PKI PKA Inhibitor (5-24) is a synthetic peptide inhibitor of PKA (cAMP-dependent protein kinase) (Ki= 2.3 nM) derived from the active site of the skeletal muscle inhibitor protein.1It mimics the protein substrate by binding to the catalytic site through the arginine-cluster basic subsite.1The prominent enzyme-substrate interaction site occurs where PKA catalytic subunit residues Tyr235and Phe239form a sandwich-like structure with residue Phe10of PKI (5-24).2