Oxygen Activation and Transfer

Abstract

The cytochrome P-450-catalyzed insertion of an oxygen into a substrate culminates a process that reduces molecular oxygen to a species equivalent, in terms of formal electron count and reactivity, to an oxygen atom. The catalytic sequence for microsomal cytochrome P-450 enzymes, which has been reviewed previously,1–4 involves the following steps (Fig. 1): (1) binding of a substrate, (2) reduction of the two flavin prosthetic groups of cytochrome P-450 reductase by NADPH, (3) transfer of one of the two electrons thus made available to cytochrome P-450, (4) binding of molecular oxygen to give a ferrous cytochrome P-450—dioxygen complex, (5) transfer of a second electron from cytochrome P-450 reductase, or of an electron from cytochrome b 5, to the complex, (6) cleavage of the oxygen-oxygen bond with concurrent incorporation of the distal oxygen atom into a molecule of water, (7) transfer of the second oxygen atom to the substrate, and (8) dissociation of the product. The catalytic cycle for mitochondrial cytochrome P-450 enzymes differs in that the transfer of electrons from cytochrome P-450 reductase to the hemeprotein is mediated by adrenodoxin, an iron—sulfur protein (see Chapter 12). The steps in the catalytic sequence in which bonds to oxygen are made or broken are addressed in this chapter. The substrate-binding step is discussed in Chapter 3, however, and the electron transfer steps in Chapters 4, 5, 11, and 12.