Reactivity and selectivity in cytochrome P450 enzymes and haem peroxidases
Bathelt, Christina Michaela #
The cytochromes P450 (CYP) play a vital role in human drug metabolism. Predicting the reactivity and selectivity of reactions catalyzed by these enzymes is essential in the development of new drugs. The nature of the active species of cytochrome P450 and the underlying reaction mechanisms are of fundamental importance for understanding biotransformations of drugs.
The active species (Compound I) is highly reactive and not accessible to direct experimental characterization for CYP. Using a combined quantum mechanical/molecular mechanical (QM/MM) approach at the B3LYP:CHARMM27 level, the electronic structure of Compound I has been characterized, here, in different enzyme environments. These calculations include, for the first time, human drug-metabolizing CYP isoforms and show that the main features of Compound I do not change between different human CYP isoforms.
Conversely, it is known that two haem peroxidases display large differences in the nature of their Compound I species in spite of their remarkably similar enzyme structure. QM/MM calculations reproduce this striking difference in electronic structure of Compound I in very good agreement with experiment.
Hydroxylation of aromatic compounds is one of the most common processes in drug metabolism. The mechanism of this reaction has been explored using QM and QM/MM techniques. Based on the detailed knowledge of the mechanism obtained, structure-reactivity relationships for substituted benzenes were established.
The anti-inflammatory drug diclofenac is metabolized with different regioselectivity by the two CYP isoforms 2C9 and 3A4. QM/MM reaction profiles reveal that the C5 carbon on the phenyl acetate ring is the intrinsically more reactive position. On the other hand, MD simulations indicate that a strong interaction of diclofenac with Arg 108 in the active site of CYP 2C9 is likely to favour reaction at the C4' carbon of the dichlorophenyl ring. These findings provide insight into the origins of the experimentally observed regioselectivity.