Biochemistry

+ + coo" Protein COO' /I \\ + NHj NH 3 NH } NH 3 Among these proteins are papain, trypsin, lysozyme, etc. The total charge of the protein molecule, naturally, depends on the pH of the medium: in an acidic medium it is positive, in an alkaline medium it is negative. The charge distribution on the surface of the molecule is uneven, different parts of it may have opposite charges stabilized by water dipoles. A shift in the pH of the medium leads to a change in the nature of the dissociation of amino acid groups, redistribution of charges on the surface of the molecule, which causes a change in the spatial structure of the protein, therefore, its biological activity. By changing the pH of the medium, it is possible to influence the dissociation constant of the protein molecule as an acid and as a base. At a pH value determined for each protein, it is possible to achieve a state where the degree of acid dissociation becomes equal to the degree of dissociation of the protein as a base, i.e., the number of negative charges is equal to the number of positive charges. The charge of such a protein molecule as a whole is practically zero. The pH at which the protein does not carry a total charge is called an isoelectric point. COO ” COOH COOH / +H 4 / +H + / Protein - COO ” — * Protein - COO ” — * Protein - COOH I I I \H 3 "NH 3 'NH 3 Protein anion Protein cation coo- coo - COO ” / +ОН' +ОН ’ / Protein - COO — > Protein - COO ” — > Protein - COOH I \ "NHj *NH 3 Protein cation I \ 'NH 3 NH 2 I \ nh 2 nh 2 Protein anion 58