[42] Criteria for the classification of protein kinases

Protein kinases catalyze the transfer of the terminal phosphate moiety of ATP to a variety of different protein substrates. Initially the classification of these enzymes was, in accord with standard enzymological nomenclature, based on specificity with respect to the protein substrate. More recently it has become recognized that cyclic AMP (cAMP) regulates the activity of some, but not all, of these enzymes, and from experimentation to date it would appear that many cAMP-regulated protein kinases exhibit broad protein substrate specificity. Thus, classical nomenclature becomes inoperable at the experimental level, although in the future cAMP-regnlated protein kinases of unique protein substrate specificity may be recognized. The activation of cAMP-regulated protein kinases occurs by a cyclic nucleotide-promoted dissociation of the holoenzyme (designated RC) to yield the active catalytic species (C) and a complex consisting of regulatory subunit (R) and cAMP.