title: Multiple sequential steps involved in the binding of inhibitors to cytochrome P450 3A4.
abstract: Cytochrome P450 (P450) 3A4 is an extensively studied human enzyme involved in the metabolism of >50% of drugs. The mechanism of the observed homotropic and heterotropic cooperativity in P450 3A4-catalyzed oxidations is not well understood, and together with the cooperative behavior, a detailed understanding of interaction of drug inhibitors with P450 3A4 is important in predicting clinical drug-drug interactions. The interactions of P450 3A4 with several structurally diverse inhibitors were investigated using both kinetic and thermodynamic approaches to resolve the steps involved in binding of these ligands. The results of pre-steady-state absorbance and fluorescence experiments demonstrate that inhibitor binding is clearly a multistep process, even more complex than the binding of substrates. Based on spectrophotometric equilibrium binding titrations as well as isothermal titration calorimetry experiments, the stoichiometry of binding appears to be 1:1 in the concentration ranges studied. Using a sequential-mixing stopped-flow approach, we were also able to show that the observed multiphasic binding kinetics is the result of sequential events as opposed to the existence of multiple enzyme populations in dynamic equilibrium that interact with ligands at different rates. We propose a three-step minimal model for inhibitor binding, developed with kinetic simulations, consistent with our previously reported model for the binding of substrates, although it is possible that even more steps are involved.
authors.affiliation: Department of Biochemistry and Center in Molecular Toxicology, Vanderbilt University School of Medicine, Nashville, Tennessee 37232-0146, USA.
authors.forename: Emre M
authors.initials: EM
authors.lastname: Isin
authors.forename: F Peter
authors.initials: FP
authors.lastname: Guengerich
mesh_terms: D051544:Cytochrome P-450 CYP3A; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System / Q000737:chemistry; D004791:Enzyme Inhibitors / Q000737:chemistry*; D006801:Humans; D007700:Kinetics; D008024:Ligands; D008956:Models, Chemical; D004364:Pharmaceutical Preparations / Q000378:metabolism; D011485:Protein Binding; D011994:Recombinant Proteins; D013057:Spectrum Analysis; D013816:Thermodynamics; D017186:Titrimetry
chemical_list: D065607:Cytochrome P-450 Enzyme Inhibitors; D004791:Enzyme Inhibitors; D008024:Ligands; D004364:Pharmaceutical Preparations; D011994:Recombinant Proteins; D003577:Cytochrome P-450 Enzyme System; D051544:Cytochrome P-450 CYP3A; C510163:CYP3A4 protein, human