Historical perspectives on protein phosphorylation and a classification system for protein kinases.

Work on the phosphorylation of proteins as a dynamic process involved in the regulation of biological processes started in the 1950s with the finding that phosphorylase a and phosphorylase b are phospho and dephospho forms of the same enzyme. The field expanded sharply in the late 1960s with the discovery of the cyclic-AMP-dependent protein kinase, and it is now clear that phosphorylation-dephosphorylation constitutes a major type of regulation almost as common as allosteric control. The protein kinases, which catalyse the phosphorylation step in various phosphorylation-dephosphorylation systems, can be divided into two main classes, the serine-threonine protein kinases and the tyrosine protein kinases. Each class can be subdivided into groups or entities depending on the nature of the agent(s) that regulate activity.