Active site structure and substrate specificity of cytochrome P450 4A1: steric control of ligand approach perpendicular to heme plane.

The active site of an engineered, expressed fusion protein containing the sequences of cytochrome P450 4A1 (lauric acid omega-hydroxylase) and NADPH-cytochrome P450 reductase, has been probed with olefinic, acetylenic, aromatic, and other analogs of its normal substrate. The fusion protein omega-hydroxylates lauric acid, epoxidizes 11-dodocenoic acid, oxidizes 11-dodecynoic acid to 1,12-dodecandioic acid, but does not oxidize 9-phenylnonanoic acid. Nevertheless, the latter is a tight-binding Type I ligand (Ks = 0.77 microM) and a potent inhibitor of lauric acid hydroxylation. These and other observations are used to construct an active site model that accounts for its remarkable substrate and inhibitor specificity.