Electronic models for cytochrome P450 oxidations.

The cytochrome P450 enzymes metabolize a wide variety of hydrophobic endogenous (1,2) and exogenous compounds (3). Their involvement in drug metabolism and xenobiotic activation has made them one of the most studied families of enzymes. A part of our efforts in the area of drug metabolism and toxicology has been to develop models to predict cytochrome P450 oxidations. In general, a complete model for an enzymatic reaction should include both protein-substrate interactions and the appropriate chemical transformations. Unfortunately, the mammalian enzymes are membrane bound, and crystal structures are not available. Therefore, exact interactions with the protein will be difficult, if not impossible to predict. For many enzymes and receptors, a general description of the binding site can be developed based on the structures of substrates and inhibitors (or agonists and antagonists). This information is used to construct a pharmacophore, a representation of the characteristics of the binding site of the protein. While pharmacophores may be developed for some of the cytochrome P450 enzymes (4,5), this may not be possible for others. Some of the interactions between drug metabolizing P450 enzymes and their substrates are likely to be nonspecific. This is expected since many P450 enzymes show very broad substrate and regioselectivity. Whereas the nonspecific nature of the substrate-P450 interactions prevents the development of structural models for these enzymes, this characteristic allows for the development of electronic models. For some small hydrophobic substrates, rotation within the P450 active site is fast, relative to the substrate oxidation step. This has been shown and described by isotope effect studies, where slowing the rate of oxidation of one position by deuterium substitution causes an increase in the rate of metabolism of another position (6,7).