QM/MM Calculations Suggest Concerted O-O Bond Cleavage and Substrate Oxidation by Nonheme Diiron Toluene/o-Xylene Monooxygenase

The nonheme diiron toluene/o-xylene monooxygenase (ToMO) is the most studied toluene monooxygenase that mediates an aromatic hydroxylation reaction. In this work, QM/MM calculations were performed to understand the reaction mechanism. It is revealed that the mu-eta(2):eta(2) peroxodiferric species is the reactive intermediate after the binding of the O-2 molecule to the reduced diferrous center. Subsequently, both a stepwise and a concerted mechanism involving the critical O-O bond cleavage and C-O bond formation were considered. The latter was calculated to be more favorable, suggesting that the formation of a high-valent diferryl Q intermediate is not needed. The isomeric formation of the phenol product was found very facile. The first step was calculated to be rate-limiting, with a barrier of 17.6 kcal/mol for the ortho-hydroxylation.

Automated summary

This study investigated the reaction mechanism of the nonheme diiron toluene/o-xylene monooxygenase (ToMO) through calculations. The findings suggest that the peroxodiferric species plays a crucial role as the reactive intermediate, and the formation of the phenol product is facile. The first step of the ortho-hydroxylation reaction was determined to be rate-limiting.