Conformational Response of N-Terminally Truncated Cytochrome P450 3A4 to Ligand Binding in Solution.

Human cytochrome P450 3A4 (CYP3A4) is a membrane-associated monooxygenase that is responsible for metabolizing >50% of the pharmaceuticals in the current market, so studying its chemical mechanism and structural changes upon ligand binding will help provide deeper insights into drug metabolism and further drug development. The best-characterized cytochrome P450 is a bacterial form, P450cam, which undergoes significant conformational changes upon binding substrate and its redox partner, putidaredoxin. In contrast, most crystal structures of CYP3A4 with or without ligands have shown few changes, although allosteric effects and multiple-substrate binding in solution are well-documented. In this study, we use double electron-electron resonance (DEER) to measure distances between spatially separated spin-labels on CYP3A4 and molecular dynamics to interpret the DEER data. These methods were applied to a soluble N-terminally truncated CYP3A4 form, and the results show that there are few changes in the average structure upon binding ketoconazole, ritonavir, or midazolam. However, binding of midazolam, but not ketoconazole or ritonavir, resulted in a significant change in the motion and/or disorder in the F/G helix region near the substrate binding pocket. These results suggest that soluble CYP3A4 behaves in a unique way in response to inhibitor and substrate binding.