Biotransformation of Bisphenol by Human Cytochrome P450 2C9 Enzymes: a Density Functional Theory Study

Bisphenol A (BPA, 2,2-bis-(4-hydroxyphenyl)propane) is used as a precursor in the synthesis of polycar-bonate and epoxy plastics; however, its availability in the environment is causing toxicity as an endocrine disrupting chemical. Metabolism of BPA and their analogs (substitutes) is generally performed by liver cytochrome P450 enzymes, and often leads to a mixture of products and some of those are toxic. To understand the product distributions of P450 activation of BPA, we have performed a computational study into the mechanisms and reactivities using large model structures of a human P450 isozyme (P450 2C9) with BPA bound. Density functional theory (DFT) calculations on mech-anisms of BPA activation by a P450 Compound I model were investigated leading to a number of possible products. The substrate binding pocket is tight and as a consequence, aliphatic hydroxylation is not feasible as the methyl substituents of BPA cannot reach Compound I well due to constraints of the substrate binding pocket. Instead, we find low-energy pathways that are initiated with phenol hydrogen atom abstraction followed by OH rebound to the phenolic ortho- or para-position. The barriers of para-rebound are well lower in energy than those for ortho-rebound and consequently our P450 2C9 model predicts dominant hydroxycumyl alcohol products. The reactions proceed through two-state-reactivity on competing doublet and quartet spin state surfaces. The calculations show fast and efficient substrate activation on a doublet spin state surface with a rate-determining electrophilic addition step, while the quartet spin state surface has multiple high-energy barriers that also can lead to various side-products including C4-aromatic hydroxylation. This work showed that product formation is more feasible on the low spin state, while the physicochemical properties of the sub-strate govern barrier heights of the rate-determining step of the reaction. Finally, the importance of the second-coordination sphere is highlighted that determines the product distributions and guides the bifurcation pathways.