In nature, there is a large number of proteins of plant and animal origin, different in chemical composition and structure, physico-chemical and biological properties. Despite the successful development of the chemistry and biochemistry of proteins, there is still no single scientifically based system for their classification.
Nor has a single criterion been developed, on the basis of which it would be possible to divide proteins into separate groups. Therefore, they are often classified according to random features – the source of secretion, the shape of protein molecules, solubility in certain solvents, localization in the relevant organs and tissues, amino acid composition, etc. However, this classification is imperfect, since in many cases completely different proteins belong to the same group.
To classify proteins, the functional principle is often used, that is, they are classified based on the main functions they perform during metabolism.
According to this principle, proteins are divided into the following groups: catalytically active, hormone proteins, gene regulatory proteins, protective, toxic, transport, membrane, contractile, receptor proteins, enzyme inhibitors, viral envelope proteins, proteins with other functions. Although functional classification also has some disadvantages, in particular when classifying bifunctional proteins, it is believed that it allows a deeper understanding of the relationship between the structure, properties and functions of protein molecules, the patterns of their evolution and interaction with other substances. But in case of illness, we advise you to take the ifa test – this is a blood test.
Now all proteins are classified mainly according to physicochemical properties and chemical composition. According to these characteristics, proteins are divided into two groups – simple (proteins) and complex (proteins). Proteins are proteins that only contain amino acid residues. Proteins are complex proteins, the molecules of which, in addition to amino acid residues, also contain other components – prosthetic groups. Proteins and proteids, in turn, are divided into a number of classes characterized by different physicochemical properties.
Simple proteins (proteins)
Simple proteins are divided into the following classes: albumins, globulins, protamines, histones, prolamins, glutelins and protenoids.
Albumins. These proteins are very widespread in the animal and plant world. They are part of the cytoplasm of cells and various body fluids — blood serum, lymph, cerebrospinal fluid. In higher animals, albumins make up the main part of the blood plasma (>50%). Albumins are also part of food products of vegetable and animal origin – milk, eggs, grains of cereals and legumes.
Depending on the localization of albumins, they are called: milk albumin — lactoalbumin (lact — milk), egg albumin — ovoalbumin, seroalbumin (serum — blood serum). The most common plant albumins are leucosin (extracted from wheat germ) and legumelin (extracted from legumes). A certain part of albumins is also contained in the vegetative parts of plants.
The molecular weight of albumins is 35–70 thousand. Their molecules have an elliptical shape, which is more compact and symmetrical than that of globulins. Albumins belong to hydrophilic proteins, they dissolve well in water and salt solutions. Their precipitation requires 100% saturation of solutions with neutral salts.
According to the chemical composition, albumins are characterized by a high content of leucine (15%), as well as a significant content of sulfur-containing amino acids, lysine, aspartic and glutamic acids and a small content of glycine. Some albumins do not contain this amino acid at all (serum albumin). One of the main functions of albumins is the regulation of osmotic processes. Their content in blood plasma is 75-80%. About 40% of albumin is included in the blood, the rest – in the extracellular fluid, muscles, skin, intestines.
Albumins play an important role in the transport of various substances, in particular those that are poorly soluble in water. When the content of albumins (albuminemia) decreases, the transport of lipids, in particular free fatty acids, from the liver to peripheral tissues is disturbed. Albumins also regulate the content of Ca2+ ions, steroid hormones, tryptophan, some drugs (dicumarin, penicillin, aspirin) in the plasma, forming complexes with these substances.
Globulins, like albumins, are quite widespread in the composition of animal and plant tissues. A particularly large amount of them is included in cereal grains, sunflower seeds, flax, cotton, and leguminous plants. For example, such globulins as bean phaseolin, pea legumin, soybean glycidin are widely known. Lactoglobulin of milk, fibrinogen of blood, etc. are the most common in animal tissues.
Unlike albumins, globulins do not dissolve in concentrated solutions of neutral salts. They are isolated from animal and plant tissues by extraction with a 10% solution of salts. When the concentration of salts increases, the solubility of globulins decreases, and in a 50% solution, they precipitate.
The molecular weight of globulins is 0.9-1.5 million. They are more coarsely dispersed and less hydrophilic than albumins, which is explained by the lower stability of their solutions. In terms of chemical composition, globulins are slightly different from albumins — they contain more glycine (~5%) and a smaller amount of sulfur-containing amino acids. The ratio between albumins and globulins in the human and animal body is always an important characteristic of its physiological state; it is within certain limits and is called the albumin-globulin coefficient. For a healthy body, the albumin-globulin ratio (A/G ratio) is 1.7-2.3.
During electrophoresis, serum proteins are divided into a number of fractions depending on their mobility, among which the albumin fraction is 54-58%, the globulin fraction is not homogeneous and is divided into α1-globulins (6-7%), α2-globulins (8-9%), β1-globulins (13-14%), γ-globulins (11-12%). With the help of modern methods, in particular, disk electrophoresis and immunoelectrophoresis, blood serum proteins can be divided into 16-19 fractions, each of which performs a certain role in metabolic processes. The α- and β-globulin fractions include the bulk of glycoproteins — proteins that play a rather important role in human and animal bodies. Thus, the α2-globulin fraction includes a protein of glycoprotein nature — ceruloplasmin, which contains 0.34% of copper in its composition and is a mediator of the content and transport of this element. The β-globulin fraction contains transferrin as the main component, which ensures the transport of iron into the tissues (especially into the reticuloendothelial system), where it is released without changing the structure of the protein (transporter). Therefore, transferrin participates in the regulation of the content of free iron in the plasma, preventing its excessive accumulation in the tissues and its loss in the urine. γ-Globulin fractions contain most of the antibodies and play an important role in immunity processes.
Protamines. This is a group of simple proteins with a characteristic composition of amino acids. They are characterized by a high content of diaminomonocarboxylic acids, so their solutions have alkaline properties. These proteins dissolve well in water and do not precipitate during boiling.
A significant amount of protamines is part of parenchymal organs and glands of internal secretion, which contain a large amount of nuclear matter. In the nuclei, protamines form nucleoprotein complexes. Protamines include chlumine, which is contained in the sperm (milk) of herrings. Its molecular weight is 5000, it has pronounced alkaline properties. The polypeptide chain of this protein contains 30 amino acid residues, of which 21 are arginine residues, the rest are alanine, valine, proline, and serine. Another representative of protamines is esalmin isolated from salmon milk. It also mainly contains arginine, proline, valine, serine and does not contain other amino acids.
Histones, like protamines, are contained in parenchymal tissues rich in nuclei, such as the liver, spleen, kidneys, and goiter. A significant amount of histones is also found in plants. They were first identified in 1910 by A. Kossel. Histones also contain a large amount of diaminomonocarboxylic acids, although much less than protamines. So, their composition includes an average of 26% arginine, 8-10% lysine, which causes an alkaline character. Depending on the composition of amino acids (lysine and histidine content), histones are divided into five fractions.
The main mass of histones is part of the chromosomes of cell nuclei, forming a complex compound with DNA – nucleohistone. It is believed that histones play an important role in stabilizing the structure of DNA. These proteins also play a certain role in the processes of protein synthesis, as they are part of the deoxyribonucleotides of the nucleus.
Prolamins. This group of simple proteins is quite widespread in the plant world. They are included in the composition of the seeds of cereal crops. Their representatives are wheat gliadin, rice orosein, barley hordein, corn zein, oat avenin, etc. The name “prolamins” was proposed due to the fact that their composition includes a significant amount of proline. In addition to proline, prolamins also contain glutamic acid and a small amount of other amino acids. Prolamins are characterized by the fact that they do not contain lysine at all.
The component composition of prolamins (gliadin) is genetically determined and determines the variety of plants. To isolate prolamins, they are extracted from crushed tissues with a 70% alcohol solution, which is then distilled off under vacuum, and the viscous mass is dissolved in alcohol and a double volume of acetone is added. At the same time, prolamins precipitate, which are separated and dried.
Glutelins, like prolamins, are proteins of plant origin, a significant amount of them are found in the green parts of plants and seeds. A characteristic feature of proteins is that they contain a large amount of glutamic acid and lysine. These proteins are poorly soluble in water and well soluble in dilute alkali solutions. Their molecular weight varies within fairly wide limits.
Proteinoids. These proteins are part of animal tissues and mainly perform mechanical and resistance functions. They do not dissolve in water, salt solutions, and solutions of acids and alkalis. These proteins are characterized by a high content of sulfur-containing amino acids. Proteinoids are part of the proteins of hair, horns, cartilage, and covering tissues. All of them are characterized by high strength and elasticity. Proteinoids are slowly broken down by enzymes of the digestive tract, so they are poorly absorbed and contribute to the processes of decay in the intestines.
Protenoids include silk fibroin, keratin, collagen, elastin, etc.
Collagen is a fibrous fibrillar protein. It makes up a significant part of the body’s proteins (~25%) and performs a rather important structural function. It is the main component of cartilage, tendons, ligaments, skin, bones. When boiled for a long time in water, collagen hydrolyzes, forming gelatin, which turns into a gel when cooled, which is used in medicine and the food industry. The amino acid composition is quite characteristic for collagen — it contains a large amount of proline, glycine, as well as oxyproline and oxylysine, which are not found in the composition of other proteins, and does not contain sulfur-containing amino acids and methionine at all. It has been established that collagen is formed from procollagen, and this process takes place with the participation of vitamin C. Collagen molecules have an elongated thread-like shape.
Keratin is part of connective and covering tissues, hair, feathers, horns. The composition of its molecules contains a large number of transverse disulfide bonds, which determines its considerable strength and resistance against the action of various factors. In particular, keratin does not dissolve in water, acids and alkalis, salt solutions, organic solutions.
If the number of disulfide bonds decreases during reduction (treatment with sulfides) or hydrolysis, the solubility of keratin increases and it turns into keratin. This is the basis for removing hair when processing hides. This process is reversible, since sulfhydryl groups are easily converted into disulfide groups. In addition to strong disulfide bonds, the keratin molecule also contains hydrogen and ionic bonds, which are easily destroyed when heated and the protein molecule shrinks in the longitudinal direction, that is, polypeptide molecules are folded into a ball.
Elastin is part of tissues that perform a structural function. The inner linings of blood vessels — arteries and veins — are built from it. The polypeptide chain of elastin includes a large amount of glycine, proline, valine and leucine. It does not contain sulfur-containing and aromatic amino acids, as well as oxylysine and methionine. Unlike collagen, elastin does not form gelatin when boiled. After long-term cooking, it can be partially broken down by hydrolytic enzymes and partially absorbed by the body. Elastin can be isolated from connective tissue by alkaline hydrolysis or heating. Native coils of elastin are built from relatively small molecules connected into fibrous strands with the help of rigid cross-links.
Fibroin and sericin are proteins of natural silk, which are secreted by the caterpillars of mulberry and oak silkworms, the glands of spiders, etc.
Their composition includes only four amino acids – glycine, alanine, serine, tyrosine. Glycine content is particularly high (~44%). These proteins are quite resistant to hydrolysis. Fibroin is a high-molecular-weight protein built from linear polypeptide chains, which are located along the axis of the fiber, tightly adhering to each other. A large number of hydrogen bonds are formed between them. In its pure form, fibroin is obtained by heating silkworm cocoons with a solution of soda or soap. Sericin dissolves under these conditions.