Biochemistry

Table 2.5 - Molecular weights of some proteins Name Molecular weight, Da Insulin 5700 Ribonuclease 13 000 Lysozyme 14 000 Trypsin 24 000 Pepsin 35 000 Egg albumin . 45 000 Human hemoglobin 65 000 Catalase 250 000 Collagen 350 000 Urease 480 000 Tobacco Mosaic Virus 40 000 000 Amphoteric properties of proteins Proteins are amphoteric electrolytes, since both acidic and basic groups are present in their molecules. The effect of a-amino and a-carboxyl groups on the acid-base properties of proteins is insignificant, since they participate in the formation of peptide bonds in protein molecules. Amphotericity to proteins is given by lateral residues of acidic and basic amino acids. The presence of dissociating -NH 2 and COOH groups determines a certain total charge of the protein molecule, which depends on the pH of the medium. Natural proteins, depending on the composition of amino acids, can be acidic or basic in nature. Monoaminodicarboxylic acids (Asp and Glu) predominate in acidic proteins. The molecule of such a protein in solution is negatively charged: Most proteins are acidic, such as albumin, globulin, casein, pepsin, etc. In the molecules of basic proteins, diaminomonocarboxylic acids (Lys, Arg, Gis) predominate. In solution, the main proteins carry a positive charge: 57-