Protein combustion reaction. Structure and properties of proteins

Squirrels- high-molecular organic compounds consisting of amino acid residues connected in a long chain by a peptide bond.

The composition of proteins in living organisms includes only 20 types of amino acids, all of which are alpha amino acids, and the amino acid composition of proteins and their order of connection with each other are determined by the individual genetic code of a living organism.

One of the features of proteins is their ability to spontaneously form spatial structures characteristic only of this particular protein.

Due to the specificity of their structure, proteins can have a variety of properties. For example, proteins with a globular quaternary structure, in particular chicken egg white, dissolve in water to form colloidal solutions. Proteins with a fibrillar quaternary structure do not dissolve in water. Fibrillar proteins, in particular, form nails, hair, and cartilage.

Chemical properties of proteins

Hydrolysis

All proteins are capable of undergoing hydrolysis reactions. In the case of complete hydrolysis of proteins, a mixture of α-amino acids is formed:

Protein + nH 2 O => mixture of α-amino acids

Denaturation

The destruction of the secondary, tertiary and quaternary structures of a protein without destroying its primary structure is called denaturation. Protein denaturation can occur under the influence of solutions of sodium, potassium or ammonium salts - such denaturation is reversible:

Denaturation occurring under the influence of radiation (for example, heating) or treatment of the protein with salts of heavy metals is irreversible:

For example, irreversible protein denaturation is observed during heat treatment of eggs during their preparation. As a result of denaturation of egg white, its ability to dissolve in water to form a colloidal solution disappears.

Qualitative reactions to proteins

Biuret reaction

If a 10% sodium hydroxide solution is added to a solution containing protein, and then a small amount of a 1% copper sulfate solution, a violet color will appear.

protein solution + NaOH (10% solution) + CuSO 4 = purple color

Xanthoprotein reaction

Protein solutions turn yellow when boiled with concentrated nitric acid:

protein solution + HNO 3 (conc.) => yellow color

Biological functions of proteins

catalytic	accelerate various chemical reactions in living organisms	enzymes
structural	cell building material	collagen, cell membrane proteins
protective	protect the body from infections	immunoglobulins, interferon
regulatory	regulate metabolic processes	hormones
transport	transfer of vital substances from one part of the body to another	hemoglobin carries oxygen
energy	supply the body with energy	1 gram of protein can provide the body with 17.6 J of energy
motor (motor)	any motor functions of the body	myosin (muscle protein)

The classification of proteins is based on their chemical composition. According to this classification, proteins are simple And complex. Simple proteins consist only of amino acids, that is, of one or more polypeptides. Simple proteins found in the human body include albumins, globulins, histones, supporting tissue proteins.

In a complex protein molecule, in addition to amino acids, there is also a non-amino acid part called prosthetic group. Depending on the structure of this group, complex proteins are distinguished such as phosphoproteins( contain phosphoric acid) nucleoproteins(contain nucleic acid), glycoproteins(contain carbohydrate) lipoproteins(contain lipoid) and others.

According to the classification, which is based on the spatial shape of proteins, proteins are divided into fibrillar And globular.

Fibrillar proteins consist of helices, that is, predominantly of secondary structure. Molecules of globular proteins have a spherical and ellipsoidal shape.

An example of fibrillar proteins is collagen – the most abundant protein in the human body. This protein accounts for 25-30% of the total number of proteins in the body. Collagen has high strength and elasticity. It is part of the blood vessels of muscles, tendons, cartilage, bones, and vessel walls.

Examples of globular proteins are albumins and globulins of blood plasma.

Physicochemical properties of proteins.

One of the main features of proteins is their high molecular weight, which ranges from 6000 to several million daltons.

Another important physicochemical property of proteins is their amphotericity,that is, the presence of both acidic and basic properties. Amphotericity is associated with the presence in some amino acids of free carboxyl groups, that is, acidic, and amino groups, that is, alkaline. This leads to the fact that in an acidic environment proteins exhibit alkaline properties, and in an alkaline environment - acidic. However, under certain conditions, proteins exhibit neutral properties. The pH value at which proteins exhibit neutral properties is called isoelectric point. The isoelectric point for each protein is individual. Proteins according to this indicator are divided into two large classes - acidic and alkaline, since the isoelectric point can be shifted either to one side or the other.

Another important property of protein molecules is solubility. Despite the large size of the molecules, proteins are quite soluble in water. Moreover, solutions of proteins in water are very stable. The first reason for the solubility of proteins is the presence of a charge on the surface of protein molecules, due to which protein molecules practically do not form aggregates that are insoluble in water. The second reason for the stability of protein solutions is the presence of a hydration (water) shell in the protein molecule. The hydration shell separates the proteins from each other.

The third important physicochemical property of proteins is salting out,that is, the ability to precipitate under the influence of water-removing agents. Salting out is a reversible process. This ability to move in and out of solution is very important for the manifestation of many vital properties.

Finally, the most important property of proteins is their ability to denaturation.Denaturation is the loss of nativeness by a protein. When we scramble eggs in a frying pan, we get irreversible denaturation of the protein. Denaturation consists of permanent or temporary disruption of the secondary and tertiary structure of a protein, but the primary structure is preserved. In addition to temperature (above 50 degrees), denaturation can be caused by other physical factors: radiation, ultrasound, vibration, strong acids and alkalis. Denaturation can be reversible or irreversible. With small impacts, the destruction of the secondary and tertiary structures of the protein occurs insignificantly. Therefore, in the absence of denaturing effects, the protein can restore its native structure. The reverse process of denaturation is called renaturation.However, with prolonged and strong exposure renaturation becomes impossible, and denaturation is thus irreversible.

Before talking about the most important physical and chemical properties of protein, you need to know what it consists of and what its structure is. Proteins are an important natural biopolymer; amino acids serve as the foundation for them.

What are amino acids

These are organic compounds that contain 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 for the formation of proteins.

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

Their synthesis does not occur in the body. Therefore, they get there along with food. If a person receives them in insufficient quantities, then the normal functioning of various systems may be disrupted. Proteins are formed through a polycondensation reaction.

Proteins and their structure

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

The structure of these compounds depends on the order in which amino acid residues alternate. As a result, it looks like this:

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

Their classification

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

Their composition is as follows:

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

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

By solubility:

• water soluble;
• fat-soluble.

A short description 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 important for the normal functioning of a living organism. In their original state, these are solid substances that either dissolve in various liquids or not.

If we talk briefly 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, construction function, etc. The physical properties of proteins depend on whether they are soluble or not. It is these features that will be written about further.

Physical properties of proteins

It has already been written above 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, a method by which proteins are isolated from mixtures, becomes possible.

Chemical properties of protein compounds

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

1. Denaturation. Protein coagulation under the influence of high temperatures, strong acids or alkalis. During denaturation, only the primary structure is preserved, 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. The result is a purple coloration.
5. Xanthoprotein reaction. The effect is carried out using concentrated nitric acid. This reaction results in a white precipitate that turns yellow when heated. And if you add an aqueous ammonia solution, an orange color appears.
6. Determination of sulfur in proteins. When the proteins burn, the smell of “burnt horn” begins to be felt. This phenomenon is explained by the fact that they contain sulfur.

So these were all the physical and chemical properties of proteins. But, of course, it is not only because of them that 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’s also worth talking about the impact they have on the body and why it won’t function fully without them. The following are the functions of proteins:

1. enzymatic. Most reactions in the body occur with the participation of enzymes that are of protein origin;
2. transport. These elements deliver other important molecules to tissues and organs. One of the most important transport proteins is hemoglobin;
3. structural. Proteins are the main building material 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 sense organs also have proteins in their structure;
6. storing. This function is performed by special proteins, which can be building materials and sources of additional energy during the development of new organisms.

Proteins can be converted into fats and carbohydrates. But they will not be able to become squirrels. Therefore, the lack of these particular compounds is especially dangerous for a living organism. The energy released is small and is inferior in this regard 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 deteriorates, rapid exhaustion and fatigue sets in. Excellent sources of protein are various varieties of wheat, meat and fish products, dairy products, 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 necessary for the normal functioning of the human body.

PROTEINS (proteins), a class of complex nitrogen-containing compounds, the most characteristic and important (along with nucleic acids) components of living matter. Proteins perform numerous 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 tissue, hair and nails. Muscle contractile proteins have the ability to change their length by 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 senses 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, and among them primarily J. von Liebig, gradually came to the conclusion that proteins represent a special class of nitrogenous compounds. The name "proteins" (from the Greek.

protos first) was proposed in 1840 by the Dutch chemist G. Mulder. PHYSICAL PROPERTIES Proteins are white in the solid state, but colorless in solution, unless they carry some kind of chromophore (colored) group, such as hemoglobin. The solubility in water varies greatly among different proteins. It also changes depending on the pH and the concentration of salts in the solution, so it is possible to select conditions under which one protein will selectively precipitate in the presence of other proteins. This "salting out" method is widely used to isolate and purify proteins. The purified protein often precipitates out of solution as crystals.

Compared to other compounds, the molecular weight of proteins is very large, ranging from several thousand to many millions of daltons. Therefore, during ultracentrifugation, proteins are sedimented, and at different rates. Due to the presence of positively and negatively charged groups in protein molecules, they move at different speeds and in an electric field. This is the basis of electrophoresis, a method used to isolate individual proteins from complex mixtures. Proteins are also purified by chromatography.

CHEMICAL PROPERTIES Structure. Proteins are polymers, i.e. molecules built like chains from repeating monomer units, or subunits, the role of which they play a -amino acids. General formula of amino acids where R a hydrogen atom or some organic group.

A protein molecule (polypeptide chain) can consist of only a relatively small number of amino acids or several thousand monomer units. The combination of amino acids in a chain is possible because each of them has two different chemical groups: an amino group with basic properties,

NH 2 , and an acidic carboxyl group, COOH. Both of these groups are affiliated with 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 two amino acids have been linked in this way, the chain can be extended by adding a third to the second amino acid, and so on. As can be seen from the above equation, when a 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 connect 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, but the differences between amino acids are determined by the nature of the group, or “side chain,” which is indicated above by the letter

R . The role of the side chain can be played by one hydrogen atom, as in the amino acid glycine, or by some bulky group, as in histidine and tryptophan. Some side chains are chemically inert, while others are markedly reactive.

Many thousands of different amino acids can be synthesized, and many different amino acids occur 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 ​​may be present as a dimer

– cystine). True, some proteins contain other amino acids in addition to the regularly occurring twenty, but they are formed as a result of modification of one of the twenty listed after it has been included in the protein.Optical activity. All amino acids, with the exception of glycine, have a The -carbon atom has four different groups attached to it. 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 as an object is to its mirror image, i.e. like the left hand to the right. One configuration is called left, or left-handed ( L ), and the other right, or dextrorotatory ( D ), since two such isomers differ in the direction of rotation of the plane of polarized light. Found only in proteins L -amino acids (the exception is glycine; it can be represented in only one form, since two of its four groups are the same), and all of them are optically active (since there is only one isomer). D -amino acids are rare in nature; they are found in some antibiotics and the cell wall of bacteria.Amino acid sequence. Amino acids in a 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 create a huge number of different proteins, just as you can create many different texts from the letters of the alphabet.

In the past, determining the amino acid sequence of a protein often took several years. Direct determination is still quite a labor-intensive 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 deciphered 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 of only amino acids are called simple. Often, however, a metal atom or some chemical compound that is not an amino acid is attached to the polypeptide chain. Such proteins are called complex. 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 indicate the nature of the attached groups: glycoproteins contain sugars, lipoproteins contain fats. If the catalytic activity of an enzyme depends on the attached group, then it is called a prosthetic group. Often a vitamin plays the role of a prosthetic group or is part of one. Vitamin A, for example, attached to one of the proteins in the retina, determines its sensitivity to light.

Tertiary structure. What is important is not so much the amino acid sequence of the protein itself (the primary structure), but the way it is laid out in space. Along the entire length of the polypeptide chain, hydrogen ions form regular hydrogen bonds, which give it the shape of a helix or layer (secondary structure). From the combination of such helices and layers, a compact form of the next order emerges: the tertiary structure of the protein. Around the bonds holding the monomer units of the chain, rotations at small angles are possible. Therefore, from a purely geometric point of view, the number of possible configurations for any polypeptide chain is infinitely large. In reality, each protein normally exists in only one configuration, determined by its amino acid sequence. This structure is not rigid, it is as if « breathes” fluctuates around a certain average configuration. The circuit is folded into a configuration in which free energy (the ability to produce work) is minimal, just as a released spring compresses only to a state corresponding to the minimum free energy. Often one part of the chain is rigidly linked to another by disulfide ( SS) 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 not yet possible to calculate the tertiary structure of a protein, even if its amino acid sequence is known. But if it is possible to obtain protein crystals, then its tertiary structure can be determined by X-ray diffraction.

In structural, contractile and some other proteins, the chains are elongated and several slightly folded chains lying nearby form fibrils; the fibrils, in turn, fold into larger formations of fibers. However, most proteins in solution have a globular shape: the chains are coiled in a globule, like yarn in a ball. Free energy with this configuration is minimal, since hydrophobic (“water-repelling”) amino acids are hidden inside the globule, and hydrophilic (“water-attracting”) amino acids are on its surface.

Many proteins are complexes of several polypeptide chains. This structure is called the quaternary structure of the protein. The hemoglobin molecule, for example, consists of four subunits, each of which is a globular protein.

Structural proteins, due to their linear configuration, form fibers that have a very high tensile strength, while the globular configuration allows the proteins to enter into specific interactions with other compounds. On the surface of the globule, when the chains are correctly laid out, cavities of a certain shape appear in which reactive chemical groups are located. If a given protein is an enzyme, then another, usually smaller, molecule of some substance enters such a cavity, just as a key enters a lock; in this case, the configuration of the electron cloud of the molecule changes under the influence of the chemical groups located 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 bind and are thereby rendered harmless. The “lock and key” model, which explains the interaction of proteins with other compounds, allows us to understand the specificity of enzymes and antibodies, i.e. their ability to react only with certain compounds.

Proteins in different types of organisms. Proteins that perform the same function in different species of plants and animals and therefore bear the same name also have a similar configuration. They, however, differ somewhat in their amino acid sequence. As species diverge from a common ancestor, some amino acids at certain positions are replaced by mutations by others. Harmful mutations that cause hereditary diseases are eliminated by natural selection, but beneficial or at least neutral ones may persist. The closer two biological species are to each other, the less differences are found in their proteins.

Some proteins change relatively quickly, others are very conserved. The latter includes, for example, cytochrome With a respiratory enzyme found in most living organisms. In humans and chimpanzees, its amino acid sequences are identical, and in cytochrome With In 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 construct a phylogenetic (family) tree, reflecting the 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 pH, by exposure to organic solvents, and even by simply shaking the solution until bubbles appear on its surface. A protein modified 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 renaturation, i.e. reacquire the original configuration. But most proteins simply turn into a mass of tangled polypeptide chains and do not restore their previous configuration.

One of the main difficulties in isolating active proteins is their extreme sensitivity to denaturation. This property of proteins finds useful application in food preservation: high temperature irreversibly denatures the enzymes of microorganisms, and the microorganisms die.

PROTEIN SYNTHESIS To synthesize protein, a living organism must have a system of enzymes capable of joining one amino acid to another. A source of information is also needed to determine which amino acids should be combined. Since there are thousands of types of proteins in the body and each of them consists on average of several hundred amino acids, the information required must be truly enormous. It is stored (similar to how a recording is stored on a magnetic tape) in the nucleic acid molecules that make up genes. Cm . also HEREDITARY; NUCLEIC ACIDS.Enzyme activation. A polypeptide chain synthesized from amino acids is not always a protein in its final form. Many enzymes are synthesized first as inactive precursors and become active only after another enzyme removes several amino acids at one end of the chain. Some of the digestive enzymes, such as trypsin, are synthesized in this inactive form; these enzymes are activated in the digestive tract as a result of the removal of the terminal fragment 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. The middle part of this chain is then removed, and the remaining fragments bind together to form the active hormone molecule. Complex proteins are formed only after a specific chemical group is attached to the protein, and this attachment often also requires an enzyme.Metabolic circulation. After feeding an animal amino acids labeled with radioactive isotopes of carbon, nitrogen or hydrogen, the label is quickly incorporated into its proteins. If labeled amino acids stop entering the body, the amount of label in proteins begins to decrease. These experiments show that the resulting proteins are not retained in the body until the end of life. All of them, with few exceptions, are in a dynamic state, constantly breaking down into amino acids and then being synthesized again.

Some proteins break down when cells die and are destroyed. This happens all the time, for example, with red blood cells and epithelial cells lining the inner surface of the intestine. In addition, the breakdown and resynthesis of proteins also occurs in living cells. Oddly enough, less is known about the breakdown of proteins than about their synthesis. It is clear, however, that the breakdown involves proteolytic enzymes similar to those that break down proteins into amino acids in the digestive tract.

The half-life of different proteins varies from several hours to many months. The only exception is the collagen molecule. Once formed, they remain stable and are not renewed or replaced. Over time, however, some of their properties change, in particular elasticity, and since they are not renewed, this results in certain age-related changes, such as the appearance of wrinkles on the skin.

Synthetic proteins. Chemists have long learned to polymerize amino acids, but the amino acids are combined in a disorderly manner, so that the products of such polymerization bear little resemblance to natural ones. True, it is possible to combine amino acids in a given order, which makes it possible to obtain some biologically active proteins, in particular insulin. The process is quite complicated, and in this way it is possible to obtain only those proteins whose molecules contain about a hundred amino acids. It is preferable instead to synthesize or isolate the nucleotide sequence of a gene corresponding to the desired amino acid sequence, and then introduce this gene into a bacterium, which will produce large quantities of the desired product by replication. This method, however, also has its drawbacks. Cm . also GENETIC ENGINEERING. PROTEIN AND NUTRITION When proteins in the body are broken down into amino acids, these amino acids can be used again to synthesize proteins. At the same time, the amino acids themselves are subject to breakdown, so they are not completely reutilized. It is also clear that during growth, pregnancy and wound healing, protein synthesis must exceed breakdown. The body continuously loses some proteins; These are the proteins of hair, nails and the surface layer of skin. Therefore, in order to synthesize proteins, each organism must receive amino acids from food. Green plants synthesize from CO 2 , water and ammonia or nitrates are all 20 amino acids found in proteins. Many bacteria are also capable of synthesizing amino acids in the presence of sugar (or some equivalent) and fixed nitrogen, but sugar is ultimately supplied by green plants. Animals have a limited ability to synthesize amino acids; they obtain amino acids by eating green plants or other animals. In the digestive tract, absorbed proteins are broken down into amino acids, the latter are absorbed, and from them proteins characteristic of a given organism are built. None of the absorbed protein is incorporated into body structures as such. The only exception is that in many mammals, some maternal antibodies can pass intact through the placenta into the fetal bloodstream, and through maternal milk (especially in ruminants) can be transferred to the newborn immediately after birth.Protein requirement. It is clear that 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 material for building its structures. The need for energy comes first. This means that when there are few carbohydrates and fats in the diet, dietary proteins are used not for the synthesis of their own proteins, but as a source of calories. During prolonged fasting, even your own proteins are used to satisfy energy needs. If there are enough carbohydrates in the diet, then protein consumption can be reduced.Nitrogen balance. On average approx. 16% of the total mass of protein is nitrogen. When the amino acids contained in proteins are broken down, the nitrogen they contain is excreted from the body in the urine and (to a lesser extent) in feces in the form of various nitrogenous compounds. It is therefore 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 normal nutrition in an adult, these amounts are equal. In a growing organism, the amount of nitrogen excreted is less than the amount received, i.e. the balance is positive. If there is a lack of protein in the diet, the balance is negative. If there are enough calories in the diet, but there are no proteins in it, the body saves proteins. At the same time, protein metabolism slows down, and the repeated utilization of amino acids in protein synthesis occurs 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 fasting can serve as a measure of daily protein deficiency. It is natural to assume that by introducing into the diet an amount of protein equivalent to this deficiency, nitrogen balance can be restored. However, it is not. After receiving this amount of protein, the body begins to use amino acids less efficiently, so some additional protein is required to restore nitrogen balance.

If the amount of protein in the diet exceeds what is necessary to maintain nitrogen balance, then there appears to be no harm. Excess amino acids are simply used as an energy source. As a particularly striking example, the Eskimos consume few carbohydrates and about ten times the amount of protein required to maintain nitrogen balance. In most cases, however, using protein as an energy source is not beneficial because a given amount of carbohydrate can produce many more calories than the same amount of protein. In poor countries, people get their calories from carbohydrates and consume minimal amounts of protein.

If the body receives the required number of calories in the form of non-protein products, then the minimum amount of protein to ensure the maintenance of nitrogen balance is approx. 30 g per day. About this much protein is contained in four slices of bread or 0.5 liters of milk. A slightly larger number is usually considered optimal; recommended from 50 to 70 g.

Essential amino acids. Until now, protein was considered as a whole. Meanwhile, in order for protein synthesis to occur, all the necessary amino acids must be present in the body. The animal’s body itself is capable of synthesizing some of the amino acids. They are called replaceable because they do not necessarily have to be present in the diet, it is only important that the overall supply of protein as a source of nitrogen is sufficient; then, if there is a shortage of non-essential amino acids, the body can synthesize them at the expense of those that are present in excess. The remaining, “essential” amino acids cannot be synthesized and must be supplied to the body through food. Essential for humans are valine, leucine, isoleucine, threonine, methionine, phenylalanine, tryptophan, histidine, lysine and arginine. (Although arginine can be synthesized in the body, it is classified as an essential amino acid because it is not produced in sufficient quantities in newborns and growing children. On the other hand, some of these amino acids from food may become unnecessary for an adult person.)

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 animals' weight gain.

Nutritional value of proteins. The nutritional value of a protein is determined by the essential amino acid that is most deficient. Let's illustrate this with an example. The proteins in 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 incomplete 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 occur, all amino acids must be present at the same time, the effect of the intake of essential amino acids can only be detected 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), that contain very few essential amino acids. Plant 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, unless it consumes a slightly larger amount of plant proteins, sufficient to provide the body with essential amino acids. Plants contain the most protein in their seeds, 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 amino acid-rich proteins to incomplete proteins, such as corn proteins, the nutritional value of the latter can be significantly increased, i.e. thereby increasing 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 can be directly consumed by humans. The third, widely used method uses the physiology of ruminants. In ruminants, in the initial part of the stomach, the so-called. The rumen is inhabited by special forms of bacteria and protozoa that convert incomplete plant proteins into more complete microbial proteins, and these, in turn, after digestion and absorption turn into animal proteins. Urea, a cheap synthetic nitrogen-containing compound, can be added to livestock feed. Microorganisms living in the rumen use urea nitrogen to convert carbohydrates (of which there is much more in the feed) into protein. About a third of all nitrogen in livestock feed can come in the form of urea, which essentially means, to a certain extent, the chemical synthesis of protein. In the USA, this method plays an important role as one of the ways to obtain protein.LITERATURE Murray R., Grenner D., Mayes P., Rodwell W. Human biochemistry, vol. 12. M., 1993
Alberts B, Bray D, Lewis J, et al. Molecular cell biology, vol. 13. M., 1994

Squirrels- These are high-molecular (molecular weight varies from 5-10 thousand to 1 million or more) natural polymers, the molecules of which are built from amino acid residues connected by an amide (peptide) bond.

Proteins are also called proteins (Greek “protos” - first, important). The number of amino acid residues in a protein molecule varies greatly and sometimes reaches several thousand. Each protein has its own inherent sequence of amino acid residues.

Proteins perform a variety of biological functions: catalytic (enzymes), regulatory (hormones), structural (collagen, fibroin), motor (myosin), transport (hemoglobin, myoglobin), protective (immunoglobulins, interferon), storage (casein, albumin, gliadin) and others.

Proteins are the basis of biomembranes, the most important component of the cell and cellular components. They play a key role in the life of the cell, constituting, as it were, the material basis of its chemical activity.

The exceptional property of protein is self-organization of structure, i.e. its ability to spontaneously create a certain spatial structure characteristic only of a given protein. Essentially, all the activities of the body (development, movement, performance of various functions, and much more) are associated with protein substances. It is impossible to imagine life without proteins.

Proteins are the most important component of human and animal food and a supplier of essential amino acids.

Protein structure

In the spatial structure of proteins, the nature of the R- radicals (residues) in amino acid molecules is of great importance. Nonpolar amino acid radicals are usually located inside the protein macromolecule and cause hydrophobic interactions; polar radicals containing ionic (ion-forming) groups are usually found on the surface of a protein macromolecule and characterize electrostatic (ionic) interactions. Polar nonionic radicals (for example, containing alcohol OH groups, amide groups) can be located both on the surface and inside the protein molecule. They participate in the formation of hydrogen bonds.

In protein molecules, α-amino acids are linked to each other by peptide (-CO-NH-) bonds:

Polypeptide chains or individual sections within a polypeptide chain constructed in this way can, in some cases, be additionally linked to each other by disulfide (-S-S-) bonds or, as they are often called, disulfide bridges.

A major role in creating the structure of proteins is played by ionic (salt) and hydrogen bonds, as well as hydrophobic interaction - a special type of contact between the hydrophobic components of protein molecules in an aqueous environment. All these bonds have varying strengths and ensure the formation of a complex, large protein molecule.

Despite the difference in the structure and functions of protein substances, their elemental composition varies slightly (in% by dry weight): carbon - 51-53; oxygen - 21.5-23.5; nitrogen - 16.8-18.4; hydrogen - 6.5-7.3; sulfur - 0.3-2.5.

Some proteins contain small amounts of phosphorus, selenium and other elements.

The sequence of amino acid residues in a polypeptide chain is called primary protein structure.

A protein molecule can consist of one or more polypeptide chains, each of which contains a different number of amino acid residues. Given the number of possible combinations, the variety of proteins is almost limitless, but not all of them exist in nature.

The total number of different types of proteins in all types of living organisms is 10 11 -10 12. For proteins whose structure is extremely complex, in addition to the primary one, higher levels of structural organization are also distinguished: secondary, tertiary, and sometimes quaternary structure.

Secondary structure most proteins possess, although not always along the entire length of the polypeptide chain. Polypeptide chains with a certain secondary structure can be differently located in space.

In formation tertiary structure In addition to hydrogen bonds, ionic and hydrophobic interactions play an important role. Based on the nature of the “packaging” of the protein molecule, they are distinguished globular, or spherical, and fibrillar, or filamentous proteins (Table 12).

For globular proteins, an a-helical structure is more typical; the helices are curved, “folded.” The macromolecule has a spherical shape. They dissolve in water and saline solutions to form colloidal systems. Most proteins in animals, plants and microorganisms are globular proteins.

For fibrillar proteins, a filamentous structure is more typical. They are generally insoluble in water. Fibrillar proteins usually perform structure-forming functions. Their properties (strength, stretchability) depend on the method of packing the polypeptide chains. Examples of fibrillar proteins are myosin and keratin. In some cases, individual protein subunits form complex ensembles with the help of hydrogen bonds, electrostatic and other interactions. In this case, it is formed quaternary structure proteins.

An example of a protein with a quaternary structure is blood hemoglobin. Only with such a structure does it perform its functions - binding oxygen and transporting it to tissues and organs.

However, it should be noted that in the organization of higher protein structures, an exclusive role belongs to the primary structure.

Protein classification

There are several classifications of proteins:

1. By degree of difficulty (simple and complex).
2. According to the shape of the molecules (globular and fibrillar proteins).
3. According to solubility in individual solvents (water-soluble, soluble in dilute saline solutions - albumins, alcohol-soluble - prolamins, soluble in dilute alkalis and acids - glutelins).
4. According to the functions performed (for example, storage proteins, skeletal proteins, etc.).

Properties of proteins

Proteins are amphoteric electrolytes. At a certain pH value (called the isoelectric point), the number of positive and negative charges in the protein molecule is equal. This is one of the main properties of protein. Proteins at this point are electrically neutral, and their solubility in water is the lowest. The ability of proteins to reduce solubility when their molecules reach electrical neutrality is used for isolation from solutions, for example, in the technology for obtaining protein products.

Hydration. The process of hydration means the binding of water by proteins, and they exhibit hydrophilic properties: they swell, their mass and volume increase. The swelling of individual proteins depends solely on their structure. The hydrophilic amide (-CO-NH-, peptide bond), amine (-NH 2) and carboxyl (-COOH) groups present in the composition and located on the surface of the protein macromolecule attract water molecules, strictly orienting them on the surface of the molecule. The hydration (aqueous) shell surrounding protein globules prevents aggregation and sedimentation, and therefore contributes to the stability of protein solutions. At the isoelectric point, proteins have the least ability to bind water; the hydration shell around the protein molecules is destroyed, so they combine to form large aggregates. Aggregation of protein molecules also occurs when they are dehydrated with the help of certain organic solvents, for example, ethyl alcohol. This leads to the precipitation of proteins. When the pH of the environment changes, the protein macromolecule becomes charged and its hydration capacity changes.

With limited swelling, concentrated protein solutions form complex systems called jellies.

Jellies are not fluid, elastic, have plasticity, a certain mechanical strength, and are able to retain their shape. Globular proteins can be completely hydrated and dissolved in water (for example, milk proteins), forming solutions with low concentrations. The hydrophilic properties of proteins, i.e. their ability to swell, form jellies, stabilize suspensions, emulsions and foams, are of great importance in biology and the food industry. A very mobile jelly, built mainly from protein molecules, is cytoplasm - raw gluten isolated from wheat dough; it contains up to 65% water. The different hydrophilicity of gluten proteins is one of the signs characterizing the quality of wheat grain and flour obtained from it (the so-called strong and weak wheat). The hydrophilicity of grain and flour proteins plays an important role in the storage and processing of grain and in baking. The dough, which is obtained in bakery production, is a protein swollen in water, a concentrated jelly containing starch grains.

Denaturation of proteins. During denaturation under the influence of external factors (temperature, mechanical stress, the action of chemical agents and a number of other factors), a change occurs in the secondary, tertiary and quaternary structures of the protein macromolecule, i.e. its native spatial structure. The primary structure, and therefore the chemical composition of the protein, does not change. Physical properties change: solubility and hydration ability decrease, biological activity is lost. The shape of the protein macromolecule changes and aggregation occurs. At the same time, the activity of certain chemical groups increases, the effect of proteolytic enzymes on proteins is facilitated, and therefore it is more easily hydrolyzed.

In food technology, thermal denaturation of proteins is of particular practical importance, the degree of which depends on temperature, duration of heating and humidity. This must be remembered when developing heat treatment regimes for food raw materials, semi-finished products, and sometimes finished products. Thermal denaturation processes play a special role in blanching plant materials, drying grain, baking bread, and producing pasta. Protein denaturation can also be caused by mechanical action (pressure, rubbing, shaking, ultrasound). Finally, the denaturation of proteins is caused by the action of chemical reagents (acids, alkalis, alcohol, acetone). All these techniques are widely used in food and biotechnology.

Foaming. The foaming process refers to the ability of proteins to form highly concentrated liquid-gas systems called foams. The stability of foam, in which protein is a foaming agent, depends not only on its nature and concentration, but also on temperature. Proteins are widely used as foaming agents in the confectionery industry (marshmallows, marshmallows, soufflés). Bread has a foam structure, and this affects its taste.

Protein molecules, under the influence of a number of factors, can be destroyed or interact with other substances to form new products. For the food industry, two important processes can be distinguished:

1) hydrolysis of proteins under the action of enzymes;

2) interaction of amino groups of proteins or amino acids with carbonyl groups of reducing sugars.

Under the influence of protease enzymes that catalyze the hydrolytic breakdown of proteins, the latter break down into simpler products (poly- and dipeptides) and ultimately into amino acids. The rate of protein hydrolysis depends on its composition, molecular structure, enzyme activity and conditions.

Protein hydrolysis. The hydrolysis reaction with the formation of amino acids in general can be written as follows:

Combustion. Proteins burn to produce nitrogen, carbon dioxide and water, as well as some other substances. Combustion is accompanied by the characteristic smell of burnt feathers.

Color reactions to proteins. For the qualitative determination of protein, the following reactions are used:

1) xantoprotein, in which the interaction of aromatic and heteroatomic cycles in a protein molecule with concentrated nitric acid occurs, accompanied by the appearance of a yellow color.

2) biuret, in which weakly alkaline solutions of proteins interact with a solution of copper (II) sulfate to form complex compounds between Cu 2+ ions and polypeptides. The reaction is accompanied by the appearance of a violet-blue color.