Protein kinase A bound to an inhibitor. This ribbon model shows a complex of the catalytic subunit of protein kinase A with an inhibitor (yellow) bearing a pseudosubstrate sequence. Notice that the inhibitor binds to the active site, a cleft between the domains of the enzyme. The bound ATP (purple)-Mn2+ (green) is in the active site adjacent to the site to which the inhibitor is bound.
[Drawn from 1ATP.pdb.]