CAMP-dependent protein kinase
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In cell biology, cAMP-dependent protein kinase (cAPK), also known as protein kinase A (PKA)(EC 2.7.1.37), refers to a family of enzymes whose activity is dependent on the level of cyclic AMP (cAMP) in the cell.
Each PKA is a holoenzyme that consists of two regulatory and two catalytic subunits. Under low levels of cAMP, the holoenzyme remains intact and is catalytically inactive. When the concentration of cAMP rises (e.g. activation of adenylate cyclases by certain G protein-coupled receptors, inhibition of phosphodiesterases which degrade cAMP), cAMP binds to the two binding sites on the regulatory subunits, which then undergo a conformational change that releases the catalytic subunits. The free catalytic subunits can then phosphorylate protein substrates at serine, threonine, or tyrosine residues. This phosphorylation usually results in a change in activity of the substrate.