Rate-Limiting Steps in Cytochrome P450 Catalysis

Abstract

Cytochrome P450 (P450) reactions are of interest because of their relevance to the oxidative metabolism of drugs, steroids, carcinogens, and other chemicals. One of the considerations about functional characterization is which steps of the catalytic cycle are ratelimiting. Detailed analysis indicates that several different steps can be ratelimiting with individual P450 reactions. NDealkylation of parasubstituted N,Ndimethylanilines is a function of the electron withdrawing/donating properties of the substituent and the oxidationreduction potential of the substrate, supporting a role in ratelimiting electron transfer from substrate to the high valent P450. In the oxidations of ethanol and acetaldehyde by human P450 2E1, a step following product formation must be the slow step (but not product release per se). Several oxidations catalyzed by human P450s 1A2 and 2D6 show slow CH bond breaking, and apparent highvalent iron complexes accumulate in the reaction steadystate. Kinetic simulations were used to test the suitability of potential schemes and to probe the effects of changes in individual reaction steps.