Insights into the Mechanism of Aromatic Ring Cleavage of Noncatecholic Compound 2-Aminophenol by Aminophenol Dioxygenase: A Quantum Mechanics/Molecular Mechanics Study

Abstract

2-Aminophenol 1,6-dioxygenase (APD) is an extradiol dioxygenase responsible for the ring cleavage of 2-aminophenol (2AP) at the position ortho to the hydroxyl substituent. To elucidate the reaction mechanism, we conducted quantum mechanical/molecular mechanical (QM/MM) calculations. The mode of binding of the substrate (monodentate or bidentate) to the iron center was found to have a crucial role in dioxygen activation. The Fe–O2 adducts with 2AP bound bidentately has a quintet ground state having a FeIII–superoxo character, while the Fe–O2 adducts with a monodentately bound substrate has been characterized as a substrate radical–FeII–superoxide. Unlike other extradiol dioxygenases that cleave catechol analogues using the superoxo moiety of the Fe–O2 adducts to attack the substrate, we found here an FeII–O(H)O intermediate formed through two sequential proton-coupled electron transfer steps from the initial FeIII–superoxo species is responsible for the attack. Importantly, the second-sphere His195 residue acts as an acid–base catalyst to mediate proton transfer (associated with electron transfer). The study presented here expands our understanding of the extradiol dioxygenases, especially those catalyzing the ring cleavage of noncatecholic substrates.