Proteins are formed as a result of the reaction. Proteins. Classification. Functions. Organization levels. Physicochemical properties

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 at the expense of 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, the chains of which are assembled from amino acid fragments. Amino acids are organic compounds containing (in accordance with the name) an amino group NH 2 and an organic acidic group, i.e. carboxyl, COOH group. Of the whole 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. V general view 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 = 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.

Table 1. AMINO ACIDS PARTICIPATING IN THE CREATION OF PROTEINS

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
ASPARAGIC 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 on a gray background, are called essential, 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.

Rice. 1 SERIAL COMPOUND OF AMINO ACIDS 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 in which they alternate 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.

Rice. 2 INSULIN MOLECULE built of 51 amino acid residues, fragments of the same amino acids are marked with the corresponding background color. Cysteine ​​amino 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 ​​amino acid molecules (Table 1) contain reactive sulfhydride groups –SH, which interact with each other to form –S – S– disulfide bridges. The role of cysteine ​​in the world of proteins is special, with its participation cross-links are formed between polymer protein molecules.

The combination of amino acids into a polymer chain occurs in a living organism under the control nucleic acids, it is they that provide a strict assembly order and regulate the fixed length of the polymer molecule ().

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 () arise between the imino groups HN and carbonyl groups CO present in the polymer chain; as a result, the protein molecule acquires a certain spatial form, 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 = O groups, between which there are two peptide fragments H-N-C = 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-shaped ribbon passing through the atoms forming a polymer chain (Fig. 4)

Rice. 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 = O groups of two or more polymer chains located 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 = O groups is of decisive importance. Since, relative to the polymer chain, the H-N and C = O groups are directed in different directions (in the figure - up and down), it becomes possible to simultaneously interact with three or more chains.

The composition of the first polypeptide chain in Fig. 5:

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 inside 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 in the form of a volumetric model. The 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 proposed to depict 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. Nowadays, this method of imaging the tertiary structure of a protein is widely used (Fig. 9, variant B). Sometimes, for more informational content, they show together a 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) do not indicate, 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.

Rice. nine DIFFERENT IMAGE OPTIONS OF CRAMBIN PROTEIN STRUCTURE.
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 certain order... The tertiary structure contains α-helices connected by single chains (Fig. 10).

Rice. ten GLOBULAR STRUCTURE OF ALBUMIN

An example of a fibrillar structure is fibroin protein. They contain a large amount of glycine, alanine and serine residues (every second amino acid residue is glycine); residues of cysteine ​​containing sulfhydride groups are absent. Fibroin, the main component of natural silk and spider webs, contains β-structures connected by single chains (Fig. 11).

Rice. eleven FIBRILLARY PROTEIN FIBROIN

The possibility of the formation of a tertiary structure of a certain type is inherent in the primary structure of the protein, i.e. predetermined by the order of alternation of amino acid residues. From certain sets of such residues, α-helices predominantly 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, which consists mainly of leucine, glutamic acid, aspartic acid, and histidine (all 20 amino acid residues in ferricin, 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 QUATERNARY 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, therefore boiled (or pasteurized) milk can last longer.

The peptide bonds H-N-C = O, which form the polymer chain of the protein molecule, are hydrolyzed in the presence of acids or alkalis, and the polymer chain is broken, 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 (in comparison with 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).

Rice. fourteen. 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 fragment of one amino acid (Fig. 15).

Rice. 15 SEQUENTIAL DEGRADATION 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 S-S disulfide bridges (formed during the interaction of cysteine ​​residues, 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) again leads to the formation of disulfide bridges (Fig. 16).

Rice. 16. SPLITTING OF DISULFIDE 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).

Rice. 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 are able to 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 a certain type of body. Enzymes are also involved in many biochemical synthesis processes, for example, in the synthesis of proteins (some proteins help to synthesize others).

Enzymes are not only highly efficient catalysts, but also selective (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 flow conditions are mild: usual Atmosphere pressure and the temperature of the 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 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 an 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 has been preserved (without the end -aza), 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).

Rice. eighteen 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 passes - the oxidation of acetaldehyde to acetic acid and the longer and stronger the intoxicating effect of ethanol ingestion is manifested. The analysis showed that more than 80% of 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 ().

Rice. 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 theory of enzymes, 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 = O + H 2 O = CO 2 + 2NH 3

The finest selectivity is shown by enzymes that distinguish between optically active antipodes - left- and dextrorotatory 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 exclusively 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).

Rice. twenty ALCOHOLDEHYDROGENASE MOLECULE

Transport proteins bind and transfer various molecules or ions across cell membranes (both inside the cell and outside), 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 by coordination with nitrogen in histidine (Table 1), which is part of the protein. The O2 molecule, which is carried by hemoglobin, attaches by means of a coordination bond to the iron atom on the side opposite to that to which histidine is attached (Fig. 21, right).

Rice. 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 transported by hemoglobin, is coordinatively attached (red dotted line) 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).

Rice. 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 with Fe in hemoglobin even faster, but, unlike O 2, forms a complex that is difficult to decompose. As a result, such hemoglobin is unable to bind O 2, which leads (when large amounts of carbon monoxide is inhaled) 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 (outer wall of the cell). Such proteins are 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).

Rice. 23 PORINE PROTEIN

Food and storage proteins, as the name suggests, serve as sources of internal nutrition, more 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, as well as 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.

Iron ions are stored in ferritin (Fig. 12), which is contained in animal tissues.

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 covering) or support - they fasten the body into a single whole 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 the body of mammals, it accounts 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 during the growth and development of the body are not renewed and are not 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 renewal does not occur, 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 ​​residues, which forms disulfide bridges, which gives high elasticity (the ability to restore its original shape after deformation) to hair and woolen fabrics.

Rice. 24. FRAGMENT OF FIBRILLARY PROTEIN KERATIN

For an irreversible change in the shape of a keratin object, you must first 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 ​​residues 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 (the horns of ungulates and the shells of turtles contain up to 18% of cysteine ​​fragments). The body of mammals contains up to 30 different types keratin.

The keratin-related fibrillar protein fibroin, secreted by silkworm caterpillars during cocoon curling, as well as 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 has a 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 almost indestructible, and silk easily crumple).

Regulatory proteins.

Regulatory proteins, more commonly referred to as, 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.

Rice. 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)

Rice. 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 to protect the body from the invasion of attacking bacteria, viruses and from the penetration of foreign proteins (the generalized name of 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 linked 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 twice as short, they contain 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.

Rice. 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 end 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 others). Thus, for each antigen, a strictly individual antibody is created as an immune response. Not a single known protein can change the structure so "plastically" 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" anew. 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 creature.

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 reached (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 some 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 that can combine with –COOH - amino acid groups. A crosslinked polystyrene with chloromethyl groups introduced into it was taken as such a polymer substrate. So that the amino acid taken for the reaction does not react 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 pre-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).

Rice. 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 has been 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 that are 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-obtain proteins and peptides, as well as their analogs, 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 the proteins are synthesized again (also with the participation of enzymes), i.e. the body is constantly renewing itself. Some proteins (collagen of the skin, 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 materials 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 ​​with an essential amino acid - methionine. Part of the proteins breaks down, releasing the energy necessary to maintain vital activity, and the nitrogen contained in them is excreted from the body 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 food, it is important to consider the quality of the protein. In the absence or low content of essential amino acids, protein is considered 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, therefore, 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 the compound feed for domestic animals, 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 represent 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 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 in the form of 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 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 monomeric 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, 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 connected 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 ​​(in proteins, cysteine ​​can 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, 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, right-handed, or dextrorotatory (D), since two such isomers differ in the direction of rotation of the plane of polarized light. Proteins contain only L-amino acids (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 bacterial cell walls.

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 "breathe" - it oscillates around a certain average configuration. The chain folds into such 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 the other by disulfide (–S – S–) bonds between two cysteine ​​residues. This is partly why cysteine ​​plays 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 adjacent slightly folded chains form fibrils; fibrils, in turn, fold into larger formations - fibers. However, most proteins in solution have a globular shape: the chains are rolled up in a globule, like yarn in a ball. Free energy with this configuration, it 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, has 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 at correct styling chains arise of a certain shape of the cavity, in which reactive chemical groups are located. If the given protein is an enzyme, then another, usually smaller, molecule of some substance enters such a cavity just like a key enters a lock; this changes the configuration of the electron cloud of the molecule under the influence of the chemical groups in the cavity, 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 types plants and animals, and therefore bearing the same name, 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 natural selection but useful or at least neutral ones can persist. The closer two biological species are to each other, the less differences are found in their proteins.

Some proteins change relatively quickly, while 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 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, comparison of amino acid sequences is often used to build 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 changing the pH, by the action of organic solvents, and even by simple agitation of the solution until bubbles appear on its surface. The protein changed in this way is called denatured; it loses its biological activity and usually becomes insoluble. Well-known examples of denatured protein are boiled eggs or whipped cream. Small proteins containing only about a hundred amino acids are capable of annealing, 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 finds useful application in food preservation: high temperature irreversibly denatures enzymes of microorganisms, and microorganisms die.

PROTEIN SYNTHESIS

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 (just as 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 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, such as trypsin; these enzymes are activated in the digestive tract as a result of the removal of the end of the chain. The hormone insulin, the molecule of which in its active form consists of two short chains, is synthesized in the form of 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 an enzyme is often required for this attachment.

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, then the amount of the label in 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 break down. This constantly happens, for example, with erythrocytes and epithelial cells lining inner surface intestines. In addition, degradation and resynthesis of proteins also take place in living cells. Ironically, less is known about the breakdown of proteins than about their synthesis. It is clear, however, that proteolytic enzymes are involved in the breakdown, 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, 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 since learned 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 given order, which allows you to get 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 the gene corresponding to the desired amino acid sequence, and then introduce this gene into the 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 enter the fetal bloodstream through the placenta in intact form, 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 extent 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 amounts 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. At the same time, protein metabolism slows down, and the re-utilization of amino acids in protein synthesis proceeds with the highest possible efficiency. However, losses are inevitable, and nitrogenous compounds are still excreted in the urine and partly 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 into the diet an amount of protein equivalent to this deficiency, 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 that some additional amount of protein is required to restore nitrogen balance.

If the amount of protein in the diet exceeds what is needed to maintain nitrogen balance, then there is probably no harm from this. Excess amino acids are simply used as an energy source. As especially a striking example you can refer to the Eskimos, who consume few carbohydrates and about ten times more protein than is required to maintain nitrogen balance. In most cases, however, the use of protein as an energy source is disadvantageous, since a certain amount of carbohydrates can produce many more calories than from the same amount of protein. In poor countries, the population gets 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 four 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. Some of the amino acids the body of the animal is able to synthesize itself. 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 from food may become unnecessary.)

This list of essential amino acids is approximately the same in other vertebrates and even 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 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 the seeds of plants, especially in the 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 part 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, a cheap synthetic nitrogen-containing compound, can be added to livestock feed. 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 a certain extent, chemical protein synthesis.

Donetsk secondary school I - III levels № 21

“Proteins. Production of proteins by amino acid polycondensation reaction. Primary, secondary and tertiary structure proteins. Chemical properties of proteins: combustion, denaturation, hydrolysis and color reactions. Biochemical functions of proteins ".

Prepared

chemistry teacher

teacher - methodologist

Donetsk, 2016

"Life is a way of existence of protein bodies"

Lesson topic. Proteins. Production of proteins by amino acid polycondensation reaction. Primary, secondary and tertiary structures of proteins. Chemical properties of proteins: combustion, denaturation, hydrolysis and color reactions. Biochemical functions of proteins.

Lesson objectives. To acquaint students with proteins as the highest degree of development of substances in nature, which led to the emergence of life; show their structure, properties and variety of biological functions; to expand the concept of the polycondensation reaction on the example of obtaining proteins, to inform schoolchildren about food hygiene, about maintaining their health. Develop logical thinking in students.

Reagents and equipment. Table "Primary, secondary and tertiary structures of proteins". Reagents: HNO3, NaOH, CuSO4, chicken protein, wool thread, chemical glassware.

Lesson method. Informational - developing.

Lesson type. Lesson in assimilation of new knowledge and skills.

During the classes

І. Organizing time.

II. Examination homework, updating and correction of basic knowledge.

Blitz Question

1. Explain the term "amino acid".

2. Name the functional groups that make up the amino acids.

3. Nomenclature of amino acids and their isomerism.

4. Why do amino acids exhibit amphoteric properties? Write the equations of chemical reactions.

5. Due to what properties of amino acids form polypeptides. Write the polycondensation reaction of amino acids.

ІІІ. Communication of the topic, the purpose of the lesson, the motivation of educational activities.

IV. Perception and primary awareness of new material.

Teacher.

"Wherever we meet life, we find that it is associated with some protein body" - wrote F. Engels in his book "Anti - Dühring". Lack of protein in food leads to a general weakening of the body, in children - to a slowdown in mental and physical development. Today, more than half of humanity does not receive the required amount of protein from food. A person needs 115 g of protein per day, protein is not stored in reserve, unlike carbohydrates and fats, so you need to monitor your diet. We are familiar with keratin - a protein that makes up hair, nails, feathers, skin - it performs a building function; familiar with the protein pepsin - it is found in gastric juice and is able to destroy other proteins during digestion; the protein thrombin is involved in blood clotting; pancreatic hormone - insulin - regulates glucose metabolism; hemoglobin transports O2 to all cells and tissues of the body, etc.

Where does this endless variety of protein molecules, the variety of their functions and their special role in life processes come from? In order to answer this question, let us turn to the composition and structure of proteins.

Do proteins contain atoms? ...

To answer this question, let's do a warm-up. Guess the riddles and explain the meaning of the answers.

1. He is everywhere and everywhere:

In the stone, in the air, in the water.

He is in the morning dew

And blue in the sky.

(oxygen)

2. I am the lightest element,

In nature, not a step without me.

And with oxygen I am in the moment

3. It is the main gas in the air,

Surrounds us everywhere.

Plant life is dying out

Without it, without fertilizers.

Lives in our cells

4. Schoolchildren went on a hike somehow

(This is an approach to the chemical problem).

At night they made a fire in the moonlight,

Songs sang about bright fire.

Put your sentiment aside:

What elements were burning in the fire?

(carbon, hydrogen)

Yes, that's right, these are the main chemical elements that make up the protein.

About these four elements can be said in the words of Schiller "The four elements, merging together, give life and build the world."

Proteins are natural polymers consisting of α - amino acid residues linked by peptide bonds.

The composition of proteins includes 20 different amino acids, hence the huge variety of proteins in their various combinations. The human body contains up to 100,000 proteins.

Historical reference.

The first hypothesis about the structure of the protein molecule was proposed in the 70s. XIX century. This was the ureid theory of protein structure.

In 1903. a peptide theory was put forward by German scientists, which gave the key to the secret of the structure of the protein. Fisher suggested that proteins are polymers of amino acids linked by a peptide bond.

The idea that proteins are polymeric formations was expressed as early as 70 - 88 years. XIX century. , Russian scientists. This theory has been confirmed in modern works.

Already the first acquaintance with proteins gives some idea of ​​the extremely complex structure their molecules. Proteins are obtained by amino acid polycondensation reaction:

https://pandia.ru/text/80/390/images/image007_47.gif "width =" 16 "height =" 18 "> H - N - CH2 - C + H - N - CH2 - C →

https://pandia.ru/text/80/390/images/image012_41.gif "height =" 20 ">

NH2 - CH - C - N - CH - C - N - CH - C -… + nH2O →

⸗ O ⸗ O ⸗ O

→ NH2 - CH - C + NH2 - CH - C + NH2 - CH - C + ...

̀ OH ̀ OH ̀ OH

4. The teacher demonstrates the experience: burning a woolen thread; the smell of burnt feathers is felt - this is how you can distinguish wool from other types of fabrics.

V. Generalization and systematization of knowledge.

1. Make a basic protein synopsis.

basis of life ← Proteins → polypeptides

(C, H, O, N) ↓ ↓ ↓ \ protein structures

chemically colored functions

cue sv-va protein reactions

2. Write the reaction equations for the formation of a dipeptide from glycine and valine.

Vi. Summing up the lesson, homework.

Learn §38 p. 178 - 184. Complete test tasks p. 183.

Before talking about the most important physical and chemical properties of a protein, you need to know what it consists of, what its structure is. Proteins are an important natural biopolymer and are based on amino acids.

What are amino acids

These are organic compounds containing carboxyl and amine groups. Thanks to the first group, they have carbon, oxygen and hydrogen, and the other - nitrogen and hydrogen. Alpha amino acids are considered the most important because they are needed to make proteins.

There are essential amino acids called proteinogenic. So they are responsible for the appearance of proteins. There are only 20 of them, and they can form countless protein compounds. Moreover, none of them will be completely identical to the other. This is possible thanks to the combinations of elements that are present in these amino acids.

Their synthesis does not take place in the body. Therefore, they get there along with food. If a person receives them in insufficient quantities, then a violation is possible normal functioning different systems. Proteins are formed through a polycondensation reaction.

Proteins and their structure

Before moving on to the physical properties of proteins, it is worth giving more precise definition this organic compound... Proteins are one of the most important bioorganic compounds that are formed thanks to amino acids and take part in many processes in the body.

The structure of these compounds depends on the order in which amino acid residues alternate. As a result, it happens as follows:

• primary (linear);
• secondary (spiral);
• tertiary (globular).

Their classification

Due to the huge variety of protein compounds and varying degrees of complexity of their composition and different structure, for convenience there are classifications that rely on these characteristics.

In terms of their composition, they are as follows:

• simple;
• complex, which in turn are subdivided into:

1. a combination of protein and carbohydrates;
2. combining proteins and fats;
3. connection of protein molecules and nucleic acids.

By solubility:

• water soluble;
• fat soluble.

Small characterization of protein compounds

Before moving on to the physical and chemical properties of proteins, it will be useful to give them a little characterization. Of course, their properties are essential for the normal functioning of a living organism. By their initial state, these are solids that either dissolve in various liquids or not.

Briefly speaking about the physical properties of proteins, then they determine many of the most important biological processes in the body. For example, such as transport of substances, building function, etc. The physical properties of proteins depend on whether they are soluble or not. It is precisely about these features that will be written further.

Physical properties of proteins

The above has already been written about their state of aggregation and solubility. Therefore, we move on to the following properties:

1. They have a large molecular weight, which depends on certain environmental conditions.
2. Their solubility has a wide range, as a result of which electrophoresis becomes possible - a method by which proteins are isolated from mixtures.

Chemical properties of protein compounds

Readers now know the physical properties of proteins. Now we need to talk about the equally important chemical ones. They are listed below:

1. Denaturation... Protein coagulation when exposed to high temperatures, strong acids or alkalis. With denaturation, only the primary structure is retained, and all biological properties of proteins are lost.
2. Hydrolysis... As a result, simple proteins and amino acids are formed, because the primary structure is destroyed. It is the basis of the digestion process.
3. Qualitative reactions for protein determination... There are only two of them, and the third is needed in order to detect sulfur in these compounds.
4. Biuret reaction. Proteins are exposed to copper hydroxide precipitate. As a result, it turns purple.
5. Xanthoprotein reaction... The impact is carried out with concentrated nitric acid. This reaction produces a white precipitate that turns yellow when heated. And if you add an aqueous ammonia solution, then an orange color appears.
6. Determination of sulfur in proteins... When proteins are burned, the smell of "burnt horn" begins to be felt. This phenomenon is explained by the fact that they contain sulfur.

So it was all physical and Chemical properties proteins. But, of course, not only because of them, they are considered the most important components of a living organism. They determine the most important biological functions.

Biological properties of proteins

We examined the physical properties of proteins in chemistry. But it is also worth telling about what effect they have on the body and why without them it will not function fully. The functions of proteins are listed below:

1. enzymatic. Most of the reactions in the body involve enzymes that are of protein origin;
2. transport. These elements carry out the delivery of other important molecules to tissues and organs. One of the most important transport proteins is hemoglobin;
3. structural. Proteins are the main building blocks for many tissues (muscle, integumentary, supporting);
4. protective. Antibodies and antitoxins are a special type of protein compounds that form the basis of immunity;
5. signal. The receptors that are responsible for the functioning of the sensory organs also have proteins in their structure;
6. storing. This function is performed by special proteins, which can be a building material and are sources of additional energy during the development of new organisms.

Proteins can be converted to fats and carbohydrates. But they cannot become proteins. Therefore, the lack of these particular compounds is especially dangerous for a living organism. The energy released during the process is small and is inferior in this respect to fats and carbohydrates. However, they are the source of essential amino acids in the body.

How to understand that there is not enough protein in the body? A person's health worsens, there is rapid exhaustion and fatigue. Excellent sources of protein are various types of wheat, meat and fish products, dairy, eggs and some types of legumes.

It is important to know not only the physical properties of proteins, but also the chemical ones, as well as what significance they have for the body from a biological point of view. Protein compounds are unique in that they are sources of essential amino acids that are needed for the normal functioning of the human body.

§ 9. PHYSICO-CHEMICAL PROPERTIES OF PROTEINS

Proteins are very large molecules, in their size they can be inferior only to individual representatives of nucleic acids and polysaccharides. Table 4 shows the molecular characteristics of some proteins.

Table 4

Molecular characteristics of some proteins

	Relative molecular weight	Number of chains	Amino acid residues
Ribonuclease
Myoglobin
Chymotrypsin
Hemoglobin
Glutamate dehydrogenase

Protein molecules can contain a very different number of amino acid residues - from 50 to several thousand; the relative molecular weights of proteins also vary greatly - from several thousand (insulin, ribonuclease) to a million (glutamate dehydrogenase) and more. The number of polypeptide chains in proteins can range from one to several tens or even thousands. Thus, the protein of the tobacco mosaic virus includes 2120 protomers.

Knowing the relative molecular weight of a protein, it is possible to roughly estimate how many amino acid residues are included in its composition. The average relative molecular weight of the amino acids forming the polypeptide chain is 128. When a peptide bond is formed, a water molecule is cleaved; therefore, the average relative weight of an amino acid residue will be 128 - 18 = 110. Using these data, it can be calculated that a protein with a relative molecular weight of 100,000 will consist of approximately 909 amino acid residues.

Electrical properties of protein molecules

The electrical properties of proteins are determined by the presence of positively and negatively charged amino acid residues on their surface. The presence of charged protein groups determines the total charge of the protein molecule. If negatively charged amino acids prevail in proteins, then its molecule in a neutral solution will have a negative charge, if positively charged ones prevail, the molecule will have a positive charge. The total charge of a protein molecule also depends on the acidity (pH) of the medium. With an increase in the concentration of hydrogen ions (an increase in acidity), the dissociation of carboxyl groups is suppressed:

and at the same time, the number of protonated amino groups increases;

Thus, with an increase in the acidity of the medium, the number of negatively charged groups on the surface of the protein molecule decreases and the number of positively charged groups increases. A completely different picture is observed with a decrease in the concentration of hydrogen ions and an increase in the concentration of hydroxide ions. The number of dissociated carboxyl groups increases

and the number of protonated amino groups decreases

So, by changing the acidity of the medium, you can also change the charge of the protein molecule. With an increase in the acidity of the medium in the protein molecule, the number of negatively charged groups decreases and the number of positively charged groups increases, the molecule gradually loses its negative and acquires a positive charge. With a decrease in the acidity of the solution, the opposite picture is observed. Obviously, at certain pH values, the molecule will be electrically neutral, i.e. the number of positively charged groups will be equal to the number of negatively charged groups, and the total charge of the molecule will be zero (Fig. 14).

The pH value at which the total charge of the protein is zero is called the isoelectric point and is denotedpI.

Rice. 14. In the state of the isoelectric point, the total charge of the protein molecule is zero

The isoelectric point for most proteins is in the pH range from 4.5 to 6.5. However, there are also exceptions. Below are the isoelectric points of some proteins:

At pH values ​​below the isoelectric point, the protein carries a total positive charge, above it, a total negative one.

At the isoelectric point, the solubility of the protein is minimal, since its molecules in this state are electrically neutral and there are no forces of mutual repulsion between them, so they can "stick together" due to hydrogen and ionic bonds, hydrophobic interactions, van der Waals forces. At pH values ​​different from pI, the protein molecules will carry the same charge - either positive or negative. As a result, there will be forces of electrostatic repulsion between the molecules, preventing their "sticking", the solubility will be higher.

Solubility of proteins

Proteins are soluble and insoluble in water. The solubility of proteins depends on their structure, pH value, salt composition of the solution, temperature and other factors and is determined by the nature of those groups that are on the surface of the protein molecule. Insoluble proteins include keratin (hair, nails, feathers), collagen (tendons), fibroin (flick, spider webs). Many other proteins are water soluble. Solubility is determined by the presence on their surface of charged and polar groups (-СОО -, -NH 3 +, -OH, etc.). Charged and polar groups of proteins attract water molecules to themselves, and a hydration shell is formed around them (Fig. 15), the existence of which determines their solubility in water.

Rice. 15. Formation of a hydration shell around a protein molecule.

The solubility of the protein is affected by the presence of neutral salts (Na 2 SO 4, (NH 4) 2 SO 4, etc.) in the solution. At low salt concentrations, protein solubility increases (Fig. 16), since under such conditions the degree of dissociation of polar groups increases and charged groups of protein molecules are screened, thereby reducing the protein-protein interaction that promotes the formation of aggregates and precipitation of the protein. At high salt concentrations, protein solubility decreases (Fig. 16) due to the destruction of the hydration shell, leading to the aggregation of protein molecules.

Rice. 16. Dependence of protein solubility on salt concentration

There are proteins that dissolve only in salt solutions and do not dissolve in clean water, such proteins are called globulins... There are other proteins - albumin, unlike globulins, they are highly soluble in pure water.
The solubility of proteins also depends on the pH of the solutions. As we have already noted, proteins have the minimum solubility at the isoelectric point, which is explained by the absence of electrostatic repulsion between protein molecules.
Under certain conditions, proteins can form gels. When a gel is formed, the protein molecules form a dense network, the interior of which is filled with a solvent. Gels form, for example, gelatin (this protein is used to make jelly) and milk proteins when making curdled milk.
Temperature also affects protein solubility. On action high temperature many proteins precipitate due to a violation of their structure, but we will talk about this in more detail in the next section.

Protein denaturation

Consider a familiar phenomenon. When the egg white is heated, it gradually becomes cloudy, and then a solid clot forms. Curled egg white - egg albumin - after cooling turns out to be insoluble, while before heating, the egg white was well soluble in water. The same phenomena occur when almost all globular proteins are heated. Those changes that occurred during heating are called denaturation... Proteins in their natural state are called native proteins, and after denaturation - denatured.
With denaturation, the native conformation of proteins is disrupted as a result of breaking weak bonds (ionic, hydrogen, hydrophobic interactions). As a result of this process, the quaternary, tertiary and secondary structures of the protein can be destroyed. At the same time, the primary structure is preserved (Fig. 17).

Rice. 17. Protein denaturation

During denaturation, hydrophobic amino acid radicals found in native proteins deep in the molecule end up on the surface, resulting in conditions for aggregation. Aggregates of protein molecules precipitate. Denaturation is accompanied by the loss of the biological function of the protein.

Protein denaturation can be caused not only by fever, but also by other factors. Acids and alkalis can cause protein denaturation: as a result of their action, ionogenic groups are recharged, which leads to the rupture of ionic and hydrogen bonds. Urea destroys hydrogen bonds, which results in the loss of their native structure by proteins. Denaturing agents are organic solvents and heavy metal ions: organic solvents destroy hydrophobic bonds, and heavy metal ions form insoluble complexes with proteins.

Along with denaturation, there is also a reverse process - renaturation. When the denaturing factor is removed, the original native structure can be restored. For example, when cooling slowly to room temperature solution is restored to the native structure and biological function of trypsin.

Proteins can also denature in the cell during normal life processes. It is quite obvious that the loss of the native structure and function of proteins is an extremely undesirable event. In this regard, special proteins should be mentioned - chaperones... These proteins are able to recognize partially denatured proteins and, by binding to them, restore their native conformation. Chaperones also recognize proteins, the process of denaturation of which has gone far, and transport them to lysosomes, where they are degraded (degraded). Chaperones also play an important role in the formation of tertiary and quaternary structures during protein synthesis.

Interesting to know! Nowadays, such a disease as mad cow disease is often mentioned. This disease is caused by prions. They can cause other diseases of a neurodegenerative nature in animals and humans. Prions are infectious agents of a protein nature. When a prion enters a cell, it causes a change in the conformation of its cellular counterpart, which itself becomes a prion. This is how the disease arises. The prion protein differs from the cellular protein in its secondary structure. The prion form of the protein is mainlyb-folded structure, and cellular -a-spiral.

Isoelectric point

Amphotericity is the acid-base properties of proteins.

Quaternary structure

Many proteins are composed of several subunits (protomers), which can have the same or different amino acid composition. In this case, proteins have quaternary structure... Proteins usually contain an even number of subunits: two, four, six. Interaction occurs due to ionic, hydrogen bonds, van der Waals forces. Adult hemoglobin HbA consists of four pairwise identical subunits ( a 2 β 2).

The quaternary structure provides many biological benefits:

a) there is a saving of genetic material., the length of the structural gene and mRNA, in which information about the primary structure of the protein is recorded, decreases.

b) it is possible to carry out the substitution of subunits, which allows you to change the activity

enzyme due to changing conditions (to adapt). Hemoglobin

the newborn consists of proteins ( a 2 γ 2). but during the first months the composition becomes like that of an adult (a 2 β 2) .

8.4. Physicochemical properties of protein

Proteins, like amino acids, are amphoteric compounds and have buffering properties.

Proteins can be divided into neutral, sour and basic.

Neutral proteins contain an equal number of groups prone to ionization: acidic and basic. The isoelectric point of such proteins is in an environment close to neutral, if the pH< pI , то белок становится положительно заряженным катионом, pH >pI, then the protein becomes a negatively charged anion.

NH 3 - protein - COOH<-->+ NH 3 - protein - COO -<-->NH 2 - protein - COO -

NS< pI aqueous solution I pH> pI

Acidic proteins contain an unequal number of groups prone to ionization: there are more carboxyl groups than amino groups. In an aqueous solution, they acquire a negative charge, and the solution becomes acidic. When an acid (H +) is added, the protein first enters the isoelectric point, and then, in an excess of acid, turns into a cation. In an alkaline medium, such a protein is negatively charged (the charge of the amino group disappears).

Sour protein

NH 3 - protein - COO - + H + + NH 3 - protein - COO - + H + + NH 3 -protein - COOH

| <--> | <--> |

COO - CUN COOH

Aqueous solution pH = p I NS< pI

An excess of acid protein

positively charged

Acidic protein in an alkaline medium is negatively charged

NH 3 - protein - COO - OH - NH 2 - protein - COO -

| <--> |

COO - COO -

pH> pI

Basic proteins contain an unequal number of groups prone to ionization: there are more amino groups than carboxyl ones. In an aqueous solution, they acquire a positive charge, and the solution becomes alkaline. When alkali (OH -) is added, the protein first enters the isoelectric point, and then, in an excess of alkali, turns into an anion. In an acidic environment, such a protein is positively charged (the charge of the carboxyl group disappears)