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Activation of PKA by cAMP. In its inactive state, PKA consists of a tetramer of two regulatory and two catalytic subunits. Each regulatory subunit has two cAMP-binding sites; cAMP binding releases the catalytic subunits, which become bound to ATP and go on to phosphorylate serine and threonine residues that possess the appropriate substrate sequence. The two cAMP-binding sites (A and B) on each regulatory subunit are shown. C, protein kinase A catalytic subunit; R, protein kinase A regulatory subunit.
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