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 polycarbonate and epoxy plastics;
however, its availability in the environment is causing toxicity as
an endocrine-disrupting chemical. Metabolism of BPA and their analogues
(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 mechanisms
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
can also lead to various side products including C4-aromatic
hydroxylation. This work shows that product formation is more feasible
on the low spin state, while the physicochemical properties of the
substrate 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.