Computational study on the catalytic reaction mechanism of heme haloperoxidase enzymes

Heme haloperoxidases are unique enzymes in biology that react H2O2 and halides on a heme center to generate hypohalide, which reacts with a substrate by halide transfer. We studied model complexes of the active site of heme haloperoxidase and investigated the reaction mechanism starting from an iron(III)-hydrogen peroxide-heme complex. We find two stepwise proton transfers by active site Glu and His residues to form Compound I and water, whereby the second proton transfer step is rate-determining. In a subsequent reaction with chloride the oxygen atom transfer is studied to form hypohalide. Overall, the free energy of activation of the second proton transfer and oxygen atom transfer to halide are similar in energy with free energies of activation around 20 kcal mol–1. The calculations show that during oxygen atom transfer from Compound I to halide, significant charge-transfer happens prior to the transition state. This implies that the reaction may be enhanced in polar environments and through second-coordination sphere effects. The studies show that the conversion of H2O2 and halide on a heme center is fast and few intermediates along the reaction mechanism will have a lifetime that is long enough to enable trapping and characterization with experimental methods. A range of active site models and density functional theory methods were tested, but little effect is seen on the mechanism and optimized geometries.