Biochemistry

charge as a whole), and consequently, precipitation of the protein from solution. A characteristic feature of the proteins obtained by salting out is the preservation by them of their natural properties andfunctions after salt removal. The solubility of proteins also depends on the solvent', its pH, composition and temperature. The addition of water-miscible organic solvents (ethanol, acetone, etc.) to a protein solution reduces the solubility of proteins. At high solvent concentrations, precipitation of the protein is observed (due to a decrease in the degree of protein hydration). In solutions, proteins have colloidal properties', they slowly diffuse, are not able to penetrate through semipermeable membranes, scatter light, and are characterized by high viscosity. The formation of colloidal solutions by proteins determines many physicochemical phenomena observed in biological fluids and organisms in general. Under certain conditions, all colloidal solutions can lose fluidity and form gels (jellies). They arise as a result of combining molecules in the form of a grid, the inner space of which is filled with a large amount of solvent, but at the same time, separation into liquid and solid phases does not occur, as in the case of coagulation. In the cells of living organisms, proteins are not only in the form of solutions, but also gels (in protoplasm, the lens of the eye, connective tissues, etc.). Fibrillar proteins are easier to gel. So, food jellies are prepared from products containing large amounts of fibrillar proteins (bones, cartilage, meat). Under the action of enzymes of microorganisms, a protein solution - milk can turn into a gel with the formation of kefir, yogurt. Protein gels have an important physiological value in the life of the body. For example, collagen proteins of bones, tendons, skin, etc., have high strength, elasticity and firmness, as they are in a gel state. The deposition of mineral salts during aging reduces their firmness and elasticity, which leads to a number of diseases. One of the properties of gels is their ability to swell - an increase in volume due to the binding of a large amount of water. Such a process is observed during the germination of seeds, during the soaking of cereals, in the manufacture of dough, etc. The reverse phenomenon of swelling (separation of water from the gel) is called syneresis. It occurs, for example, during prolonged storage of kefir. Due to the presence of hydrophilic and hydrophobic groups, proteins can affect the solubility of other substances, acting as emulsifiers. It is known that emulsifiers are substances that stabilize water-oil emulsions. In the human body in an emulsified state are fats in the blood and lymph. The protein forms a thin film on the surface of the droplets of fat (due to hydrophobic interactions), which attracts water (due to polar groups) and prevents the adhesion of fatty particles. 60