Electron Transfer Partners of Cytochrome P450

Abstract

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 b^ may also couple with some members of the Class II P450s family, notably CYP3A4, to enhance the rate of catalysis Although P450 redox partners are usually expressed independently, "self-sufficient" P450 monooxygenase systems have also evolved through the fusion of P450 and CPR genes. These fusion molecules are found in bacteria and fungi, the bestknown example being P450 BM3, a fatty acid (0-2 hydroxylase from Bacillus megaterium, which comprises a soluble P450 with a fiised carboxylterminal CPR module (recently reviewed by Munro^). BM3 has the highest catalytic activity known for a P450 monooxygenase^ and was for many years the only naturally occurring ftised system known until the identification of a eukaryotic membrane-bound equivalent fatty acid hydroxylase, CYP505A1, from the phytopathogenic fungus Fusarium oxysporurrP. A number of novel P450 systems are starting to emerge from the large numbers of genome sequencing projects now underway^. In this chapter, we review the most recent advances being made in understanding the function of P450 redox partners and the electron transfer process. Special attention is paid to CPR, which occupies a particularly important position because of its central involvement in human drug metabolism.