Inactive PKA exists as a tetramer consisting of a regulatory (R) subunit dimer and two catalytic (C) subunits. This PKA holoenzyme complex is tethered to cell membranes and organelles through association with A-Kinase Anchoring proteins (AKAPs) . The addition of cAMP causes a conformational change in the anchored R subunits that releases the C subunits to phosphorylate downstream substrates..