Synergistic Substrate and Oxygen Activation in Salicylate Dioxygenase Revealed by QM/MM Simulations

摘要

Salicylate 1,2-dioxygenase (SDO) is the first enzyme to be discovered to catalyze the oxidative cleavage of a monohydroxylated aromatic compound, namely salicylate, instead of the well-known electron-rich substrates. We have investigated the mechanism of dioxygen activation in SDO by QM/MM calculations. Our study reveals that the non-heme Fe-II center in SDO activates salicylate and O-2 synergistically through a strong covalent interaction to facilitate the reductive cleavage of O-2. A covalent salicylate-Fe-II-O-2 complex is the reactive oxygen species in this case, and its electronic structure is best described as being between the two limiting cases, Fe-II-O-2 and Fe-II-O-2(.-), with partial electron transfer from the activated salicylate to O-2 via the Fe center. Thus SDO employs a synergistic strategy of substrate and oxygen activation to carry out the catalytic reaction, which is unprecedented in the family of iron dioxygenases. Moreover, O-2 activation in SDO happens without the assistance of a proton source. Our study essentially shows a new mechanistic possibility for O-2 activation.