cAMP-dependent protein kinase (cAPK) or protein kinase A (PKA)is used synonymously for a family of enzymes that consist of two regulatory (R) and two catalytic subunits which under low levels of cAMP is catalytically inactive. When cAMP rises it bind sequentially to the two kinetically different binding sites on the regulatory subunits which undergo an conformational change that releases the catalytic subunits. The free catalytic subunits can then phosphorylate proteins with a certain amino acid motif.