cAMP-dependent PKA is a ubiquitous serine/theonine protein kinase present in a variety of tissues （e.g. brain， skeletal muscle， heart）. The intracellular cAMP level regulates cellular responses by altering the interaction between the catatytic C and regulatory R subunits of PKA. The inactive tetrameric PKA holoenzyme R2C2 is activated when cAMP binds to R2， which dissociates the tetramer to R2 cAMP 4 and two active catalytic subunits. Free Catalytic subunits of PKA can phosphorylate a wide variety of intracellular target proteins. In response to hormone- induced high cAMP levels， PKA phosphorylates glycogen synthetase （inhibition of the enzyme activity） and phosphorylase kinase to block glycogen synthesis. Different isoforms of catalytic and regulatory subunits suggest specific functions.
