Biochemistry

The isoelectric point is a characteristic constant of the protein and depends on the amino acid composition of the protein molecule. The higher the acid / base amino acid ratio in a protein, then lower its pH. For most intracellular proteins, the pH is in the pH range from 5.5 to 7.0, which indicates a negative charge of the cytoplasm as a whole. The excess of negative charges inside the cell is balanced by inorganic cations. The value of the isoelectric point is very important for understanding the stability of proteins in solutions, since in it proteins are the least stable due to electroneutrality. Uncharged protein particles can stick together and precipitate. Protein solubility Proteins are hydrophilic substances that are highly soluble in water. By hydration is meant the binding of water dipoles to ionic and polar groups. The ionic groups in charge-bearing proteins are all -NH 2 and -COOH groups. In a dissociated state, they attract water molecules due to ion-dipole interactions. Nonionic (uncharged) polar side groups of amino acids (-OH, -NH, -CO) form weak hydrogen bonds with water. Water bound by ionic groups is very firmly connected to the protein macromolecule; it is possible to separate it with great difficulty. Water molecules bound to non-ionic groups, protein retained less firmly. The stability of protein solutions is given by the charge of the protein molecule and the hydration shell. Each protein molecule has a total charge of one sign, which prevents the aggregation of molecules. Therefore, all factors contributing to the conservation of charge and hydration shell facilitate the solubility of the protein and its stability in solution. The more polar amino acids in a protein, the more water molecules bind. Hydrated water can be up to 1/5 of the mass of protein. The solubility of proteins in water increases at low concentrations of neutral salts. This is explained by the following: salt ions, interacting with opposite charges in a protein, screen charged groups of protein molecules and thereby reduce protein interaction. An increase in the concentration of neutral salts (an increase in the ionic strength of the solution) has the opposite effect. The process of protein precipitation by neutral salts is called salting out. At high concentrations of ions in solution, they pull water molecules from the polar groups of the protein onto themselves. In this case, the hydration shell of the protein molecule is partially removed, i.e., one of the factors of protein stability in solution is violated. It is possible that neutralization of protein charges by salt ions occurs simultaneously, which leads to a change in the second stability factor (a change in the protein 59