no renewal, age-related changes appear - an increase in the fragility of cartilage and tendons, the appearance of wrinkles on the skin.
The articular ligaments contain elastin, a structural protein that is easily stretched in two dimensions. The protein resilin has the greatest elasticity, which is located in the places of the hinged attachment of the wings of some insects.
Horny formations - hair, nails, feathers, consisting mainly of the protein keratin (Fig. 24). Its main difference is a noticeable content of cysteine \u200b\u200bresidues, which forms disulfide bridges, which gives high elasticity (the ability to restore its original shape after deformation) to hair, as well as woolen fabrics.
Figure: 24. FRAGMENT OF FIBRILLARY PROTEIN KERATIN
For an irreversible change in the shape of a keratin object, you first need to destroy the disulfide bridges with the help of a reducing agent, give a new shape, and then re-create the disulfide bridges with the help of an oxidizing agent (Fig. 16), this is how, for example, perming hair is done.
With an increase in the content of cysteine \u200b\u200bresidues in keratin and, accordingly, an increase in the number of disulfide bridges, the ability to deform disappears, but at the same time a high strength appears (horns of ungulates and tortoiseshells contain up to 18% of cysteine \u200b\u200bfragments). Mammals contain up to 30 different types of keratin.
The keratin-related fibrillar protein fibroin, secreted by silkworm caterpillars when curling a cocoon, and by spiders when weaving a web, contains only β-structures connected by single chains (Fig. 11). Unlike keratin, fibroin does not have transverse disulfide bridges, it is very tensile strength (strength per unit cross section some specimens have spider webs higher than steel cables). Due to the lack of cross-linking, fibroin is inelastic (it is known that woolen fabrics are almost indestructible, and silk fabrics easily wrinkle).
Regulatory proteins.
Regulatory proteins, often called hormones, are involved in various physiological processes. For example, the hormone insulin (Fig. 25) consists of two α-chains connected by disulfide bridges. Insulin regulates metabolic processes with the participation of glucose, its absence leads to diabetes.
Figure: 25 PROTEIN INSULIN
In the pituitary gland of the brain, a hormone is synthesized that regulates the growth of the body. There are regulatory proteins that control the biosynthesis of various enzymes in the body.
The contractile and motor proteins give the body the ability to contract, change shape and move, especially in the muscles. 40% of the mass of all proteins contained in muscles is myosin (mys, myos, greek... - muscle). Its molecule contains both a fibrillar and a globular part (Fig. 26)
Figure: 26 MYOSIN MOLECULE
Such molecules are combined into large aggregates containing 300–400 molecules.
When the concentration of calcium ions in the space surrounding the muscle fibers changes, a reversible change in the conformation of molecules occurs - a change in the shape of the chain due to the rotation of individual fragments around the valence bonds. This leads to muscle contraction and relaxation, the signal to change the concentration of calcium ions comes from the nerve endings in the muscle fibers. Artificial muscle contraction can be caused by the action of electrical impulses, leading to a sharp change in the concentration of calcium ions, this is the basis for the stimulation of the heart muscle to restore the work of the heart.
Protective proteins allow you to protect the body from the invasion of attacking bacteria, viruses and from the penetration of foreign proteins (the generalized name for foreign bodies - antigens). The role of protective proteins is played by immunoglobulins (their other name is antibodies); they recognize antigens that have entered the body and firmly bind to them. In mammals, including humans, there are five classes of immunoglobulins: M, G, A, D and E, their structure, as the name suggests, is globular, in addition, they are all built in a similar way. The molecular organization of antibodies is shown below using the example of class G immunoglobulin (Fig. 27). The molecule contains four polypeptide chains united by three S-S disulfide bridges (in Fig. 27 they are shown with thickened valence bonds and large S symbols), in addition, each polymer chain contains intrachain disulfide bridges. Two large polymer chains (highlighted in blue) contain 400-600 amino acid residues. The other two chains (highlighted in green) are almost half as long, containing approximately 220 amino acid residues. All four chains are arranged in such a way that the end H 2 N-groups are directed in the same direction.
Figure: 27 SCHEMATIC IMAGE OF THE IMMUNOGLOBULIN STRUCTURE
After contact of the body with a foreign protein (antigen), the cells of the immune system begin to produce immunoglobulins (antibodies), which accumulate in the blood serum. At the first stage, the main work is done by the sections of the chains containing the terminal H 2 N (in Fig. 27, the corresponding sections are marked in light blue and light green). These are antigen capture areas. In the process of immunoglobulin synthesis, these areas are formed in such a way that their structure and configuration correspond as much as possible to the structure of the approaching antigen (like a key to a lock, like enzymes, but the tasks in this case are different). Thus, a strictly individual antibody is created for each antigen as an immune response. Not a single known protein can change the structure so "plasticly" depending on external factors, in addition to immunoglobulins. Enzymes solve the problem of structural correspondence to the reagent in a different way - with the help of a gigantic set of various enzymes, counting on all possible cases, and immunoglobulins each time rebuild the "working tool". Moreover, the hinge region of the immunoglobulin (Fig. 27) provides the two capture regions with some independent mobility, as a result, the immunoglobulin molecule can "find" the two most convenient sites for capture in the antigen in order to securely fix it, this resembles the actions of a crustacean.
Next, a chain of sequential reactions of the body's immune system turns on, immunoglobulins of other classes are connected, as a result, a foreign protein is deactivated, and then the destruction and removal of the antigen (foreign microorganism or toxin).
After contact with the antigen, the maximum concentration of immunoglobulin is achieved (depending on the nature of the antigen and the individual characteristics of the organism itself) within several hours (sometimes several days). The body retains the memory of such a contact, and with a repeated attack with the same antigen, immunoglobulins accumulate in the blood serum much faster and in greater quantities - acquired immunity arises.
The above classification of proteins is to a certain extent arbitrary, for example, the protein thrombin, mentioned among the protective proteins, is essentially an enzyme that catalyzes the hydrolysis of peptide bonds, that is, belongs to the class of proteases.
Protective proteins are often referred to as snake venom proteins and toxic proteins from certain plants, since their task is to protect the body from damage.
There are proteins whose functions are so unique that it is difficult to classify them. For example, the monellin protein found in one African plant is very sweet in taste and has become the subject of research as a non-toxic substance that can be used in place of sugar to prevent obesity. The blood plasma of some Antarctic fish contains proteins with antifreeze properties, which prevents the blood of these fish from freezing.
Artificial synthesis of proteins.
The condensation of amino acids leading to the polypeptide chain is a well-studied process. You can carry out, for example, the condensation of any one amino acid or a mixture of acids and get, respectively, a polymer containing the same units, or different units alternating in a random order. Such polymers have little resemblance to natural polypeptides and have no biological activity. The main task is to combine amino acids in a strictly defined, predetermined order in order to reproduce the sequence of amino acid residues in natural proteins. American scientist Robert Merrifield proposed an original method to solve this problem. The essence of the method is that the first amino acid is attached to an insoluble polymer gel, which contains reactive groups capable of combining with –COOH - amino acid groups. Crosslinked polystyrene with chloromethyl groups introduced into it was taken as such a polymer substrate. To prevent the amino acid taken for the reaction from reacting with itself and so that it does not attach with the H 2 N-group to the support, the amino group of this acid is previously blocked with a bulky substituent [(C 4 H 9) 3] 3 OC (O) -group. After the amino acid has attached to the polymer support, the blocking group is removed, and another amino acid is introduced into the reaction mixture, in which the H 2 N group is also pre-blocked. In such a system, only the interaction of the H 2 N-group of the first amino acid and the –COOH group of the second acid is possible, which is carried out in the presence of catalysts (phosphonium salts). Then the whole scheme is repeated by introducing the third amino acid (Fig. 28).
Figure: 28. SCHEME OF SYNTHESIS OF POLYPEPTIDE CHAINS
In the last step, the resulting polypeptide chains are separated from the polystyrene support. Now the whole process is automated, there are automatic peptide synthesizers operating according to the described scheme. This method is used to synthesize many peptides used in medicine and agriculture. It was also possible to obtain improved analogs of natural peptides with selective and enhanced action. Some small proteins are synthesized, such as insulin hormone and some enzymes.
There are also methods of protein synthesis that copy natural processes: they synthesize fragments of nucleic acids tuned to obtain certain proteins, then these fragments are inserted into a living organism (for example, a bacterium), after which the body begins to produce the desired protein. In this way, significant amounts of hard-to-reach proteins and peptides, as well as their analogues, are now obtained.
Proteins as food sources.
Proteins in a living organism are constantly split into the original amino acids (with the indispensable participation of enzymes), some amino acids pass into others, then proteins are synthesized again (also with the participation of enzymes), i.e. the body is constantly renewed. Some proteins (skin collagen, hair) are not renewed, the body constantly loses them and synthesizes new ones instead. Proteins as food sources perform two main functions: they supply the body with building material for the synthesis of new protein molecules and, in addition, provide the body with energy (sources of calories).
Carnivorous mammals (including humans) get the necessary proteins from plant and animal food. None of the proteins obtained from food is incorporated into the body unchanged. In the digestive tract, all absorbed proteins are broken down to amino acids, and already from them proteins necessary for a particular organism are built, while of the 8 essential acids (Table 1), the other 12 can be synthesized in the body if they are not supplied in sufficient quantities with food, but essential acids must be supplied with food without fail. The body receives sulfur atoms in cysteine \u200b\u200bwith the essential amino acid - methionine. Some of the proteins break down, releasing the energy necessary to maintain life, and the nitrogen they contain is excreted in the urine. Usually, the human body loses 25-30 g of protein per day, so protein food must be constantly present in the right amount. The minimum daily protein requirement is 37 g for men and 29 g for women, but the recommended intake is almost twice as high. When evaluating foods, it is important to consider the quality of the protein. In the absence or low content of essential amino acids, the protein is considered to be of low value, therefore, such proteins should be consumed in greater quantities. So, proteins of legumes contain little methionine, and proteins of wheat and corn have a low content of lysine (both amino acids are essential). Animal proteins (excluding collagens) are classified as complete foods. A complete set of all essential acids contains milk casein, as well as cottage cheese and cheese made from it, so a vegetarian diet, if it is very strict, i.e. "Dairy-free", requires increased consumption of legumes, nuts and mushrooms to supply the body with essential amino acids in the right amount.
Synthetic amino acids and proteins are also used as food products, adding them to feeds that contain small amounts of essential amino acids. There are bacteria that can process and assimilate oil hydrocarbons, in this case, for the full synthesis of proteins, they need to be fed with nitrogen-containing compounds (ammonia or nitrates). The protein obtained in this way is used as feed for livestock and poultry. A set of enzymes, carbohydrases, are often added to animal feed, which catalyze the hydrolysis of difficult-to-decompose components of carbohydrate food (cell walls of cereals), as a result of which plant food is absorbed more fully.
Mikhail Levitsky
PROTEINS (article 2)
(proteins), a class of complex nitrogen-containing compounds, the most characteristic and important (along with nucleic acids) components of living matter. Proteins have many and varied functions. Most proteins are enzymes that catalyze chemical reactions. Many hormones that regulate physiological processes are also proteins. Structural proteins such as collagen and keratin are the main components of bone, hair and nails. The contractile proteins of muscles have the ability to change their length, using chemical energy to perform mechanical work. Proteins include antibodies that bind and neutralize toxic substances. Some proteins that can respond to external influences (light, smell) serve as receptors in the sense organs that perceive irritation. Many proteins located inside the cell and on the cell membrane perform regulatory functions.
In the first half of the 19th century. many chemists, among them in the first place J. von Liebig, gradually came to the conclusion that proteins are a special class of nitrogenous compounds. The name “proteins” (from the Greek protos - the first) was proposed in 1840 by the Dutch chemist G. Mulder.
PHYSICAL PROPERTIES
Proteins are white in solid state, and colorless in solution, unless they carry some chromophore (colored) group, such as hemoglobin. Water solubility varies greatly between proteins. It also changes depending on the pH and on the concentration of salts in the solution, so that conditions can be selected under which one protein will selectively precipitate in the presence of other proteins. This "salting-out" method is widely used for the isolation and purification of proteins. Purified protein often precipitates out of solution as crystals.
In comparison with other compounds, the molecular weight of proteins is very high - from several thousand to many millions of daltons. Therefore, during ultracentrifugation, proteins are precipitated, and moreover, at different rates. Due to the presence of positively and negatively charged groups in protein molecules, they move at different speeds and in an electric field. This is the basis of electrophoresis - a method used to isolate individual proteins from complex mixtures. Protein purification is also carried out by chromatography.
CHEMICAL PROPERTIES
Structure.
Proteins are polymers, i.e. molecules built, like chains, from repeating monomeric units, or subunits, the role of which is played by alpha-amino acids. General amino acid formula
where R is a hydrogen atom or some organic group.
A protein molecule (polypeptide chain) can consist of only a relatively small number of amino acids or several thousand monomer units. The connection of amino acids in a chain is possible because each of them has two different chemical groups: an amino group with basic properties, NH2, and an acidic carboxyl group, COOH. Both of these groups are attached to the a-carbon atom. The carboxyl group of one amino acid can form an amide (peptide) bond with the amino group of another amino acid:
After the two amino acids have joined in this way, the chain can be extended by adding a third to the second amino acid, etc. As you can see from the above equation, when the peptide bond is formed, a water molecule is released. In the presence of acids, alkalis or proteolytic enzymes, the reaction proceeds in the opposite direction: the polypeptide chain is split into amino acids with the addition of water. This reaction is called hydrolysis. Hydrolysis occurs spontaneously, and energy is required to combine amino acids into a polypeptide chain.
A carboxyl group and an amide group (or a similar imide group - in the case of the amino acid proline) are present in all amino acids, the differences between amino acids are determined by the nature of that group, or "side chain", which is indicated above by the letter R. The role of the side chain can be played by one a hydrogen atom, like the amino acid glycine, and some bulky grouping, like histidine and tryptophan. Some side chains are chemically inert, while others are markedly reactive.
Many thousands of different amino acids can be synthesized, and many different amino acids are found in nature, but only 20 types of amino acids are used for protein synthesis: alanine, arginine, asparagine, aspartic acid, valine, histidine, glycine, glutamine, glutamic acid, isoleucine, leucine, lysine , methionine, proline, serine, tyrosine, threonine, tryptophan, phenylalanine and cysteine \u200b\u200b(in proteins, cysteine \u200b\u200bcan be present in the form of a dimer - cystine). True, some proteins also contain other amino acids besides the regularly occurring twenty, but they are formed as a result of modification of any of the twenty listed after it has been incorporated into the protein.
Optical activity.
All amino acids, with the exception of glycine, have four different groups attached to the alpha carbon. From the point of view of geometry, four different groups can be attached in two ways, and accordingly there are two possible configurations, or two isomers, related to each other, like an object to its mirror image, i.e. like the left hand to the right. One configuration is called left-handed or levogyrate (L), and the other is called right-handed or dextrorotatory (D), since two such isomers differ in the direction of rotation of the plane of polarized light. Only L-amino acids are found in proteins (the exception is glycine; it can be represented only in one form, since it has two of the four groups that are the same), and they all have optical activity (since there is only one isomer). D-amino acids are rare in nature; they are found in some antibiotics and in the cell wall of bacteria.
Amino acid sequence.
The amino acids in the polypeptide chain are not arranged randomly, but in a certain fixed order, and it is this order that determines the functions and properties of the protein. By varying the order of the 20 types of amino acids, you can get a huge number of different proteins, just as you can make up many different texts from the letters of the alphabet.
In the past, it often took several years to determine the amino acid sequence of a protein. Direct definition and now it is a rather laborious task, although devices have been created that allow it to be carried out automatically. It is usually easier to determine the nucleotide sequence of the corresponding gene and deduce the amino acid sequence of the protein from it. To date, the amino acid sequences of many hundreds of proteins have already been determined. The functions of the decoded proteins are usually known, and this helps to imagine the possible functions of similar proteins formed, for example, in malignant neoplasms.
Complex proteins.
Proteins consisting only of amino acids are called simple proteins. Often, however, a metal atom or some chemical compound other than an amino acid is attached to the polypeptide chain. These proteins are called complex proteins. An example is hemoglobin: it contains iron porphyrin, which determines its red color and allows it to act as an oxygen carrier.
The names of most complex proteins contain an indication of the nature of the attached groups: there are sugars in glycoproteins, and fats in lipoproteins. If the catalytic activity of the enzyme depends on the attached group, then it is called a prosthetic group. Often, some vitamin plays the role of a prosthetic group or is part of it. Vitamin A, for example, attached to one of the retinal proteins, determines its sensitivity to light.
Tertiary structure.
It is not so much the amino acid sequence of the protein itself (primary structure) that is important, but the way of its packing in space. Along the entire length of the polypeptide chain, hydrogen ions form regular hydrogen bonds, which give it the shape of a spiral or a layer (secondary structure). The combination of such helices and layers gives rise to a compact form of the next order - the tertiary structure of the protein. Rotations through small angles are possible around the bonds holding the monomeric links of the chain. Therefore, from a purely geometric point of view, the number of possible configurations for any polypeptide chain is infinitely large. In reality, however, each protein normally exists in only one configuration, determined by its amino acid sequence. This structure is not rigid, it seems to “breathe” - it oscillates around a certain average configuration. The chain folds into a configuration in which free energy (the ability to perform work) is minimal, just as a released spring is compressed only to a state corresponding to a minimum of free energy. Quite often, one part of the chain is rigidly linked to another by disulfide (–S – S–) bonds between two cysteine \u200b\u200bresidues. This is partly why cysteine \u200b\u200bplays a particularly important role among amino acids.
The complexity of the structure of proteins is so great that it is still impossible to calculate the tertiary structure of a protein, even if its amino acid sequence is known. But if it is possible to obtain crystals of a protein, then its tertiary structure can be determined by X-ray diffraction.
In structural, contractile, and some other proteins, the chains are elongated and several slightly folded chains lying side by side form fibrils; fibrils, in turn, fold into larger formations - fibers. However, most proteins in solution have a globular shape: the chains are coiled in a globule, like yarn in a ball. Free energy in this configuration is minimal, since hydrophobic ("water repelling") amino acids are hidden inside the globule, and hydrophilic ("water attracting") are on its surface.
Many proteins are complexes of several polypeptide chains. This structure is called the quaternary protein structure. The hemoglobin molecule, for example, consists of four subunits, each of which is a globular protein.
Structural proteins, due to their linear configuration, form fibers with a very high tensile strength, while the globular configuration allows proteins to enter into specific interactions with other compounds. On the surface of the globule, with the correct stacking of the chains, cavities of a certain shape appear, in which reactive chemical groups are located. If a given protein is an enzyme, then another, usually smaller, molecule of some substance enters such a cavity just like a key enters a lock; in this case, the configuration of the electron cloud of the molecule under the influence of the chemical groups in the cavity changes, and this forces it to react in a certain way. In this way, the enzyme catalyzes the reaction. Antibody molecules also have cavities in which various foreign substances are bound and thereby rendered harmless. The “key and lock” model, which explains the interaction of proteins with other compounds, makes it possible to understand the specificity of enzymes and antibodies; their ability to react only with certain compounds.
Proteins in different types of organisms.
Proteins that perform the same function in different plant and animal species and therefore bear the same name also have a similar configuration. They, however, differ somewhat in their amino acid sequence. As species diverge from a common ancestor, some amino acids at certain positions are replaced by others as a result of mutations. Harmful mutations that cause hereditary diseases are discarded by natural selection, but beneficial or at least neutral ones can remain. The closer two biological species are to each other, the less differences are found in their proteins.
Some proteins change relatively quickly, others are very conservative. The latter include, for example, cytochrome c, a respiratory enzyme found in most living organisms. In humans and chimpanzees, its amino acid sequences are identical, while in the cytochrome c of wheat, only 38% of the amino acids were different. Even comparing humans and bacteria, the similarity of cytochromes with (the differences affect 65% of amino acids here) can still be seen, although the common ancestor of bacteria and humans lived on Earth about two billion years ago. Nowadays, amino acid sequence comparisons are often used to construct a phylogenetic (genealogical) tree reflecting evolutionary relationships between different organisms.
Denaturation.
The synthesized protein molecule, folding, acquires its characteristic configuration. This configuration, however, can be destroyed by heating, by a change in pH, by organic solvents, and even by simple agitation of the solution until bubbles appear on its surface. The protein thus modified is called denatured; it loses its biological activity and usually becomes insoluble. Familiar examples of denatured protein are boiled eggs or whipped cream. Small proteins, containing only about a hundred amino acids, are able to renature, i.e. re-acquire the original configuration. But most of the proteins are simply converted into a mass of entangled polypeptide chains and does not restore the previous configuration.
One of the main difficulties in isolating active proteins is associated with their extreme sensitivity to denaturation. This property of proteins is useful in preserving food: high temperature irreversibly denatures enzymes of microorganisms, and microorganisms die.
SYNTHESIS OF PROTEINS
For protein synthesis, a living organism must have a system of enzymes capable of attaching one amino acid to another. A source of information is also needed that would determine which amino acids should be combined. Since there are thousands of types of proteins in the body, and each of them consists of an average of several hundred amino acids, the information required must be truly enormous. It is stored (similar to how a tape is stored) in the nucleic acid molecules that make up the genes.
Enzyme activation.
A polypeptide chain synthesized from amino acids is not always a protein in its final form. Many enzymes are synthesized at first in the form of inactive precursors and become active only after another enzyme has removed several amino acids at one end of the chain. In this inactive form, some of the digestive enzymes are synthesized, for example trypsin; these enzymes are activated in the digestive tract by removing the end of the chain. The hormone insulin, the molecule of which in its active form consists of two short chains, is synthesized as one chain, the so-called. proinsulin. Then the middle part of this chain is removed, and the remaining fragments bind to each other, forming an active hormone molecule. Complex proteins are formed only after a certain chemical group is attached to the protein, and this attachment often requires an enzyme.
Metabolic circulation.
After feeding the animal amino acids labeled with radioactive isotopes of carbon, nitrogen or hydrogen, the label is quickly incorporated into its proteins. If the labeled amino acids cease to enter the body, the amount of the label in the proteins begins to decrease. These experiments show that the proteins formed are not stored in the body until the end of life. All of them, with a few exceptions, are in a dynamic state, constantly decaying to amino acids, and then synthesized again.
Some proteins break down when cells die and are destroyed. This constantly happens, for example, with erythrocytes and epithelial cells lining the inner surface of the intestine. In addition, protein breakdown and resynthesis also take place in living cells. Oddly enough, less is known about the breakdown of proteins than about their synthesis. It is clear, however, that the breakdown involves proteolytic enzymes similar to those that break down proteins into amino acids in the digestive tract.
The half-life of different proteins is different - from several hours to many months. The only exception is collagen molecules. Once formed, they remain stable, are not renewed or replaced. Over time, however, some of their properties change, in particular elasticity, and since they are not renewed, certain age-related changes are the result of this, for example, the appearance of wrinkles on the skin.
Synthetic proteins.
Chemists have long learned how to polymerize amino acids, but amino acids combine in this disordered manner, so that the products of such polymerization are not very similar to natural ones. True, it is possible to combine amino acids in a given order, which makes it possible to obtain some biologically active proteins, in particular insulin. The process is quite complicated, and in this way it is possible to obtain only those proteins, the molecules of which contain about a hundred amino acids. It is preferable to instead synthesize or isolate the nucleotide sequence of a gene corresponding to the desired amino acid sequence, and then introduce this gene into a bacterium, which will produce a large amount of the desired product by replication. This method, however, also has its drawbacks.
PROTEIN AND NUTRITION
When proteins in the body are broken down into amino acids, these amino acids can be used again to synthesize proteins. At the same time, the amino acids themselves are subject to degradation, so that they are not completely reused. It is also clear that during growth, pregnancy and wound healing, protein synthesis must exceed decay. The body is constantly losing some proteins; these are proteins of hair, nails and the surface layer of the skin. Therefore, for the synthesis of proteins, each organism must receive amino acids from food.
Sources of amino acids.
Green plants synthesize all 20 amino acids found in proteins from CO2, water and ammonia or nitrates. Many bacteria are also able to synthesize amino acids in the presence of sugar (or some equivalent) and fixed nitrogen, but sugar is ultimately supplied by green plants. In animals, the ability to synthesize amino acids is limited; they get amino acids by eating green plants or other animals. In the digestive tract, the absorbed proteins are broken down into amino acids, the latter are absorbed, and proteins characteristic of the given organism are already built from them. No absorbed protein is incorporated into the structures of the body as such. The only exception is that in many mammals, part of the maternal antibodies can pass through the placenta into the fetal bloodstream intact, and through breast milk (especially in ruminants) be transferred to the newborn immediately after birth.
Protein requirements.
It is clear that in order to maintain life, the body must receive a certain amount of protein from food. However, the size of this need depends on a number of factors. The body needs food both as a source of energy (calories) and as a material for building its structures. In the first place is the need for energy. This means that when there are few carbohydrates and fats in the diet, dietary proteins are used not to synthesize their own proteins, but as a source of calories. With prolonged fasting, even one's own proteins are spent on meeting energy needs. If there are enough carbohydrates in the diet, then protein intake can be reduced.
Nitrogen balance.
On average approx. 16% of the total protein mass is nitrogen. When the amino acids that were part of the proteins are broken down, the nitrogen contained in them is excreted from the body in the urine and (to a lesser extent) in the feces in the form of various nitrogenous compounds. Therefore, it is convenient to use an indicator such as nitrogen balance to assess the quality of protein nutrition, i.e. the difference (in grams) between the amount of nitrogen entering the body and the amount of nitrogen excreted per day. With a normal diet in an adult, these quantities are equal. In a growing organism, the amount of excreted nitrogen is less than the amount received, i.e. the balance is positive. With a lack of protein in the diet, the balance is negative. If there are enough calories in the diet, but proteins are completely absent in it, the body conserves proteins. In this case, protein metabolism slows down, and the re-utilization of amino acids in protein synthesis is as efficient as possible. However, losses are inevitable, and nitrogenous compounds are still excreted in the urine and partially in the feces. The amount of nitrogen excreted from the body per day during protein starvation can serve as a measure of the daily lack of protein. It is natural to assume that by introducing an amount of protein equivalent to this deficiency into the diet, it is possible to restore the nitrogen balance. However, it is not. Having received this amount of protein, the body begins to use amino acids less efficiently, so some additional protein is required to restore nitrogen balance.
If the amount of protein in the diet exceeds what is required to maintain nitrogen balance, then there is apparently no harm from this. Excess amino acids are simply used as an energy source. As a particularly striking example, we can cite the Eskimos, who are low in carbohydrates and about ten times more protein than is required to maintain nitrogen balance. In most cases, however, using protein as a source of energy is disadvantageous, since a certain amount of carbohydrates can provide many more calories than the same amount of protein. In poor countries, the population receives the necessary calories from carbohydrates and consumes the minimum amount of protein.
If the body receives the required number of calories in the form of non-protein foods, then the minimum amount of protein that maintains the nitrogen balance is approx. 30 g per day. About 4 slices of bread or 0.5 liters of milk contains about the same amount of protein. A slightly larger amount is usually considered optimal; recommended from 50 to 70 g.
Essential amino acids.
Until now, protein has been viewed as a whole. Meanwhile, in order for protein synthesis to proceed, all the necessary amino acids must be present in the body. The body of the animal itself is able to synthesize some of the amino acids. They are called non-essential because they do not have to be present in the diet - it is only important that the overall intake of protein as a source of nitrogen is sufficient; then, with a shortage of nonessential amino acids, the body can synthesize them at the expense of those that are present in excess. The rest, "irreplaceable", amino acids cannot be synthesized and must enter the body with food. Valine, leucine, isoleucine, threonine, methionine, phenylalanine, tryptophan, histidine, lysine and arginine are indispensable for humans. (Although arginine can be synthesized in the body, it is classified as an essential amino acid, since it is not produced in sufficient quantities in newborns and growing children. On the other hand, for a mature person, the intake of some of these amino acids in the diet may become unnecessary.)
This list of essential amino acids is approximately the same in other vertebrates and even in insects. The nutritional value of proteins is usually determined by feeding them to growing rats and monitoring the weight gain of the animals.
The nutritional value of proteins.
The nutritional value of protein is determined by the essential amino acid that is most lacking. Let us illustrate this with an example. The proteins of our body contain on average approx. 2% tryptophan (by weight). Let's say that the diet includes 10 g of protein containing 1% tryptophan, and that there are enough other essential amino acids in it. In our case, 10 g of this defective protein is essentially equivalent to 5 g of complete protein; the remaining 5 g can only serve as a source of energy. Note that since amino acids are practically not stored in the body, and in order for protein synthesis to proceed, all amino acids must be present at the same time, the effect of the intake of essential amino acids can be detected only if all of them enter the body at the same time.
The average composition of most animal proteins is close to the average composition of proteins in the human body, so we are unlikely to face an amino acid deficiency if our diet is rich in foods such as meat, eggs, milk and cheese. However, there are proteins, such as gelatin (a product of collagen denaturation), which contain very few essential amino acids. Vegetable proteins, although they are better than gelatin in this sense, are also poor in essential amino acids; they are especially low in lysine and tryptophan. Nevertheless, a purely vegetarian diet cannot be considered harmful at all, if only a slightly larger amount of plant proteins is consumed, sufficient to provide the body with essential amino acids. Most of the protein is found in plants in seeds, especially in seeds of wheat and various legumes. Young shoots such as asparagus are also rich in protein.
Synthetic proteins in the diet.
By adding small amounts of synthetic essential amino acids or proteins rich in them to deficient proteins, such as maize proteins, it is possible to significantly increase the nutritional value of the latter, i.e. thereby, as it were, to increase the amount of protein consumed. Another possibility is to grow bacteria or yeast on petroleum hydrocarbons with the addition of nitrates or ammonia as a nitrogen source. The microbial protein obtained in this way can serve as feed for poultry or livestock, or it can be directly consumed by humans. The third, widely used, method uses the features of the physiology of ruminants. In ruminants in the initial section of the stomach, the so-called. In the rumen, special forms of bacteria and protozoa live, which convert defective plant proteins into more complete microbial proteins, and these, in turn, after being digested and absorbed, turn into animal proteins. Urea can be added to livestock feed - a cheap synthetic nitrogen-containing compound. The microorganisms inhabiting the rumen use urea nitrogen to convert carbohydrates (which are much more abundant in the feed) into protein. About a third of all nitrogen in livestock feed can come in the form of urea, which in fact means, to some extent, chemical protein synthesis.
Protein molecules are made up of amino acid residues linked into a chain by a peptide bond.
Peptide bond occurs when proteins are formed as a result of the interaction of the amino group ( -NH2) of one amino acid with a carboxyl group ( -Un) another amino acid.
A dipeptide (a chain of two amino acids) and a water molecule are formed from two amino acids.
Tens, hundreds, and thousands of amino acid molecules combine with each other to form giant protein molecules.
In protein molecules, groups of atoms are repeated many times -CO-NH-; they are called amide, or in protein chemistry peptide groups... Accordingly, proteins are referred to as naturally occurring high molecular weight polyamides or polypeptides.
The total number of naturally occurring amino acids reaches 300, but some of them are quite rare.
Among the amino acids, a group of 20 most important stands out. They are found in all proteins and are named alpha amino acids.
The entire variety of proteins in most cases is formed by these twenty alpha amino acids. Moreover, for each protein, the sequence in which the residues of its constituent amino acids are connected to each other is strictly specific. The amino acid composition of proteins is determined by the genetic code of the organism.
Proteins and peptides
AND proteinsand peptides Are compounds built from amino acid residues. The differences between them are quantitative.
It is conventionally believed that:
· peptides contain up to 100 amino acid residues in a molecule
(which corresponds to a molecular weight of up to 10,000), and
· proteins - over 100 amino acid residues
(molecular weight from 10,000 to several million).
In turn, in the group of peptides, it is customary to distinguish:
· oligopeptides (low molecular weight peptides),
containing no more than 10
amino acid residues, and
· polypeptides, the chain of which includes up to 100 amino acid residues.
For macromolecules with the number of amino acid residues approaching or slightly exceeding 100, the concepts of polypeptides and proteins are practically not differentiated and are often synonymous.
Protein structure. Organization levels.
A protein molecule is an extremely complex entity. The properties of a protein depend not only on the chemical composition of its molecules, but also on other factors. For example, from the spatial structure of a molecule, from the bonds between atoms that make up a molecule.
Allocate four levels the structural organization of the protein molecule.
Primary structure
The primary structure is the sequence of the arrangement of amino acid residues in the polypeptide chains.
The sequence of amino acid residues in a chain is the most important characteristic of a protein. It is she who determines its main properties.
Each person's protein has its own unique primary structure associated with the genetic code.
Secondary structure.
Secondary structure is related to the spatial orientation of polypeptide chains.
Its main types:
Alpha helix,
· Betta-structure (looks like a folded sheet).
The secondary structure is fixed, as a rule, by hydrogen bonds between the hydrogen and oxygen atoms of the peptide groups, which are 4 units apart from each other.
Hydrogen bonds, as it were, sew the helix, keeping the polypeptide chain in a twisted state.
Tertiary structure
PROTEINS (proteins), a class of complex nitrogen-containing compounds, the most characteristic and important (along with nucleic acids) components of living matter. Proteins have many and varied functions. Most proteins are enzymes that catalyze chemical reactions. Many hormones that regulate physiological processes are also proteins. Structural proteins such as collagen and keratin are the main components of bone, hair and nails. The contractile proteins of muscles have the ability to change their length, using chemical energy to perform mechanical work. Proteins include antibodies that bind and neutralize toxic substances. Some proteins that can respond to external influences (light, smell) serve as receptors in the sense organs that perceive irritation. Many proteins located inside the cell and on the cell membrane perform regulatory functions.In the first half of the 19th century. many chemists, among them in the first place J. von Liebig, gradually came to the conclusion that proteins are a special class of nitrogenous compounds. The name "proteins" (from the Greek.
protos - the first) was proposed in 1840 by the Dutch chemist G. Mulder. PHYSICAL PROPERTIES Proteins are white in solid state, and colorless in solution, unless they carry some chromophore (colored) group, such as hemoglobin. Water solubility varies greatly between proteins. It also changes depending on the pH and on the concentration of salts in the solution, so that conditions can be selected under which one protein will selectively precipitate in the presence of other proteins. This "salting-out" method is widely used for the isolation and purification of proteins. Purified protein often precipitates out of solution as crystals.In comparison with other compounds, the molecular weight of proteins is very high - from several thousand to many millions of daltons. Therefore, during ultracentrifugation, proteins are precipitated, and moreover, at different rates. Due to the presence of positively and negatively charged groups in protein molecules, they move at different speeds and in an electric field. This is the basis of electrophoresis - a method used to isolate individual proteins from complex mixtures. Protein purification is also carried out by chromatography.
CHEMICAL PROPERTIES Structure. Proteins are polymers, i.e. molecules built like chains from repeating monomeric units, or subunits, the role of which they play a -amino acids. General amino acid formulawhere R - a hydrogen atom or some organic group.A protein molecule (polypeptide chain) can consist of only a relatively small number of amino acids or several thousand monomer units. The combination of amino acids in a chain is possible because each of them has two different chemical groups: an amino group with basic properties,
NH 2 , and an acidic carboxyl group, COOH. Both of these groups are attached to a -atom carbon. The carboxyl group of one amino acid can form an amide (peptide) bond with the amino group of another amino acid:
After the two amino acids have joined in this way, the chain can be extended by adding a third to the second amino acid, etc. As you can see from the above equation, when the peptide bond is formed, a water molecule is released. In the presence of acids, alkalis or proteolytic enzymes, the reaction proceeds in the opposite direction: the polypeptide chain is split into amino acids with the addition of water. This reaction is called hydrolysis. Hydrolysis occurs spontaneously, and energy is required to combine amino acids into a polypeptide chain. A carboxyl group and an amide group (or a similar imide group - in the case of the amino acid proline) are present in all amino acids, the differences between amino acids are determined by the nature of that group, or "side chain", which is indicated above by the letter
R ... The role of the side chain can be played by one hydrogen atom, like in the amino acid glycine, and some bulky grouping, like in histidine and tryptophan. Some side chains are chemically inert, while others are markedly reactive.Many thousands of different amino acids can be synthesized, and many different amino acids are found in nature, but only 20 types of amino acids are used for protein synthesis: alanine, arginine, asparagine, aspartic acid, valine, histidine, glycine, glutamine, glutamic acid, isoleucine, leucine, lysine , methionine, proline, serine, tyrosine, threonine, tryptophan, phenylalanine and cysteine \u200b\u200b(in proteins, cysteine \u200b\u200bcan be present as a dimer
cystine). True, some proteins also contain other amino acids besides the regularly occurring twenty, but they are formed as a result of modification of any of the twenty listed after it has been incorporated into the protein.Optical activity. All amino acids, with the exception of glycine, to a Four different groups are attached to the carbon atom. From the point of view of geometry, four different groups can be attached in two ways, and accordingly there are two possible configurations, or two isomers, related to each other, like an object to its mirror image, i.e. like the left hand to the right. One configuration is called left, or levorotatory (L ), and the other - right, or dextrorotatory (D ), since two such isomers differ in the direction of rotation of the plane of polarized light. Proteins contain onlyL -amino acids (the exception is glycine; it can be represented only in one form, since it has two of the four groups are the same), and they all have optical activity (since there is only one isomer).D -amino acids are rare in nature; they are found in some antibiotics and in the cell wall of bacteria.Amino acid sequence. The amino acids in the polypeptide chain are not arranged randomly, but in a certain fixed order, and it is this order that determines the functions and properties of the protein. By varying the order of the 20 types of amino acids, you can get a huge number of different proteins, just as you can make up many different texts from the letters of the alphabet.In the past, it often took several years to determine the amino acid sequence of a protein. Direct determination is still a rather laborious task, although devices have been created that allow it to be carried out automatically. It is usually easier to determine the nucleotide sequence of the corresponding gene and deduce the amino acid sequence of the protein from it. To date, the amino acid sequences of many hundreds of proteins have already been determined. The functions of the decoded proteins are usually known, and this helps to imagine the possible functions of similar proteins formed, for example, in malignant neoplasms.
Complex proteins. Proteins consisting only of amino acids are called simple proteins. Often, however, a metal atom or some chemical compound other than an amino acid is attached to the polypeptide chain. These proteins are called complex proteins. An example is hemoglobin: it contains iron porphyrin, which determines its red color and allows it to act as an oxygen carrier.The names of most complex proteins contain an indication of the nature of the attached groups: there are sugars in glycoproteins, and fats in lipoproteins. If the catalytic activity of the enzyme depends on the attached group, then it is called a prosthetic group. Often, some vitamin plays the role of a prosthetic group or is part of it. Vitamin A, for example, attached to one of the retinal proteins, determines its sensitivity to light.
Tertiary structure. It is not so much the amino acid sequence of the protein itself (primary structure) that is important, but the way of its packing in space. Along the entire length of the polypeptide chain, hydrogen ions form regular hydrogen bonds, which give it the shape of a spiral or a layer (secondary structure). The combination of such helices and layers gives rise to a compact form of the next order - the tertiary structure of the protein. Rotations through small angles are possible around the bonds holding the monomeric links of the chain. Therefore, from a purely geometric point of view, the number of possible configurations for any polypeptide chain is infinitely large. In reality, however, each protein normally exists in only one configuration, determined by its amino acid sequence. This structure is not rigid, it seems to be « breathes ”- fluctuates around a certain average configuration. The chain folds into a configuration in which free energy (the ability to perform work) is minimal, just as a released spring is compressed only to a state corresponding to a minimum of free energy. Often, one part of the chain is rigidly linked to another by disulfide (-S – S–) bonds between two cysteine \u200b\u200bresidues. This is partly why cysteine \u200b\u200bplays a particularly important role among amino acids.The complexity of the structure of proteins is so great that it is still impossible to calculate the tertiary structure of a protein, even if its amino acid sequence is known. But if it is possible to obtain crystals of a protein, then its tertiary structure can be determined by X-ray diffraction.
In structural, contractile, and some other proteins, the chains are elongated and several slightly folded chains lying side by side form fibrils; fibrils, in turn, fold into larger formations - fibers. However, most proteins in solution have a globular shape: the chains are coiled in a globule, like yarn in a ball. Free energy in this configuration is minimal, since hydrophobic ("water repelling") amino acids are hidden inside the globule, and hydrophilic ("water attracting") are on its surface.
Many proteins are complexes of several polypeptide chains. This structure is called the quaternary protein structure. The hemoglobin molecule, for example, consists of four subunits, each of which is a globular protein.
Structural proteins, due to their linear configuration, form fibers with a very high tensile strength, while the globular configuration allows proteins to enter into specific interactions with other compounds. On the surface of the globule, with the correct stacking of the chains, cavities of a certain shape appear, in which reactive chemical groups are located. If a given protein is an enzyme, then another, usually smaller, molecule of some substance enters such a cavity just like a key enters a lock; in this case, the configuration of the electron cloud of the molecule under the influence of the chemical groups in the cavity changes, and this forces it to react in a certain way. In this way, the enzyme catalyzes the reaction. Antibody molecules also have cavities in which various foreign substances are bound and thereby rendered harmless. The “key and lock” model, which explains the interaction of proteins with other compounds, makes it possible to understand the specificity of enzymes and antibodies; their ability to react only with certain compounds.
Proteins in different types of organisms. Proteins that perform the same function in different plant and animal species and therefore bear the same name also have a similar configuration. They, however, differ somewhat in their amino acid sequence. As species diverge from a common ancestor, some amino acids at certain positions are replaced by others as a result of mutations. Harmful mutations that cause hereditary diseases are discarded by natural selection, but beneficial or at least neutral ones can remain. The closer two biological species are to each other, the less differences are found in their proteins.Some proteins change relatively quickly, others are very conservative. The latter include, for example, cytochrome with - a respiratory enzyme found in most living organisms. In humans and chimpanzees, its amino acid sequences are identical, and in cytochrome with wheat turned out to be different only 38% of amino acids. Even comparing humans and bacteria, the similarity of cytochromes with (differences affect 65% of the amino acids here) can still be seen, although the common ancestor of bacteria and humans lived on Earth about two billion years ago. Nowadays, amino acid sequence comparisons are often used to construct a phylogenetic (genealogical) tree reflecting evolutionary relationships between different organisms.
Denaturation. The synthesized protein molecule, folding, acquires its characteristic configuration. This configuration, however, can be destroyed by heating, by a change in pH, by organic solvents, and even by simple agitation of the solution until bubbles appear on its surface. The protein thus modified is called denatured; it loses its biological activity and usually becomes insoluble. Familiar examples of denatured protein are boiled eggs or whipped cream. Small proteins, containing only about a hundred amino acids, are able to renature, i.e. re-acquire the original configuration. But most of the proteins are simply converted into a mass of entangled polypeptide chains and does not restore the previous configuration.One of the main difficulties in isolating active proteins is associated with their extreme sensitivity to denaturation. This property of proteins is useful in preserving food: high temperature irreversibly denatures enzymes of microorganisms, and microorganisms die.
SYNTHESIS OF PROTEINS For protein synthesis, a living organism must have a system of enzymes capable of attaching one amino acid to another. A source of information is also needed that would determine which amino acids should be combined. Since there are thousands of types of proteins in the body, and each of them consists of an average of several hundred amino acids, the information required must be truly enormous. It is stored (similar to how a tape is stored) in the nucleic acid molecules that make up the genes. Cm . also HERITAGE; NUCLEIC ACIDS.Enzyme activation. A polypeptide chain synthesized from amino acids is not always a protein in its final form. Many enzymes are synthesized at first in the form of inactive precursors and become active only after another enzyme has removed several amino acids at one end of the chain. In this inactive form, some of the digestive enzymes are synthesized, for example trypsin; these enzymes are activated in the digestive tract by removing the end of the chain. The hormone insulin, the molecule of which in its active form consists of two short chains, is synthesized as one chain, the so-called. proinsulin. Then the middle part of this chain is removed, and the remaining fragments bind to each other, forming an active hormone molecule. Complex proteins are formed only after a certain chemical group is attached to the protein, and this attachment often requires an enzyme.Metabolic circulation. After feeding the animal amino acids labeled with radioactive isotopes of carbon, nitrogen or hydrogen, the label is quickly incorporated into its proteins. If the labeled amino acids cease to enter the body, the amount of the label in the proteins begins to decrease. These experiments show that the proteins formed are not stored in the body until the end of life. All of them, with a few exceptions, are in a dynamic state, constantly decaying to amino acids, and then synthesized again.Some proteins break down when cells die and are destroyed. This constantly happens, for example, with erythrocytes and epithelial cells lining the inner surface of the intestine. In addition, protein breakdown and resynthesis also take place in living cells. Oddly enough, less is known about the breakdown of proteins than about their synthesis. It is clear, however, that the breakdown involves proteolytic enzymes similar to those that break down proteins into amino acids in the digestive tract.
The half-life of different proteins is different - from several hours to many months. The only exception is collagen molecules. Once formed, they remain stable, are not renewed or replaced. Over time, however, some of their properties change, in particular elasticity, and since they are not renewed, certain age-related changes are the result of this, for example, the appearance of wrinkles on the skin.
Synthetic proteins. Chemists have long learned how to polymerize amino acids, but amino acids combine in this disordered manner, so that the products of such polymerization are not very similar to natural ones. True, it is possible to combine amino acids in a given order, which makes it possible to obtain some biologically active proteins, in particular insulin. The process is quite complicated, and in this way it is possible to obtain only those proteins, the molecules of which contain about a hundred amino acids. It is preferable to instead synthesize or isolate the nucleotide sequence of a gene corresponding to the desired amino acid sequence, and then introduce this gene into a bacterium, which will produce a large amount of the desired product by replication. This method, however, also has its drawbacks. Cm . see also GENE ENGINEERING. PROTEIN AND NUTRITION When proteins in the body are broken down into amino acids, these amino acids can be used again to synthesize proteins. At the same time, the amino acids themselves are subject to degradation, so that they are not completely reused. It is also clear that during growth, pregnancy and wound healing, protein synthesis must exceed decay. The body is constantly losing some proteins; these are proteins of hair, nails and the surface layer of the skin. Therefore, for the synthesis of proteins, each organism must receive amino acids from food. Green plants are synthesized from CO2 , water and ammonia or nitrates are all 20 amino acids found in proteins. Many bacteria are also able to synthesize amino acids in the presence of sugar (or some equivalent) and fixed nitrogen, but sugar is ultimately supplied by green plants. In animals, the ability to synthesize amino acids is limited; they get amino acids by eating green plants or other animals. In the digestive tract, the absorbed proteins are broken down into amino acids, the latter are absorbed, and proteins characteristic of the given organism are already built from them. No absorbed protein is incorporated into the structures of the body as such. The only exception is that in many mammals, part of the maternal antibodies can pass through the placenta into the fetal bloodstream intact, and through breast milk (especially in ruminants) be transferred to the newborn immediately after birth.Protein requirements. It is clear that in order to maintain life, the body must receive a certain amount of protein from food. However, the size of this need depends on a number of factors. The body needs food both as a source of energy (calories) and as a material for building its structures. In the first place is the need for energy. This means that when there are few carbohydrates and fats in the diet, dietary proteins are used not to synthesize their own proteins, but as a source of calories. With prolonged fasting, even one's own proteins are spent on meeting energy needs. If there are enough carbohydrates in the diet, then protein intake can be reduced.Nitrogen balance. On average approx. 16% of the total protein mass is nitrogen. When the amino acids that were part of the proteins are broken down, the nitrogen contained in them is excreted from the body in the urine and (to a lesser extent) in the feces in the form of various nitrogenous compounds. Therefore, it is convenient to use an indicator such as nitrogen balance to assess the quality of protein nutrition, i.e. the difference (in grams) between the amount of nitrogen entering the body and the amount of nitrogen excreted per day. With a normal diet in an adult, these quantities are equal. In a growing organism, the amount of excreted nitrogen is less than the amount received, i.e. the balance is positive. With a lack of protein in the diet, the balance is negative. If there are enough calories in the diet, but proteins are completely absent in it, the body conserves proteins. In this case, protein metabolism slows down, and the re-utilization of amino acids in protein synthesis is as efficient as possible. However, losses are inevitable, and nitrogenous compounds are still excreted in the urine and partially in the feces. The amount of nitrogen excreted from the body per day during protein starvation can serve as a measure of the daily lack of protein. It is natural to assume that by introducing an amount of protein equivalent to this deficiency into the diet, it is possible to restore the nitrogen balance. However, it is not. Having received this amount of protein, the body begins to use amino acids less efficiently, so some additional protein is required to restore nitrogen balance.If the amount of protein in the diet exceeds what is required to maintain nitrogen balance, then there is apparently no harm from this. Excess amino acids are simply used as an energy source. As a particularly striking example, we can cite the Eskimos, who are low in carbohydrates and about ten times more protein than is required to maintain nitrogen balance. In most cases, however, using protein as a source of energy is disadvantageous, since a certain amount of carbohydrates can provide many more calories than the same amount of protein. In poor countries, the population receives the necessary calories from carbohydrates and consumes the minimum amount of protein.
If the body receives the required number of calories in the form of non-protein foods, then the minimum amount of protein that maintains the nitrogen balance is approx. 30 g per day. About 4 slices of bread or 0.5 liters of milk contains about the same amount of protein. A slightly larger amount is usually considered optimal; recommended from 50 to 70 g.
Essential amino acids. Until now, protein has been viewed as a whole. Meanwhile, in order for protein synthesis to proceed, all the necessary amino acids must be present in the body. The body of the animal itself is able to synthesize some of the amino acids. They are called non-essential because they do not have to be present in the diet - it is only important that the overall intake of protein as a source of nitrogen is sufficient; then, with a shortage of nonessential amino acids, the body can synthesize them at the expense of those that are present in excess. The rest, "irreplaceable", amino acids cannot be synthesized and must enter the body with food. Valine, leucine, isoleucine, threonine, methionine, phenylalanine, tryptophan, histidine, lysine and arginine are indispensable for humans. (Although arginine can be synthesized in the body, it is classified as an essential amino acid, since it is not produced in sufficient quantities in newborns and growing children. On the other hand, for a mature person, the intake of some of these amino acids in the diet may become unnecessary.)This list of essential amino acids is approximately the same in other vertebrates and even in insects. The nutritional value of proteins is usually determined by feeding them to growing rats and monitoring the weight gain of the animals.
The nutritional value of proteins. The nutritional value of protein is determined by the essential amino acid that is most lacking. Let us illustrate this with an example. The proteins of our body contain on average approx. 2% tryptophan (by weight). Let's say that the diet includes 10 g of protein containing 1% tryptophan, and that there are enough other essential amino acids in it. In our case, 10 g of this defective protein is essentially equivalent to 5 g of complete protein; the remaining 5 g can only serve as a source of energy. Note that, since amino acids are practically not stored in the body, and in order for protein synthesis to proceed, all amino acids must be present at the same time, the effect of the intake of essential amino acids can be detected only if all of them enter the body simultaneously. The average composition of most animal proteins is close to the average composition of proteins in the human body, so we are unlikely to face an amino acid deficiency if our diet is rich in foods such as meat, eggs, milk and cheese. However, there are proteins, such as gelatin (a product of collagen denaturation), which contain very few essential amino acids. Vegetable proteins, although they are better than gelatin in this sense, are also poor in essential amino acids; they are especially low in lysine and tryptophan. Nevertheless, a purely vegetarian diet cannot be considered harmful at all, if only a slightly larger amount of plant proteins is consumed, sufficient to provide the body with essential amino acids. Most of the protein is found in plants in seeds, especially in seeds of wheat and various legumes. Young shoots such as asparagus are also rich in protein.Synthetic proteins in the diet. By adding small amounts of synthetic essential amino acids or proteins rich in them to deficient proteins, such as maize proteins, it is possible to significantly increase the nutritional value of the latter, i.e. thereby, as it were, to increase the amount of protein consumed. Another possibility is to grow bacteria or yeast on petroleum hydrocarbons with the addition of nitrates or ammonia as a nitrogen source. The microbial protein obtained in this way can serve as feed for poultry or livestock, or it can be directly consumed by humans. The third, widely used, method uses the features of the physiology of ruminants. In ruminants in the initial section of the stomach, the so-called. In the rumen, special forms of bacteria and protozoa live, which convert defective plant proteins into more complete microbial proteins, and these, in turn, after being digested and absorbed, turn into animal proteins. Urea can be added to livestock feed - a cheap synthetic nitrogen-containing compound. The microorganisms inhabiting the rumen use urea nitrogen to convert carbohydrates (which are much more abundant in the feed) into protein. About a third of all nitrogen in livestock feed can come in the form of urea, which in fact means, to some extent, chemical protein synthesis. In the United States, this method plays an important role as one of the ways to obtain protein. LITERATURE Murray R., Grenner D., Meyes P., Rodwell W. Human biochemistry, vols. 1-2. M., 1993Alberts B., Bray D., Luce J. et al. Molecular cell biology, vols. 1-3. M., 1994
Proteins are biopolymers, the monomers of which are alpha-amino acid residues interconnected through peptide bonds. The amino acid sequence of each protein is strictly defined; in living organisms it is encrypted by means of a genetic code, on the basis of which the biosynthesis of protein molecules occurs. 20 amino acids are involved in the construction of proteins.
There are the following types of structure of protein molecules:
- Primary. It is an amino acid sequence in a linear chain.
- Secondary. This is a more compact packing of polypeptide chains by forming hydrogen bonds between peptide groups. There are two variants of the secondary structure - alpha-helix and beta-folding.
- Tertiary. It is a folding of a polypeptide chain into a globule. In this case, hydrogen, disulfide bonds are formed, and the stabilization of the molecule is also realized due to hydrophobic and ionic interactions of amino acid residues.
- Quaternary. A protein consists of several polypeptide chains that interact with each other through non-covalent bonds.
Thus, the amino acids connected in a certain sequence form a polypeptide chain, the individual parts of which fold into a spiral or form folds. Such elements of the secondary structures form globules, forming the tertiary structure of the protein. Individual globules interact with each other, forming complex protein complexes with a quaternary structure.
Protein classification
There are several criteria by which protein compounds can be classified. By composition, simple and complex proteins are distinguished. Complex protein substances contain in their composition non-amino acid groups, the chemical nature of which can be different. Depending on this, there are:
- glycoproteins;
- lipoproteins;
- nucleoproteins;
- metalloproteins;
- phosphoproteins;
- chromoproteins.
There is also a classification according to the general type of structure:
- fibrillar;
- globular;
- membrane.
Proteins are called simple (one-component) proteins, consisting only of amino acid residues. Depending on their solubility, they are divided into the following groups:
This classification is not entirely accurate, because according to recent studies, many simple proteins are associated with a minimum amount of non-protein compounds. So, some proteins include pigments, carbohydrates, and sometimes lipids, which makes them more like complex protein molecules.
Physicochemical properties of protein
The physicochemical properties of proteins are determined by the composition and amount of amino acid residues included in their molecules. The molecular weights of polypeptides vary greatly: from several thousand to a million or more. The chemical properties of protein molecules are varied, including amphotericity, solubility, and denaturation.
Amphotericity
Since proteins include both acidic and basic amino acids, the molecule will always contain free acidic and free basic groups (COO- and NH3 +, respectively). The charge is determined by the ratio of basic and acidic amino acid groups. For this reason, proteins are charged “+” if the pH decreases, and vice versa, “-” if the pH increases. In the case when the pH corresponds to the isoelectric point, the protein molecule will have zero charge. Amphotericity is important for biological functions, one of which is the maintenance of blood pH.
Solubility
The classification of proteins by their solubility property has already been given above. The solubility of protein substances in water is explained by two factors:
- charge and mutual repulsion of protein molecules;
- formation of a hydration shell around the protein - water dipoles interact with charged groups on the outer part of the globule.
Denaturation
The physicochemical property of denaturation is the process of destruction of the secondary, tertiary structure of a protein molecule under the influence of a number of factors: temperature, the action of alcohols, salts of heavy metals, acids and other chemical agents.
Important! The primary structure is not destroyed by denaturation.
Chemical properties of proteins, qualitative reactions, reaction equations
The chemical properties of proteins can be considered by the example of reactions of their qualitative detection. Qualitative reactions make it possible to determine the presence of a peptide group in a compound:
1. Xanthoprotein. When a high concentration of nitric acid acts on the protein, a precipitate is formed, which turns yellow when heated.
2. Biuret. When exposed to a weakly alkaline solution of a protein of copper sulfate, complex compounds are formed between copper ions and polypeptides, which is accompanied by the staining of the solution in a violet-blue color. The reaction is used in clinical practice to determine the concentration of protein in serum and other biological fluids.
Another important chemical property is the detection of sulfur in protein compounds. For this purpose, an alkaline protein solution is heated with lead salts. This produces a black precipitate containing lead sulfide.
The biological significance of protein
Due to their physical and chemical properties, proteins perform a large number of biological functions, the list of which includes:
- catalytic (enzyme proteins);
- transport (hemoglobin);
- structural (keratin, elastin);
- contractile (actin, myosin);
- protective (immunoglobulins);
- signal (receptor molecules);
- hormonal (insulin);
- energy.
Proteins are important for the human body, since they participate in the formation of cells, provide muscle contraction in animals, and carry many chemical compounds with blood serum. In addition, protein molecules are a source of essential amino acids and carry out a protective function, participating in the production of antibodies and the formation of immunity.
TOP 10 little-known facts about protein
- The study of proteins began in 1728, when the Italian Jacopo Bartolomeo Beccari isolated protein from flour.
- Recombinant proteins are now widely used. They are synthesized by modifying the bacterial genome. In particular, insulin, growth factors and other protein compounds that are used in medicine are obtained in this way.
- Protein molecules have been found in Antarctic fish that prevent blood from freezing.
- Resilin protein is characterized by ideal elasticity and is the basis for the attachment points for insect wings.
- The body contains unique chaperone proteins that are able to restore the correct native tertiary or quaternary structure of other protein compounds.
- The nucleus of the cell contains histones - proteins that take part in the compaction of chromatin.
- The molecular nature of antibodies - special protective proteins (immunoglobulins) - has been actively studied since 1937. Tiselius and Kabat used electrophoresis and proved that the gamma fraction was increased in immunized animals, and after absorption of serum by the provoking antigen, the distribution of proteins by fractions returned to the picture of an intact animal.
- Egg white is a vivid example of the implementation of the reserve function by protein molecules.
- In the collagen molecule, every third amino acid residue is formed by glycine.
- In the composition of glycoproteins, 15-20% are carbohydrates, and in the composition of proteoglycans, their share is 80-85%.
Conclusion
Proteins are the most complex compounds, without which it is difficult to imagine the vital activity of any organism. More than 5000 protein molecules have been isolated, but each individual has its own set of proteins and this differs from other individuals of its own species.
The most important chemical and physical properties of proteins
updated: March 21, 2019 by the author: Scientific Articles.Ru
