Biological Chemistry - Berezov, T. T., Korovkin, B. F. 1998

Chemistry of Conjugated Proteins
Phosphoproteins

This Class of Proteins includes milk caseinogen, in which phosphoric acid content reaches 1%; vitellin, vitellinin, and phosvitin isolated from chicken egg yolk; Ovalbumin discovered in chicken egg white; ichthulin found in fish roe, and others. A large amount of Phosphoproteins is found in Central Nervous system Cells. Phosphoproteins occupy a special position in the biochemistry of phosphorus-containing compounds not only due to their unique structural Organization, but also because of their wide range of Functions in METABOLISM. A characteristic feature of the phosphoprotein Structure is that phosphoric acid is linked to the protein molecule via an ester bond through the hydroxyl groups of ß-hydroxy Amino Acids, primarily Serine and to a lesser extent Threonine. Typically, there are 2–4 phosphate residues per protein molecule.

Fig. 2.4. Ionic type of bonding between proteins and Phospholipids.

Recent data indicate that phosphoproteins are synthesized in cells As a result of post-translational modification, undergoing phosphorylation mediated by protein Kinases. This process is discussed in detail in Chapter 14. Here we only note the essential role of a specific protein kinase, which catalyzes the phosphorylation of the Tyrosine OH group, in The Biosynthesis of oncoproteins. Thus, the level of phosphoproteins in a Cell largely depends on the regulatory action of Enzymes that catalyze phosphorylation (protein kinases) and dephosphorylation (protein Phosphatases). It should be noted that phosphoproteins contain organically bound, labile phosphate, which is absolutely essential for The Cell to perform A number of biological functions. In addition, they serve as a valuable source of energy and structural material during Embryogenesis and subsequent postnatal GROWTH AND DEVELOPMENT of the Organism.

It is particularly noteworthy that certain Key Enzymes regulating intracellular metabolic processes also exist in both phosphorylated and dephosphorylated forms. This emphasizes The Importance of the phosphorylation–dephosphorylation cycle in the chemical modification of macromolecules involved in integrated metabolic pathways.