A comment on the functional specificities of cyclic AMP-dependent and cyclic GMP-dependent protein kinases.

Cyclic AMP-dependent and cyclic GMP-dependent protein kinases (protein kinases A and G, respectively) utilize the same phosphate acceptor proteins when assayed in in vitro systems. Nevertheless, protein kinase A phosphorylates preferentially free histone, whereas protein kinase G greatly favors the histone which is associated with polydeoxyribonucleotide. On the other hand, when cytoplasmic soluble substrates such as phosphorylase kinase are used, the reactions are always more favorable for protein kinase A rather than for protein kinase G. Available evidence implies that the topographic relationship between enzyme and substrate may be an important determining factor for the functional specificities of these two classes of protein kinases.