Allosteric modulation of substrate motion in cytochrome P450 3A4-mediated xylene oxidation.
Many cytochrome P450 enzymes (CYPs) exhibit allosteric behavior reflecting a complex ligand-binding process involving numerous factors: conformational selection, protein-protein interactions, substrate/effector/protein structure, and multiple-ligand binding. The interplay of CYP plasticity and rigidity contributes to substrate/product selectivity and to allosterism. Detailed evidence describing how protein motion modulates product selectivity is incomplete as are descriptions of effector-induced modulation of substrate dynamics. Our intent was to discover details of allosteric behavior and CYP3A4 flexibility and rigidity by investigating substrate motion using low-molecular weight ligands. Steady state kinetics and product ratios were measured for oxidation of m-xylene-(2)H3 and p-xylene; intramolecular isotope effects were measured for m-xylene-(2)H3 oxidation as a function of m-xylene-(2)H3 and p-xylene concentration. Biphasic kinetic plots indicated homotropic cooperative behavior with xylene isomers. Selectivity for aromatic hydroxylation over benzylic hydroxylation of m-xylene-(2)H3 supports a model in which the region near the CYP3A4 active oxidizing species limits substrate dynamics. p-Xylene impedes the motion of m-xylene-(2)H3 substrates that have access to the active oxidizing species: (kH/kD)obs values for m-xylene-(2)H3 decreased with p-xylene concentration. m-Xylene-(2)H3 and p-xylene do not have simultaneous access to the active oxidizing species: deuterium-labeled and unlabeled p-xylene exhibited similar effects on the (kH/kD)obs values for m-xylene-(2)H3 oxidation. p-Xylene and m-xylene-(2)H3 bind at different sites: m-xylene-(2)H3 oxidation rates and product selectivity were consistent across the p-xylene concentration range. Overall, this study indicates that the intramolecular isotope effect experimental design provides a unique opportunity to investigate allosteric mechanisms as it provides information about substrate motion when the enzyme is primed to oxidize substrates.
Chemistry Department, Pacific University Oregon , Forest Grove, Oregon 97116, United States.
W Kurtis
WK
Childers
John P
JP
Harrelson
D000494:Allosteric Regulation / Q000502:physiology*; D051544:Cytochrome P-450 CYP3A / Q000378:metabolism*; D003903:Deuterium; D006801:Humans; D006900:Hydroxylation; D007700:Kinetics; D008024:Ligands; D011487:Protein Conformation; D014992:Xylenes / Q000378:metabolism*
D008024:Ligands; D014992:Xylenes; C031286:4-xylene; D003903:Deuterium; D051544:Cytochrome P-450 CYP3A; C510163:CYP3A4 protein, human; C031285:3-xylene