Models and Mechanisms of Cytochrome P450 Action

Abstract

The reactions catalyzed by the cytochrome P450 family of enzymes have challenged and intrigued chemists for more than two decades. Alkane hydroxylation and olefin epoxidation, particularly, have attracted a sustained worldwide effort, the allure deriving from both a desire to understand the details of biological oxygen activation and transfer and, as well, the sense that catalysts for the practical application of these principles to organic synthesis and to large-scale process chemistry could be of considerable economic value. Cytochrome P450 is able to incorporate one of the two oxygen atoms of an O2 molecule into a broad variety of substrates with concomitant reduction of the other oxygen atom by two electrons to H2O.1 Cytochrome P450 enzymes have been isolated from numerous mammalian tissues (e.g., liver, kidney, lung, intestine, adrenal cortex), insects, plants, yeasts, and bacteria.2 Cytochrome P450 is known to catalyze hydroxylations, epoxidations, N-, S-, and O-dealkylations, N-oxidations, sulfoxidations, dehalogenations, and other reactions.1 The reactive site of all of these enzymes is extraordinarily simple, containing only an iron protoporphyrin IX (1) (Fig. 1) with cysteinate as the fifth ligand, leaving the sixth coordination site to bind and activate molecular oxygen. The local environment of oxygen binding and activation is also very simple, with mostly hydrophobic protein residues and a single threonine hydroxyl which is essential for catalysis for some but not all P450s. The principal catalytic cycle of cytochrome P450 has been much discussed and often reviewed, but the essential features have been agreed upon now for some time. The essential steps involve (Scheme I): (1) binding of the substrate, (2) reduction of the ferric, resting cytochrome P450 to the ferrous state, (3) binding of molecular oxygen to give a ferrous cytochrome P450-dioxygen complex, (4) transfer of the second electron to this complex to give a peroxoiron(III) complex, (5) protonation and cleavage of the O-O bond with the concurrent incorporation of the distal oxygen atom into a molecule of water and the formation of a reactive iron-oxo species, (6) oxygen atom transfer from this oxo complex to the bound substrate, and (7) dissociation of the product. While the first three steps of the enzymatic process have been monitored spectroscopically, the transfer of the second electron, the O-O bond cleavage, and the oxidation of the substrate occur too rapidly and have yet to be observed.3