Mechanistic insights into dioxygen activation, oxygen atom exchange and substrate epoxidation by AsqJ dioxygenase from quantum mechanical/molecular mechanical calculations

Herein, we use in-protein quantum mechanical/molecular mechanical (QM/MM) calculations to elucidate the mechanism of dioxygen activation, oxygen atom exchange and substrate epoxidation processes by AsqJ, an FeII/α-ketoglutarate-dependent dioxygenase (α-KGD) using a 2-His-1-Asp facial triad. Our results demonstrated that the whole reaction proceeds through a quintet surface. The dioxygen activation by AsqJ leads to a quintet penta-coordinated FeIV–oxo species, which has a square pyramidal geometry with the oxo group trans to His134. This penta-coordinated FeIV–oxo species is not the reactive one in the substrate epoxidation reaction since its oxo group is pointing away from the target CC bond. Instead, it can undergo the oxo group isomerization followed by water binding or the water binding followed by oxygen atom exchange to form the reactive hexa-coordinated FeIV–oxo species with the oxo group trans to His211. The calculated parameters of Mössbauer spectra for this hexa-coordinated FeIV–oxo intermediate are in excellent agreement with the experimental values, suggesting that it is most likely the experimentally trapped species. The calculated energetics indicated that the rate-limiting step is the substrate CC bond activation. This work improves our understanding of the dioxygen activation by α-KGD and provides important structural information about the reactive FeIV–oxo species.