THEORETICAL MODELS FOR CYTOCHROME P-450 MEDIATED HYDROGEN ATOM ABSTRACTION

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The ubiquitous superfamily of enzymes, the cytochrome P-450s, continue to be the focus of many diverse research efforts. Interest in these heme containing monooxygenases stem from their ability to catalyze the oxidation of a wide variety of lipophilic endogenous (i.e. steroids, prostaglandins and fatty acids) and exogenous (drugs and environmental contaminants) compounds. Because of their importance in drug metabolism and toxicity, predictive models for cytochrome P450 oxidations could be extremely useful. We have used molecular modeling techniques to develop a predictive model for cytochrome P-450 hydrogen abstraction reactions. Of several model oxygen radicals studied, the p-nitrosophenoxy radical has the most appropriate transition state symmetry for use as a model for P-450 mediated hydrogen atom abstractions. Using this model, a linear correlation was observed between H and a combination of H(R) and either the modified Swain-Lupton resonance parameter or the ionization potential of the radical formed. The latter relationship gave an estimated standard deviation of the predicted H of 0.8 kcal/mol. This suggests that it may be possible to obtain an estimate of the relative ability for any carbon-hydrogen bond to undergo P-450 mediated hydrogen atom abstraction by calculating the relative stability and ionization potential of the resulting radical. In addition reactants and products for 54 hydroxylation and desaturation reactions were modeled and used to predict the relative tendency for each reaction to occur.