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Yunusov M.A. assistant
Department of Biological Chemistry Andijan State Medical Institute
SOME PHYSICOCHEMICAL INDICATORS OF PROTEINS
Abstract: The article provides information on the solubility of proteins and the factors affecting it, as well as the essence of their content.
Key words: protein, hydrophilic, collagen, gelatin, neutral salts, effect of temperature, globulin, pepsin, muscle phosphorylase, polyanion, salting.
Proteins are hydrophilic, water-loving colloids. Dry protein dissolved in water swells, like all high-molecular hydrophilic compounds, and then the protein molecules begin to slowly move into the solution. During folding, water molecules pass into the protein and connect with its polar groups. The solid structure of the polypeptide chain swells. Swollen protein can be considered a reverse solution. Further absorption of water causes the protein molecule to separate from the total mass and dissolve. But snoring does not always lead to melting; some proteins, for example, collagen, remain in a swollen state even if they absorb a lot of water.
The phenomenon of melting occurs on the basis of hydration of proteins, that is, binding of water molecules to proteins. Hydrated protein is tightly bound to water molecules, and it is very difficult to break it. This is not simple adsorption, but shows the electrostatic binding of water molecules with the polar groups of acidic and positively charged basic amino acids.
Part of the hydrated protein is connected with peptide groups that are connected with water molecules by means of hydrogen bonds. For example, proteins with a non-polar side chain bind to water and dissolve. An example of this is that non-polar amino acids stored in collagen bind a large amount of water. The polypeptide chain is elongated under the influence of water bound to peptide groups. But the bonds (bridges) between the chains do not allow the protein molecules to break apart and pass into the solution. When the product containing collagen is heated, the interchain bonds in the collagen fibers are broken, and the separated polypeptide chain goes into solution. Partially hydrolyzed soluble collagen is called gelatin. Gelatin is close to collagen in terms of its chemical composition, it swells easily and forms a viscous solution in water. Gel formation is the main characteristic of gelatin. Aqueous solutions of gelatin are used in practical medicine as plasma substitutes and blood-stopping substances, and gel formation is used in pharmaceutical practice to prepare capsules.
Factors affecting protein dissolution. The solubility of proteins depends on their amino acid composition (polar amino acids are more soluble than non-
polar amino acids), structural properties (globular proteins are more soluble than fibrillar proteins) and the quality of the solvent. For example, plant proteins -prolamins dissolve in 60-80% alcohol, albumins - in water and weak solutions of salts; collagen and keratin are insoluble in most solvents.
The stability of protein solutions depends on the charge and hydrated shell of the protein molecule. There is an organic connection between protein charge or the number of polar amino acids in it and hydration: the more polar amino acids in the protein, the more water is bound (per 1 g of protein). In some cases, the hydrated shell of the protein increases, and the hydrated water can dissolve 1/5 of its weight.
Some proteins are highly hydrated but poorly soluble. For example, collagen binds more water than soluble globular proteins, but does not dissolve. Its dissolution is hindered by structural features - cross bonds between polypeptide chains.
The solubility of proteins depends on the number of hydrophilic groups in their molecules, the size and shape of their molecules, and the sum of their charges. The solubility of proteins decreases at the isoelectric point. Because there is no electrostatic force between molecules that pushes them apart.
Effect of neutral salts. Neutral salts (Na2SO4, MgSO4, (NH4)2SO4) increase the solubility of even proteins that are insoluble in pure water, for example, euglobulins. Salt ions interact with oppositely charged protein molecules and break the salt bridges between protein molecules. An excess amount of salt (increasing the ionic strength of the solution) has the opposite effect.
Effect of environmental pH. The pH of the environment affects the charge of the protein, as well as its solubility. A protein is not stable at its isoelectric point, that is, when the sum of its charges is zero. The loss of charge makes it easier for protein molecules to approach, stick and settle. Therefore, the solubility and stability of the protein is at the lowest level at the pH of the environment at its isoelectric point.
Effect of temperature. There is no strict relationship between protein solubility and temperature. But proteins such as globulin, pepsin, muscle phosphorylase are better in water and salt solutions with temperature increase; proteins such as muscle aldolase and hemoglobin are poorly soluble.
Effect of different charged proteins. If polyanionic - acidic proteins are added to polycation - basic proteins, aggregates are formed. In such cases, as a result of the neutralization of charges, stability disappears and proteins settle down. Sometimes, this feature is used to isolate the desired protein from a mixture of proteins.
Water-absorbing agents, organic solvents - ethyl alcohol, methyl alcohol, acetone, alkaline metals - thick solutions of neutral salts destroy the protein's water film and reduce its solubility. When organic liquids - ammonium sulfate,
sodium sulfate, sodium chloride, sodium phosphate and other solutions are added to the protein solution, the protein usually precipitates.
Salting. When various salts are added to protein solutions, their precipitation is called salting. In this condition, the protein molecules are free from the hydrate shell that stabilizes them, and they easily combine with each other and form large aggregates. Salting often does not change the native (initial, natural) state of the protein, when salt ions are separated from the precipitate by dialysis, the protein goes back into solution. Therefore, the method of salting with ammonium sulfate and sodium sulfate is widely used for extracting proteins without destroying their structure. Different protein solutions precipitate when saturated with salt to different degrees. Therefore, by saturating a solution consisting of a mixture of proteins with a concentrated solution of ammonium sulfate, some proteins can be precipitated separately. For example, when blood serum is semi-saturated with ammonium sulfate, globulins are released, if the globulins are filtered and salt powder is added until the solution is fully saturated, albumins are precipitated.
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