Mechanisms of Cytochrome P450-Catalyzed Oxidations

Enzymes are complex biological catalysts and are critical to life. Most oxidations of chemicals are catalyzed by cytochrome P450 (P450, CYP) enzymes, which generally utilize mixed-function oxidase stoichiometry, utilizing pyridine nucleotides as electron donors: NAD(P)H + O2 + R → NAD(P)+ + RO + H2O (where R is a carbon substrate and RO is an oxidized product). The catalysis of oxidations is largely understood in the context of the heme iron–oxygen complex generally termed Compound I, formally FeO3+, whose basis was in peroxidase chemistry. Many X-ray crystal structures of P450s are now available (≥822 structures from ≥146 different P450s) and have helped in understanding catalytic specificity. In addition to hydroxylations, P450s catalyze more complex oxidations, including C–C bond formation and cleavage. Enzymes derived from P450s by directed evolution can even catalyze more unusual reactions, e.g., cyclopropanation. Current P450 questions under investigation include the potential role of the intermediate Compound 0 (formally FeIII–O2–) in catalysis of some reactions, the roles of high- and low-spin forms of Compound I, the mechanism of desaturation, the roles of open and closed structures of P450s in catalysis, the extent of processivity in multistep oxidations, and the role of the accessory protein cytochrome b5. More global questions include exactly how structure drives function, prediction of catalysis, and roles of multiple protein conformations.