title: Analysis of homotropic and heterotropic cooperativity of diazepam oxidation by CYP3A4 using site-directed mutagenesis and kinetic modeling.
abstract: The structural basis for the cooperativity of diazepam oxidation catalyzed by human cytochrome P450 3A4 (CYP3A4) and 40 mutants has been investigated. An ordered two-site model in which substrates bind first to a catalytic/effector site and then to the catalytic site was used to explain sigmoidal kinetics for temazepam formation but hyperbolic kinetics for nordiazepam formation. In this model diazepam binds to the enzyme-substrate complex with a greater affinity (K(S2)=140 microM) than to free enzyme (K(S1)=960 microM). Residues 107, 119, 211, 301, 304, 309, 369, 370, and 373 play an important role in determining regioselectivity of diazepam oxidation. Interestingly, S119F and A370F displayed sigmoidal kinetics for nordiazepam formation, whereas I301F exhibited hyperbolic kinetics for both products. In the presence of increasing concentrations of testosterone, K(S1) for diazepam decreased, whereas K(S2) increased. The data suggest that three sites exist within the active pocket.
authors.affiliation: Department of Pharmacology and Toxicology, University of Texas Medical Branch, Galveston 77555-1031, USA.
authors.forename: You Ai
authors.initials: YA
authors.lastname: He
authors.forename: Fabienne
authors.initials: F
authors.lastname: Roussel
authors.forename: James R
authors.initials: JR
authors.lastname: Halpert
mesh_terms: D001665:Binding Sites; D002384:Catalysis; D020134:Catalytic Domain; D002851:Chromatography, High Pressure Liquid; D051544:Cytochrome P-450 CYP3A; D003577:Cytochrome P-450 Enzyme System / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D003975:Diazepam / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D004926:Escherichia coli / Q000201:enzymology; D018757:GABA Modulators / Q000494:pharmacology; D006639:Histidine / Q000378:metabolism; D006801:Humans; D007700:Kinetics; D008956:Models, Chemical; D016297:Mutagenesis, Site-Directed; D009154:Mutation; D010100:Oxygen / Q000378:metabolism; D010957:Plasmids / Q000378:metabolism; D011485:Protein Binding; D011994:Recombinant Proteins / Q000378:metabolism; D013379:Substrate Specificity
chemical_list: D018757:GABA Modulators; D011994:Recombinant Proteins; D006639:Histidine; D003577:Cytochrome P-450 Enzyme System; C104464:CYP3A protein, human; D051544:Cytochrome P-450 CYP3A; C510163:CYP3A4 protein, human; D003975:Diazepam; D010100:Oxygen