Classification of Protein Phosphatases Involved in Cellular Regulation

The discovery that glycogen phosphorylase was controlled by a phosphorylation-dephosphorylation mechanism [1] was the first example of enzyme regulation by reversible covalent modification, and phosphorylase kinase [2] and glycogen synthase [3] were the second and third enzymes in which this phenomenon was identified. However, enzyme regulation by phosphorylation-dephosphorylation is not confined to glycogen metabolism and over 20 enzymes are now known to be controlled in this manner [4]. An important generality that seems to be emerging is that enzymes in biodegradative pathways are activated by phosphorylation whereas enzymes in biosynthetic pathways are inactivated by phosphorylation [5, 6] (Table 1). Two implications of this finding are that different enzymes may be regulated by the same protein kinases and protein phosphatases, or that different protein kinases and protein phosphatases may respond to the same effector molecules. These ideas are already established in the case of cyclic AMP-dependent protein kinase (cAMP-PrK) and Ca2+-calmodulin dependent protein kinases.