K252a is a general kinase inhibitor that inhibits PKA (Ki = 18 nM), PKC (Ki = 25 nM), PKG (Ki = 20 nM), CaMK (Ki = 1.8 nM), and phosphorylase kinase (IC50 = 1.7 nM). It functions by acting competitively with ATP and noncompetitively with the substrate.