Electron transfer partners of cytochrome P450

Cytochromes P450 contain a heme center where the activation of molecular oxygen occurs, resulting in the insertion of a single atom of oxygen into an organic substrate with the concomitant reduction of the other atom to water. The monooxygenation reaction requires a coupled and stepwise supply of electrons, which are derived from NAD(P)H and supplied via a redox partner. P450s are generally divided into two major classes (Class I and Class II) according to the different types of electron transfer systems they use. P450s in the Class I family include bacterial and mitochondrial P450s, which use a two-component shuttle system consisting of an iron-sulfur protein (ferredoxin) and ferredoxin reductase (Figure 4.1). The Class II enzymes are the microsomal P450s, which receive electrons from a single membrane-bound enzyme, NADPH cytochrome P450 reductase (CPR), which contains FAD and FMN cofactors (Figure 4.1). Cytochrome b1 may also couple with some members of the Class II P450s family, notably CYP3A4, to enhance the rate of catalysis1.