背景资料：Cyclic AMP-dependent protein kinase A (PKA) is an essential enzyme in the signaling pathway of the second messenger cAMP. Through phosphorylation of target proteins, PKA controls many biochemical events in the cell including regulation of metabolism, ion transport, and gene transcription. The PKA holoenzyme is composed of 2 regulatory and 2 catalytic subunits and dissociates from the regulatory subunits upon binding of cAMP.