Myosin Phosphorylation

The regulation of many biochemical pathways by reversible covalent phosphorylation of key enzymes and nonenzymic proteins is recognized as an important type of mechanism involved in numerous intracellular processes. These processes include glycogenolysis, gluconeogenesis, glycolysis, lipolysis, nucleic acid transcription, protein translation, and contractile-protein interactions (Adelstein and Eisenberg, 1980; M. Barany and K. Barany, 1980; Krebs and Beavo, 1979; Stull, 1980; Stull et al., 1980a,b). Protein phosphorylation consists of the transfer of the terminal phosphate of a nucleoside triphosphate (usually ATP) to serine or threonine amino acid residues in a particular protein and is catalyzed by a group of enzymes referred to as protein kinases. The introduction of a charged phosphate moiety into a protein may result in marked changes in the enzymatic properties and therefore may provide an important means for regulating particular metabolic pathways or physiological processes. Protein kinases may be classified according to the specific agent that serves to activate and serve as second messengers. These would include the cyclic AMP (cAMP)-dependent protein kinases, cGMP-dependent protein kinases, and calcium-dependent protein kinases. The cyclic-nucleotidedependent protein kinases appear to be multifunctional in their protein-substrate recognition, whereas the calcium-dependent protein kinases such as phosphorylase kinase and myosin light-chain kinase are more restricted.