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any of the conjugated proteins containing a phosphoryl group bonded to the amino acid residues of the polypeptide chain. Usually, the phosphoryl group (—PO32–) is bonded to the phosphoprotein molecule through residues of the amino acids serine and threonine; in the mitochondria in animal tissues, phosphoproteins have been found in which the phosphoryl group is bonded to the protein through the imidazole ring of histidine. The transfer of the phosphoryl residue to the protein is catalyzed by the enzyme protein kinase, one of the phosphotransferases; here, adenosine triphosphate is the phosphate donor. Alkalies bring about a nonenzymatic separation of the phosphoryl group; in the presence of acids, however, phosphoproteins are relatively stable.
Phosphoproteins include casein, a basic protein in milk, and ovalbumin and vitellin, which are present in eggs. Also included are phosphorylated modifications of histones, RNA polymerases, certain phosphotransferases, and phosphatases. Phosphoproteins are very common in living organisms, where they figure in metabolism, in the regulation of the activity of cell nuclei, in ion transport, and in oxidation processes in mitochondria.
REFERENCELisovskaia, N. P., and N. B. Livanova. Fosfoproteiny. Moscow, 1960.
V. V. ZUEVSKII