ANTHONY Y. H. LU COMMEMORATIVE ISSUE MOLECULAR BASIS OF P450 INHIBITION AND ACTIVATION Implications for Drug Development and Drug Therapy

Three-dimensional homology models of cytochromes P450 (P450) 2B1 and P450 3A4 have been utilized along with site-directed mu-tagenesis to elucidate the molecular determinants of substrate spec-ificity. Most of the key residues identified in 2B enzymes fall within five substrate recognition sites (SRSs) and have counterparts in bacterial P450 residues that regulate substrate binding or access. Docking of inhibitors into 2B models has provided a plausible explanation for changes in susceptibility to mechanism-based inactivation that ac-company particular amino acid side-chain replacements. These stud-ies provide a basis for predicting drug interactions due to P450 inhi-bition and for rational inhibitor design. In addition, the location of P450 3A4 residues capable of influencing homotropic stimulation by sub-strates and heterotropic stimulation by flavonoids has been identified. Steroid hydroxylation by the wild-type enzyme exhibits sigmoidal kinetics, indicative of positive cooperativity. Based on the 3A4 mode