Which of the proteins are simple. Simple and complex proteins. Structure, functions, properties, characteristics, examples of complex proteins. Foods with slow proteins

The structure of simple proteins is represented only a polypeptide chain(albumin, insulin). However, it must be understood that many simple proteins (for example, albumin) do not exist in a "pure" form, they are always associated with some non-protein substances. They are classified as simple proteins only for the reason that the bonds with the non-protein group weak and when highlighting in vitro they are free from other molecules - a simple protein.

Albumins

In nature, albumins are part of not only blood plasma (serum albumin), but also egg white (ovalbumin), milk (lactalbumin), and are storage proteins in the seeds of higher plants.

Globulins

A group of diverse blood plasma proteins with a molecular weight of up to 100 kDa, subacid or neutral. They are poorly hydrated, less stable in solution than albumins and are easier to precipitate, which is used in clinical diagnostics in "sedimentary" samples (thymol, Veltman). Although they are usually classified as simple, many globulins contain carbohydrate or other non-protein components.

At electrophoresis Serum globulins are divided into at least 4 fractions - α 1-globulins, α 2-globulins, β-globulins and γ-globulins.

Electrophoregram pattern (top) of blood serum proteins
and the resulting proteinogram (below)

Since globulins include a variety of proteins, their functions are varied:

Part of α-globulins has antiprotease activity, which protects blood proteins and extracellular matrix from premature destruction, for example, α 1 -antitrypsin, α 1 -antichymotrypsin, α 2 -macroglobulin.

Some globulins are capable of binding certain substances: transferrin (carries iron ions), ceruloplasmin (contains copper ions), haptoglobin (hemoglobin carrier), hemopexin (heme transport).

γ-Globulins are antibodies and provide immune defense of the body.

Histones

Histones are intranuclear proteins weighing about 24 kDa. They have pronounced basic properties, therefore, at physiological pH values, they are positively charged and bind to deoxyribo-nucleic acid (DNA), forming deoxyribo-nucleoproteins. There are 5 types of histones - very rich in lysine (29%) histone H1, other histones H2a, H2b, H3, H4 are rich in lysine and arginine (up to 25% in total).

Amino acid radicals in histones can be methylated, acetylated, or phosphorylated. This changes the net charge and other properties of proteins.

There are two functions of histones:

1. regulation of genome activity, namely, they interfere with transcription.

2. Structural - stabilize the spatial structure of DNA.

Histones in complex with DNA form nucleosomes - octahedral structures composed of histones H2a, H2b, H3, H4. Histone H1 is bound to the DNA molecule, preventing it from slipping off the histone octamer. DNA wraps around the nucleosome 2.5 times, after which it wraps around the next nucleosome. Thanks to this stacking, a 7-fold reduction in DNA size is achieved.

Thanks to histones and the formation of more complex structures, the size of DNA eventually decreases thousands of times: in fact DNA length reaches 6-9 cm (10 -1), and the size of chromosomes is only a few micrometers (10–6).

Protamines

These are proteins weighing from 4 kDa to 12 kDa, they are found in the nuclei of the spermatozoa of many organisms, in the sperm of fish (milk) they make up the bulk of the protein. Protamines are histone substitutes and serve to organize chromatin in sperm. Compared with histones, protamines have a sharply increased content of arginine (up to 80%). Also, unlike histones, protamines have only a structural function, they do not have a regulatory function, chromatin in spermatozoa is inactive.

Collagen

Collagen is a fibrillar protein with a unique structure that forms the basis of the intercellular substance of the connective tissue of tendons, bones, cartilage, skin, but, of course, it is also found in other tissues.

The polypeptide chain of collagen consists of 1000 amino acids and is called the α-chain. There are about 30 variants of the α-chain of collagen, but they all have one common feature - to a greater or lesser extent include a repeating triplet [ Gly-X-Y], where X and Y are any amino acids except glycine. Pregnant X more often located proline or, much less often, 3-hydroxyproline, pregnant Y meets proline And 4-hydroxyproline. Also in position Y often found alanine, lysine And 5-oxylysin. Other amino acids account for about a third of the total number of amino acids.

The rigid cyclic structure of proline and hydroxyproline does not allow the formation of a right-handed α-helix, but forms a so-called. "proline fracture". Due to this break, a left-handed helix is ​​formed, where there are 3 amino acid residues per turn.

Hydroxylation plays an important role in collagen synthesis. lysine And proline included in the primary chain, carried out with the participation of ascorbic acid. Also, collagen usually contains monosaccharide (galactose) and disaccharide (glucose-galactose) molecules associated with the OH groups of some oxylysin residues.

Stages of collagen molecule synthesis

synthesized molecule collagen built of 3 polypeptide chains woven together into a tight bundle - tropocollagen(length 300 nm, diameter 1.6 nm). Polypeptide chains are firmly linked to each other through the ε-amino groups of lysine residues. Tropocollagen forms large collagen fibrils with a diameter of 10-300 nm. The transverse striation of the fibril is due to the displacement of tropocollagen molecules relative to each other by 1/4 of their length.

Collagen fibrils are very strong, they are stronger than steel wire of equal cross section. In the skin, fibrils form an irregularly woven and very dense network. For example, dressed leather is almost pure collagen.

Hydroxylation of proline is carried out iron-containing enzyme prolyl hydroxylase which requires vitamin C (ascorbic acid). Ascorbic acid protects prolyl hydroxylase from inactivation, maintaining the reduced state iron atom in the enzyme. Collagen synthesized in the absence of ascorbic acid is insufficiently hydroxylated and cannot form fibers that are normal in structure, which leads to skin damage and vascular fragility, and manifests itself as scurvy.

Hydroxylation of lysine is carried out by the enzyme lysylhydroxylase. It is sensitive to the influence of homogentisic acid (tyrosine metabolite), with the accumulation of which (diseases alkaptonuria) collagen synthesis is disrupted, and arthrosis develops.

The half-life of collagen is calculated in weeks and months. Plays a key role in its exchange collagenase cleaving tropocollagen 1/4 of the distance from the C-terminus between glycine and leucine.

As the body ages, an increasing number of cross-links are formed in tropocollagen, which makes the collagen fibrils in the connective tissue more rigid and brittle. This leads to increased bone fragility and a decrease in the transparency of the cornea of ​​​​the eye in old age.

As a result of the breakdown of collagen, hydroxyproline. With damage to the connective tissue (Paget's disease, hyperparathyroidism), the excretion of hydroxyproline increases and has diagnostic value.

Elastin

Structurally, elastin is similar to collagen. It is located in the ligaments, the elastic layer of blood vessels. The structural unit is tropoelastin with a molecular weight of 72 kDa and a length of 800 amino acid residues. It has much more lysine, valine, alanine and less hydroxyproline. The absence of proline causes the presence of helical elastic regions.

A characteristic feature of elastin is the presence of a peculiar structure - desmosine, which, with its 4 groups, combines protein chains into systems that can stretch in all directions.

α-Amino groups and α-carboxyl groups of desmosine are included in the peptide bonds of one or more protein chains.

Protein is a macromolecule that cells abound in. Each of them performs a specific function, but not all of them are the same, therefore they have a certain classification that defines different types of proteins. This classification is useful to consider.

Definition of proteins: What is a protein?

Protein, from the Greek "πρωτεῖος", are biomolecules formed by linear chains of amino acids.

Due to their physicochemical properties, proteins can be classified as simple proteins (holoproteins) formed only by amino acids or their derivatives; conjugated proteins (heteroproteins) formed by amino acids, accompanied by various substances, and derivatives of proteins, substances formed by denaturation and cleavage of the previous ones.

Proteins are essential for life, especially for their plastic function (they make up 80% of the dehydrated protoplasm of every cell), but also for their bioregulatory functions (they are part of enzymes) and protection (antibodies are proteins).

Proteins play a vital role for life and are the most versatile and diverse biomolecules. They are necessary for the growth of the body and perform a huge number of different functions, including:

• Construction of fabrics. This is the most important function of a protein (for example: collagen)
• Contrability (actin and myosin)
• Enzymatic (for example: sucrase and pepsin)
• Homeostatic: cooperates in maintaining pH (because they act as a chemical buffer)
• Immunological (antibodies)
• Wound scarring (eg, fibrin)
• Protective (eg, thrombin and fibrinogen)
• Signal transduction (eg, rhodopsin).

Proteins are made up of amino acids. The proteins of all living beings are determined mainly by their genetics (with the exception of some antimicrobial peptides of non-ribosomal synthesis), that is, genetic information largely determines which proteins represent the cell, tissue and organism.

Proteins are synthesized depending on how the genes that code for them are regulated. Therefore, they are susceptible to signals or external factors. The set of proteins expressed in this case is called the proteome.

Five basic properties that allow the existence and ensure the function of proteins:

1. PH buffer (known as buffer effect): They act as pH buffers due to their amphoteric nature, meaning they can behave like acids (donating electrons) or like bases (accepting electrons).
2. Electrolytic capacity: determined by electrophoresis, an analytical method in which if proteins are transferred to the positive pole, it is because their molecule has a negative charge and vice versa.
3. Specificity: Each protein has a specific function, which is determined by its primary structure.
4. Stability: A protein must be stable in the environment where it performs its function. To do this, most aqueous proteins create a packaged hydrophobic core. This is due to the half-life and turnover of the protein.
5. Solubility: It is necessary to solvate the protein, which is achieved by exposing the surface of the protein to residues with the same degree of polarity. It is maintained as long as there are strong and weak ties. If temperature and pH increase, solubility is lost.

Protein denaturation

If changes in pH, changes in concentration, molecular agitation, or sudden changes in temperature occur in a protein solution, the solubility of proteins can be reduced to the point of precipitation. This is due to the fact that the bonds that support the globular conformation are broken, and the protein assumes a filamentous conformation. Thus, the layer of water molecules does not completely cover the protein molecules, which tend to bind to each other, leading to the formation of large particles that precipitate.

In addition, its biocatalytic properties disappear when the active center changes. Proteins in this state cannot perform the activity for which they were designed, in short, they do not function.

This conformation is called denaturation. Denaturation does not affect peptide bonds: when returning to normal states, it may happen that the protein restores a primitive conformation, which is called renaturation.

Examples of denaturation are excision of milk as a result of casein denaturation, precipitation of egg white when ovalbumin is denatured by the action of heat, or fixation of combed hair as a result of heat acting on hair keratins.

Protein classification

according to the shape

Fibrous proteins: they have long polypeptide chains and an atypical secondary structure. They are insoluble in water and in aqueous solutions. Some examples of this are keratin, collagen and fibrin.

globular proteins: are characterized by folding their chains into a tight or compact spherical shape, leaving the hydrophobic groups on the protein and hydrophilic groups exposed, making them soluble in polar solvents such as water. Most enzymes, antibodies, some hormones, and transport proteins are examples of globular proteins.

Mixed proteins: they have a fibrillar part (usually at the center of the protein) and another globular part (at the end).

According to the chemical composition

Simple proteins or holoproteins: when hydrolyzed, only amino acids are produced. Examples of such substances are insulin and collagen (spherical and fibrous), albumins.

Conjugated or heteroproteins: these proteins contain polypeptide chains and a prosthetic group. The non-amino acid part is called the prosthetic group and can be a nucleic acid, a lipid, a sugar, or an inorganic ion. Examples of this are myoglobin and cytochrome. Conjugated proteins or heteroproteins are classified according to the nature of their prosthetic group:

• Nucleoproteins: nucleic acids.
• Lipoproteins: phospholipids, cholesterol and triglycerides.
• Metalloproteins: The group consists of metals.
• Chromoproteins: These are proteins conjugated to a chromophore group (a colored substance containing a metal).
• Glycoproteins: A group made up of carbohydrates.
• Phosphoproteins: Proteins conjugated to a phosphate containing radical other than a nucleic acid or phospholipid.

Plant-based protein sources, such as legumes, are of lower quality than animal-based proteins because they provide less important amino acids, which is compensated by a suitable mixture of both.

An adult should consume protein in accordance with lifestyle, that is, the more physical activity, the more protein sources will be required than sedentary.

In old age, still looking contradictory, there is no need for a lower protein intake, but it is recommended to increase their amount, because tissue regeneration is very important at this stage. In addition, we must take into account the possible emergence of chronic diseases that can degrade proteins.

Here we will tell you which foods are the best sources of protein:

Products with animal proteins

• Eggs: This is a good source of protein because it contains excellent quality albumin, as it contains a large amount of essential amino acids.
• Fish (salmon, herring, tuna, cod, trout…).
• Milk.
• Dairy products, cheese or yogurt.
• Red meat, turkey, tenderloin and chicken.

These foods contain proteins with a lot of essential amino acids (those that cannot be synthesized by the body, so they must come from food).

Foods with plant proteins

• Legumes (lentils, beans, chickpeas, peas…) should be supplemented with other foods such as potatoes or rice.
• Green leafy vegetables (cabbage, spinach…).
• Nuts such as pistachios or almonds (provided they are not roasted or salted).
• Seitan, quinoa, soybeans, seaweed.

Protein digestion is usually initiated in the stomach when pepsinogen is converted to pepsin by hydrochloric acid and is continued by trypsin and chymotrypsin in the intestine.

Dietary proteins are degraded into ever smaller peptides, and into amino acids and their derivatives, which are absorbed by the gastrointestinal epithelium. The absorption rate of individual amino acids is highly dependent on the protein source. For example, the digestibility of many amino acids in humans differs between soy protein and milk protein, and between individual milk proteins such as beta-lactoglobulin and casein.

For milk proteins, approximately 50% of the protein consumed is digested in the stomach or small intestine, and 90% is already digested when the ingested food reaches the ileum.
In addition to their role in protein synthesis, amino acids are also an important source of nitrogen nutrition. Proteins, like carbohydrates, contain four kilocalories per gram, while lipids contain nine kcal. Alcohols - seven kcal. Amino acids can be converted to glucose through a process called gluconeogenesis.

What are proteins

Principles of protein classification

At present, many different protein preparations have been isolated from organs and tissues of humans, animals, plants, and microorganisms. Protein preparations have also been isolated from individual parts of the cell (for example, from nuclei, ribosomes, etc.), from non-cellular substances (blood serum, chicken egg protein). The resulting drugs have different names. However, for a systematic study, proteins must be divided into groups, i.e., classified. But this encounters certain difficulties. If in organic chemistry substances are classified on the basis of their chemical structure, then in biological chemistry the structure of most proteins has not yet been studied in all details. In addition, it is very difficult to classify proteins based on their chemical structure alone. It is also impossible to give a sufficiently substantiated classification of proteins according to their functions in the body. Very often, proteins that are similar in structure have completely different biological functions (for example, hemoglobin and enzymes such as catalase, peroxidase, and cytochromes).

Several great opportunities for the classification of proteins are provided in the study of the physicochemical properties of protein substances. The unequal solubility of proteins in water and other solvents, the different concentrations of salts necessary for the salting out of proteins - these are usually the signs that make it possible to classify a number of proteins. At the same time, some already known features in the chemical structure of proteins and, finally, their origin and role in the body are taken into account.

The entire vast class of protein substances is usually divided into two large groups: simple proteins, or proteins, and complex proteins, or proteids. During hydrolysis, simple proteins decompose only into amino acids, while complex ones, along with amino acids, give compounds of another type, for example: carbohydrates, lipids, heme, etc. Thus, complex proteins, or proteids, consist of the actual protein substance (protein part or simple protein) in combination with other non-protein substances.

Simple proteins, or proteins, include protamines, histones, albumins, globulins, prolamins, glutelins, proteinoids and other proteins that do not belong to any of the listed groups, for example, many enzyme proteins, muscle protein - myosin, etc. A group of complex proteins , or proteids, are usually also divided into several subgroups depending on the nature of the non-protein components contained in them.

However, such a classification has a very relative value. Recent research has established that many simple proteins are actually associated with a small number of certain non-protein compounds. Thus, some proteins could be attributed to the group of complex proteins, since they turned out to be associated with a small amount of carbohydrates, sometimes lipids, pigments, etc. At the same time, it is rather difficult to accurately characterize some complex proteins from a chemical point of view. . For example, lipoproteins in some cases represent such fragile complexes that they could be considered more like adsorption compounds of simple proteins with lipids than as individual chemicals.

Simple proteins

The simplest proteins are protamines and histones. They have a weakly basic character, while the vast majority of others are acidic. The main nature of protamines and histones is due to the fact that their molecules contain a large number of diaminomonocarboxylic amino acids, such as lysine and arginine. In these acids, one a-amino group is linked by a peptide bond to the carboxyl, while the other remains free. It also determines the weakly alkaline environment of protein solutions. In accordance with their basic character, histones and protamines exhibit a number of special properties not found in other proteins. So, these proteins are at the isoelectric point when the medium is alkaline. This is why protamines and histones "coagulate" when boiled only when alkali is added.

Protamines, first isolated by F. Miescher, are found in large quantities in the spermatozoa of fish. They are characterized by a very high content of essential amino acids (up to 80%), especially arginine. In addition, protamines lack amino acids such as tryptophan, methionine, cysteine, and most protamines also lack tyrosine and phenylalanine. Protamines are relatively small proteins. They have a molecular weight of 2,000 to 12,000. It was not possible to isolate them from the nuclei of muscle cells.

Histones are less basic than protamines. They contain only 20-30% diaminomonocarboxylic acids. The amino acid composition of histones is much more diverse than that of protamines, but they also lack tryptophan or have a very small amount of it. The composition of histones also includes modified, altered amino acid residues, for example: O-phosphoserine, methylated derivatives of arginine and lysine, lysine derivatives acetylated at the free amino group.

Many histones are found in the thymus gland, the nuclei of cells of glandular tissues. Histones are not homogeneous proteins and can be divided into a number of fractions that differ in chemical composition and biological properties from each other. The classification of histones is based on the relative amounts of lysine and arginine. Histone H1 is very rich in lysine. Histone H2 is characterized by a moderate content of this amino acid, and there are two types of this histone - H2A and H2B. Histone H3 is moderately rich in arginine and contains cysteine. Histone H4 is rich in arginine and glycine.

Histones of the same type obtained from different animals and plants have very similar amino acid sequences. Such conservatism in evolution seems to serve to preserve a sequence that provides essential and specific functions. This is best supported by the fact that the amino acid sequences of histone H4 from pea seedlings and bovine thymus differ in only two of the 102 amino acid residues present in the molecule.

Due to the presence of a large number of free amino groups, protamines and histones form ionic bonds with phosphoric acid residues, which are part of DNA, and contribute to the compact folding of the DNA double helix in the formed complex of DNA with these proteins. The complex of DNA with histones - chromatin contains DNA and histones in approximately equal quantitative proportions.

In addition to interacting with DNA, histones also interact with each other. Extraction with sodium chloride from chromatin was used to isolate a tetramer consisting of two H3 histone molecules and two H4 histone molecules. Under the same conditions, histones H2A and H2B can be isolated together as a dimer. The current model of chromatin structure suggests that one tetramer and two dimers interact with 200 base pairs of DNA, which is roughly a region of about 70 nm in length. In this case, a spherical structure with a diameter of 11 nm is formed. It is believed that chromatin is a mobile chain composed of such units. This conjectural model is confirmed by various research methods.

Albumins and globulins are well-studied proteins found in all animal tissues. The bulk of the proteins found in blood plasma, milk serum, egg white, etc., consists of albumins and globulins. Their ratio in various tissues is kept within certain limits.

Albumins and globulins differ from each other in their physicochemical properties. One of the common methods for separating albumins and globulins is their salting out with ammonium sulfate. If an amount of ammonium sulfate is added to a protein solution, which is contained in the same volume of a saturated solution of this salt diluted in half, globulins are released from the solution. If they are filtered and crystalline ammonium sulfate is continued to be added to the filtrate until it is completely saturated, albumin precipitates. Thus, globulins precipitate in a half-saturated ammonium sulfate solution, while albumins precipitate in a saturated solution.

The study of albumins and globulins revealed other differences in their physicochemical properties. It turned out that albumins are able to dissolve in distilled water, while a small amount of salt must be added to water to dissolve globulins. Based on this, it is possible to separate globulins from albumins by dialysis of the protein solution. To do this, a protein solution placed in a bag of a semi-permeable material, such as cellophane, is immersed in distilled water. The protein solution is gradually desalted, and the globulins precipitate. They are separated from the albumins remaining in the solution. Globulins can also be precipitated with saturated sodium sulfate solution, while albumins dissolve in it.

In large quantities, albumins and globulins are isolated from the blood of donors for therapeutic purposes. Human blood albumin preparations are used to administer to patients who have lost a lot of blood as blood substitutes. Y-globulin preparations are used both for the prevention and treatment of certain infectious diseases. At present, for the isolation of albumin and globulin preparations from the blood of donors, methods have been developed for the separate precipitation of these proteins, based on their different solubility in solutions containing ethyl alcohol in various concentrations in the cold. Using this method, highly purified preparations of albumin and various fractions of globulins are obtained, which are further used for medicinal purposes.

Among the simple proteins of plant origin, glutelins and prolamins are of interest. They are found in cereal seeds, forming the bulk of gluten. Gluten can be isolated in the form of a sticky mass by grinding flour with water and gradually washing away the starch with a slow stream of water. The adhesive properties of starch paste depend on the presence of gluten in it. The more gluten a grain contains, the more valuable the grain is considered. Glutelins include, for example, oryzenin, obtained from rice, and glutenin, obtained from wheat.

One of the most important prolamins and the most characteristic protein of the endosperm of wheat grain is gliadin. Gliadin is insoluble in water and saline solutions, but, unlike other proteins, it dissolves in an alcohol solution (70%) and is extracted from the grain with it. Other representatives of prolamins include hordein, obtained from barley, and zein, from corn. These proteins, like gliadin, are extracted from gluten with an alcohol solution (70-80%). All prolamins are characterized by a relatively high content of proline.

A distinctive feature of supporting tissue proteins is their complete insolubility in water, saline solutions, diluted acids and alkalis. They were united under the general name of proteinoids, which means protein-like. These proteins are fibrillar or fibrous proteins, the particles of which are in the form of more or less elongated fibers or filaments. Due to the insolubility of proteinoids in water, they are not affected by digestive juice enzymes. Proteinoids are generally unsuitable for nutrition. These include, for example, proteins of horns, hooves, wool, hair, etc. At the same time, a number of proteins of supporting tissues are able to be digested by digestive juices. These are proteins of bone tissue, tendons, cartilage.

Of the individual representatives of proteinoids, collagen, which is part of the connective tissue, is of great interest (Fig. 1). The simplest method of obtaining it is the treatment of bones with dilute hydrochloric acid. In this case, the minerals go into solution, and the collagen remains. The biological precursor of collagen is procollagen. It is found along with collagen in skin and other tissues. This protein was isolated in crystalline form. It differs from collagen both in its amino acid composition (it contains a lot of proline amino acids, while collagen contains hydroxyproline), and in that it is cleaved by all enzymes that hydrolyze proteins.

The protein substance of tendons and ligaments is called elastin. This proteinoid is slightly more susceptible to the action of digestive juices than collagen.

Keratins are characteristic proteinoids of hair, horns, nails, epidermis and coat. They contain relatively large amounts of cysteine ​​and cystine.

Fibroins are proteinoids produced in the spinning glands of insects: spiders, caterpillars of some butterflies (silkworms), etc. Silk fibroin, which makes up the bulk of the silk thread, is released in liquid form, but then quickly hardens. Silk threads used in the manufacture of fabrics are fibroin freed from sericin glue.

Complex proteins

The most important complex proteins are nucleoproteins, chromoproteins, glycoproteins, phosphoproteins, lipoproteins. The group of complex proteins includes proteins, which, in addition to the protein part, include one or another non-protein group - the prosthetic group. It is released during the hydrolysis of proteins along with the products of hydrolytic cleavage of the protein molecule - amino acids. So, nucleoproteins give nucleic acids and their decay products during hydrolysis, glycoproteins - carbohydrates and substances similar to carbohydrates, phosphoproteins - phosphoric acid, chromoproteins - a colored group, most often heme, lipoproteins - various lipids. Complex protein enzymes can also be split into a protein portion and a non-protein prosthetic group. All these prosthetic groups, more or less strongly associated with the protein component of a complex protein, are in most cases well studied from a chemical point of view.

Rice. 1. Scheme of the structure of collagen.

Among complex proteins, nucleoproteins are of great interest. The significance of nucleoproteins is determined primarily by the fact that these proteins, as their name shows, make up the bulk of an extremely important part of the cell - the cell nucleus. The nucleus is the control center of the cell's vital activity. Processes such as cell division, transmission of hereditary information, control of protein biosynthesis, are carried out with the participation of nuclear structures. Nucleoproteins, or rather deoxyribonucleoproteins, can be isolated from the thymus, spleen, spermatozoa, nuclear erythrocytes of birds and some other tissues. In their composition, in addition to the protein part, there is deoxyribonucleic acid, which is responsible for the storage and transmission of hereditary information.

At the same time, another type of nucleoproteins - ribonucleoproteins - are predominantly part of the cytoplasm of cells, taking a direct part in the formation of the most important biological systems, primarily the protein biosynthesis system. In a cell, ribonucleoproteins are an integral part of the cell organelle - the ribosome.

Deoxyribonucleic acid (DNA) is part of chromatin, a complex nucleoprotein that makes up chromosomes. In addition, there are several types of ribonucleic acid (RNA) in the cell. There is informational RNA (mRNA), which is synthesized when information is read from DNA and on which a polypeptide chain is then synthesized; transfer RNA (tRNA), which delivers amino acids to mRNA, and ribosomal RNA (rRNA), which is part of cell organelles - ribosomes, which form complexes with mRNA. Protein synthesis occurs in these complexes with the participation of all three types of RNA and amino acids.

Nucleic acids, which are part of nucleotides, are also of great interest as components of viruses that occupy an intermediate position between complex protein molecules and the smallest pathogens. Many viruses can be obtained in crystalline form. These crystals are a collection of viral particles, and those, in turn, consist of a protein "case" and a spiralized nucleic acid molecule inside it (Fig. 2). The protein "case" (virus envelope) is built from a large number of subunits - protein molecules interconnected by ionic and hydrophobic bonds. Moreover, the connection between the protein shell and the nucleic acid of viral particles is very fragile. When some viruses enter the cell, the protein shell remains on the surface, the nucleic acid enters the cell and infects it. With the participation of this nucleic acid, the proteins of the virus and the viral nucleic acid are synthesized in the cell, which ultimately leads to the formation of a large number of new viral particles and the death of the infected cell. All this makes it possible to consider a viral particle - a giant molecule of a complex protein-nucleoprotein - a kind of supermolecular structure. Viruses are an intermediate link between chemicals and complex biological systems. Viruses, like nucleoproteins, seem to fill the gap between "chemistry" and "biology", between matter and being.

The protein components of the complex proteins of the cell nucleus, in addition to the basic proteins already known to us, histones and protamines, are also some acidic proteins, the so-called non-histone chromatin proteins, whose main function is to regulate the activity of deoxyribonucleic acid, as the main keeper of genetic information.

Rice. 2. Tobacco mosaic virus: 1 - RNA helix; 2 - protein subunits forming a protective case.

Chromoproteins are complex proteins that consist of a simple protein and a colored chemical compound associated with it. This compound can belong to a wide variety of types of chemicals, but most often such an organic compound also forms a complex with a metal - iron, magnesium, cobalt.

Chromoproteins include such important proteins as hemoglobins, with the help of which oxygen is transferred from blood to tissues, and myoglobin, a protein in the muscle cells of vertebrates and invertebrates. Myoglobin is four times smaller than hemoglobin. It takes oxygen from hemoglobin and supplies it to muscle fibers. In addition, hemocyanin, which carries oxygen in many invertebrates, belongs to chromoproteins. This giant protein contains copper instead of iron, as in hemoglobin, and therefore has a blue color. Therefore, the blood of crustaceans, squids, octopuses is blue, in contrast to the red blood of animals.

Plants contain a green chromoprotein - chlorophyll. Its non-protein part is very similar to the non-protein part of hemoglobin, only instead of iron it contains magnesium. Plants use chlorophyll to capture the energy of sunlight and use it for photosynthesis.

Phosphoproteins are complex proteins, during the hydrolysis of which, along with amino acids, a more or less significant amount of phosphoric acid is obtained. Milk caseinogen is the most important representative of this group of proteins. In addition to caseinogen, the group of phosphoproteins includes ovovitellin - a protein isolated from eggs, ichthulin - a protein obtained from fish caviar, and some others. Of great interest are phosphoproteins found in brain cells. It has been established that the phosphorus of these proteins has a very high renewal rate.

Glycoproteins are complex proteins whose non-protein group is derived from carbohydrates. The separation of the carbohydrate component from glycoproteins is often accompanied by complete or partial hydrolysis of the glycoprotein. Thus, during the hydrolysis of various glycoproteins

along with amino acids, the products of carbohydrate group hydrolysis are obtained: mannose, galactose, fucose, xosamines, glucuronic, neuraminic acids, etc. The composition of the prosthetic group of various glycoproids usually contains not all of the listed substances, some glycoproteins, the carbohydrate part is loosely bound to the protein component and is easily removed from it off. The prosthetic groups of some glycoproteins, collectively known as mucopolysaccharides (the more modern name is glycosaminoglycals), occur in tissues and in free form. Such important mucopolysaccharides are hyaluronic and chondroitinsulfuric acids, which are part of the connective tissue.

Glycoproteins are part of all tissues and are respectively named: chondromucoids (from cartilage), steomucoids (from bones), ovomucoids (from egg white), mucin (in saliva). They are also present in ligaments and tendons and are of great importance. For example, the high viscosity of saliva, associated with the presence of mucin in it, facilitates the slippage of food into the stomach, protecting the oral mucosa from mechanical damage and irritation by chemicals.

Currently, it is customary to divide all glycoproteins into two large groups: glycoproteins proper and polysaccharide-protein complexes. The first have a small number of different monosaccharide residues, devoid of a repeating unit and attached covalently to the polypeptide chain. Most whey proteins are glycoproteins. It is believed that these heteropolysaccharide chains are like postcards for serum proteins, according to which proteins are recognized by certain tissues. At the same time, heteropolysaccharide chains located on the surface of cells are the addresses to which these proteins follow in order to get into the cells of that particular tissue, not another.

Polysaccharide-protein complexes have a large number of carbohydrate residues in the polysaccharide part, repeating units can always be distinguished in it, in some cases the protein-carbohydrate bond is covalent, in others it is electrostatic. Of the polysaccharide-protein complexes, proteoglycans play an important role. They form the extracellular basis of the connective tissue and can account for up to 30% of the dry mass of the tissue. These are substances containing a large number of negatively charged groups, many different heteropolysaccharide side chains covalently linked to the polypeptide backbone. Unlike conventional glycoproteins, which contain several percent carbohydrates, proteoglycans contain up to 95% or more carbohydrates. In their physicochemical properties, they are more reminiscent of polysaccharides than proteins. Polysaccharide groups of proteoglycans can be obtained in good yield after treatment with proteolytic enzymes. Proteoglycans perform several biological functions: firstly, mechanical, as they protect the articular surfaces and serve as a lubricant; secondly, they are a sieve that retains large molecular particles and contributes to the penetration of only low molecular weight particles through the proteoglycan barrier; thirdly, they bind cations, and so strongly that even the K + and Na + cations associated with proteoglycans almost do not dissociate and their ionic properties do not appear. Ca 2+ cations are not just bound by proteoglycans, but also contribute to the unification of their molecules.

The cell membranes of microorganisms contain polysaccharide-protein complexes even more durable. These complexes contain peptides instead of proteins, and therefore they are called peptidoglycans. Almost the entire cell wall is one giant sac-like macromolecule - peptidoglycan, and its structure may vary somewhat depending on the type of bacterium. If the carbohydrate part of peptidoglycan in bacteria of different species is almost the same, then in the protein part, both amino acids and their sequences vary depending on the type of bacteria. The bonds between carbohydrates and peptides in peptidoglycans are covalent and very strong.

Complex lipoprotein proteins consist of a protein part and a lipid-fat part associated with it in various ratios. Lipoproteins are usually insoluble in ether, benzene, chloroform and other organic solvents. However, compounds of lipids with proteins are known, which, in terms of their physicochemical properties, are already closer to typical lipids and lipoids, i.e., fat-like substances, than to proteins. Such substances are called proteolipids.

A number of proteins have the ability to combine with lipids to form more or less stable complexes: albumins, some fractions of globulins, proteins of cell membranes and some cell microstructures. In a living organism, simple proteins can be associated with various lipids and lipoids. Most often, the bond between the protein and lipid in such cases is non-covalent, but nevertheless it is strong, and even when treated with organic solvents under mild conditions, the lipids do not separate from the protein. This is possible only with the denaturation of the protein part.

Lipoproteins play an important role in the formation of the structural components of the cell, especially in the formation of various cell membranes: mitochondrial, microsomal, etc. A lot of lipoproteins are part of the nervous tissue. They are isolated from both white and gray matter of the brain. There are also lipoproteins in the blood of humans and animals.

Among proteins endowed with catalytic functions - enzymes, one can also find not only simple, but also complex proteins, consisting of a protein component and a non-protein group. These proteins include enzymes that catalyze various redox processes. The non-protein groups of some of them are similar in structure and properties to the non-protein groups of hemoglobin - heme and have a pronounced color, which allows them to be attributed to the group of chromoproteins. There are a number of enzyme proteins that contain atoms of one or another metal (iron, copper, zinc, etc.) directly associated with the protein structure. These complex enzyme proteins are called metalloproteins.

Iron-containing proteins include ferritin, transferrin, hemosiderin. Transferrin is a water-soluble iron protein with a molecular weight of about 90,000, located mainly in the blood serum in the β-globulin fraction. The protein contains 0.13% iron; this is about 150 times less than in ferritin. Iron is attached to the protein via the hydroxyl groups of tyrosine. Transferrin is the physiological carrier of iron in the body.

A number of enzymes are known, the activity of which depends on the presence of metals in the composition of the protein molecule. These are alcohol dehydrogenase containing zinc, phosphohydrolases including magnesium, cytochrome oxidase containing copper, and other enzymes.

In addition to the listed groups of proteins, more complex supramolecular complexes can be distinguished, which simultaneously contain proteins, lipids, carbohydrates, and nucleic acids. Brain tissue, for example, contains liponucleoproteins, lipoglycoproteins, lipoglyconucleoproteins.

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Proteins, depending on the chemical structure, are divided into simple and complex. Simple proteins break down only into amino acids upon hydrolysis. During the hydrolysis of complex proteins, along with amino acids, a substance of a non-protein nature is formed - a prosthetic group. The classification of simple proteins is based on their solubility.

Albumins- water-soluble proteins with high hydrophilicity, precipitate at 100% saturation with ammonium sulfate. This is a group of similar blood plasma proteins with a molecular weight of about 40-70 kDa, contain a lot of glutamic acid and therefore have acidic properties and a high negative charge at physiological pH. Easily adsorb polar and non-polar molecules, they are a transporter protein in the blood for many substances, primarily for bilirubin and long-chain fatty acids. These proteins include egg protein, germ proteins of cereals and legumes. Albumins contain all essential amino acids.

Globulins- dissolve in saline solutions, most often 2-10% sodium chloride solution is used to extract globulins. They are precipitated with 50% ammonium sulfate solution. This is a group of various blood plasma proteins with a molecular weight of 100-150 or more kDa, subacid or neutral. They are poorly hydrated, less stable in solution and easier to precipitate compared to albumins. The proteins of legume and oilseed seeds are mainly represented by globulins; legumin - peas and lentils, phaseolin - beans; glycine - soybeans. They make up almost half of human blood proteins, determine the immune properties of the body (immunoglobulins), blood clotting (prothrombin, fibrinogen), participate in the transfer of iron to tissues and other processes.

Many albumins and globulins have an enzymatic effect.

Prolamins. This group of proteins is characteristic exclusively for cereal seeds. A characteristic feature of prolamines is their solubility in 60–80% aqueous ethanol, while all other simple proteins usually precipitate under these conditions. These proteins contain significant amounts of proline and glutamic acid . They do not contain lysine or contain it in trace amounts. Wheat prolamins - gliadins, barley - hordein, maize - zein are well studied. Prolamins are complexes of proteins differing in composition and molecular weight.

Glutelins are usually found with prolamins. These proteins also contain significant amounts glutamic acid , which means they are acidic proteins. They dissolve in alkalis (usually 0.2% NaOH). Glutelins are not homogeneous proteins, but mixtures of different proteins with similar properties. The most studied are wheat glutelin and rice orezenin.

Wheat glutenin and gliadin form a complex called gluten. Gluten flour affects the structural and mechanical properties of the dough, and therefore the quality of the bread.

Protamines are the smallest molecular weight proteins. These proteins are found in fish milk. 2/3 of these proteins consist of arginine, therefore they have a basic character. Protamines do not contain sulfur.

Histones are also basic proteins. They include lysine and arginine, the content of which, however, does not exceed 20–30%. Histones are contained in the chromosomes of cell nuclei, they are involved in stabilizing the spatial structure of DNA. From solutions they are precipitated with ammonia.

Simple proteins include histones, protamines, albumins and globulins, prolamins and glutelins, proteinoids.

Histones- tissue proteins of numerous organisms associated with chromatin DNA. These are proteins of small molecular weight (11-24 thousand Da). According to their electrochemical properties, they belong to proteins with pronounced basic properties (polycationic proteins), the IEP in histones ranges from 9 to 12. Histones have only a tertiary structure, they are concentrated mainly in the cell nuclei. Histones are associated with DNA as part of deoxyribonucleoproteins. The histone-DNA bond is electrostatic, since the histones have a large positive charge, while the DNA strand is negative. The composition of histones is dominated by diaminomonocarboxylic amino acids arginine, lysine.

There are 5 types of histones. The division is based on a number of features, the main of which is the ratio of lysine and arginine in fractions, four histones H2A, H2B, H3 and H4 form an octameric protein complex, which is called the "nucleosome core". The DNA molecule "winds" on the surface of the histone octamer, making 1.75 turns (about 146 base pairs). Such a complex of histone proteins with DNA serves as the main structural unit of chromatin, it is called "nucleosome" .

The main function of histones is structural and regulatory. The structural function is that histones are involved in the stabilization of the spatial structure of DNA, and consequently, of chromatin and chromosomes. The regulatory function is the ability to block the transfer of genetic information from DNA to RNA.

Protamines- original biological substitutes for histones, but differ from them in composition and structure. These are the lowest molecular weight proteins (M - 4-12 thousand Da), have pronounced basic properties due to the high content of arginine in them (80%).

Like histones, protamines are polycationic proteins. They bind to DNA in sperm chromatin and are found in fish milk.

Salmin - protamine from salmon milk.

Mackerel - from mackerel milk.

Protamines make sperm DNA compact, i.e. perform, like histones, a structural function, but do not perform a regulatory one.

^ Albumins and globulins.

Albumins (A) and globulins (D).

A and G proteins that are found in all tissues. Serum is richest in these proteins. The content of albumins in it is 40-45 g / l, globulins 20-30 g / l, i.e. albumins account for more than half of the blood plasma proteins.

Albumins-proteins of relatively small molecular weight (15-70 thousand Yes); they have a negative charge and acidic properties, IET - 4.7, contain a lot of glutamine amino acid. These are highly hydrated proteins, so they precipitate only at high concentrations of water-removing substances.

Due to their high hydrophilicity, small molecular size, and significant concentration, albumins play an important role in maintaining the osmotic pressure of the blood. If the albumin concentration is below 30 g/l, the osmotic pressure of the blood changes, which leads to edema. About 75-80% of the osmotic pressure of the blood is accounted for by albumin.

A characteristic property of albumins is their high adsorption capacity. They adsorb polar and non-polar molecules, performing a transport role. These are non-specific carriers; they transport hormones, cholesterol, bilirubin, medicinal substances, calcium ions. The binding and transport of long chain fatty acids is the main physiological function of serum albumins. Albumins are synthesized mainly in the liver and are quickly updated, their half-life is 7 days.

Globulins- proteins with a higher molecular weight than albumins. Globulins are weakly acidic or neutral proteins (IET = 6 - 7.3). Some of the globulins have the ability to specifically bind substances (specific carriers).

It is possible to fractionate blood serum proteins into albumins and globulins by salting out with (NH 4) 2 SO 4 . In a saturated solution, albumins precipitate as a lighter fraction, in a semi-saturated solution, globulins.

In the clinic, the method of fractionation of blood serum proteins by electrophoresis has become widespread. With electrophoretic separation of blood serum proteins, 5-7 fractions can be distinguished: The nature and degree of changes in the protein fractions of blood serum in various pathological conditions is of great interest for diagnostic purposes. A decrease in albumins is observed as a result of a violation of their synthesis, with a deficiency of plastic material, a violation of the synthetic function of the liver, and kidney damage. The content of globulins increases in chronic infectious processes.

^ Prolamins and glutelins.

This is a group of vegetable proteins that are found exclusively in the gluten of the seeds of cereal plants, where they act as storage proteins. A characteristic feature of prolamins is that they are insoluble in water, saline solutions, alkalis, but soluble in 70% ethanol solution, while all other proteins precipitate. The most studied proteins are gliadin (wheat) and zein (corn). It has been established that prolamins contain 20-25% glutamic acid and 10-15% proline. These proteins, such as gliadin, are normally broken down in humans, but sometimes the enzyme that breaks down this protein is not present at birth. Then this protein is converted into decay products that have a toxic effect. Celiac disease develops - intolerance to vegetable proteins.

Glutelins are also vegetable proteins that are insoluble in water, salt solutions, and ethanol. They are soluble in weak alkalis.

Proteinoids.

Proteins of supporting tissues (bones, cartilage, tendons, ligaments), keratins - proteins of hair, horns, hooves, collagens - connective tissue proteins, elastin - protein of elastic fibers.

All these proteins are fibrillar, they are not hydrolyzed in the gastrointestinal tract. Collagen makes up 25-33% of the total amount of protein in the body of an adult or 6% of body weight. The peptide chain of collagen contains about 1000 amino acid residues, of which every 3rd amino acid is glycine, 20% are proline and hydroxyproline, 10% are alanine. During the formation of secondary and tertiary structures, this protein cannot form typical a-helices, since the amino acids proline and hydroxyproline can form only one hydrogen bond. Therefore, the polypeptide chain in the area where these amino acids are located is easily bent, since it is not held, as usual, by a second hydrogen bond.

Elastin - it is the main structural component of elastic fibers, which are contained in tissues with significant elasticity (blood vessels, ligaments, lungs). The properties of elasticity are manifested by the high extensibility of these tissues and the rapid restoration of their original shape and size after the removal of the load. Elastin contains many hydrophobic amino acids (glycine, valine, alanine, leucine, proline).