Catalytic Mechanism of Salicylate Dioxygenase: QM/MM Simulations Reveal the Origin of Unexpected Regioselectivity of the Ring Cleavage.

Salicylate 1,2-dioxygenase (SDO) was the first enzyme discovered, in the family of iron dioxygenases, to catalyze the ring cleavage of a monohydroxylated aromatic compound, salicylate, without a proton donor. Salicylate is not electron-rich like the familiar dihydroxy or aromatic substrates with an electron-donating group that are utilized in the well-known dioxygenases. SDO carries out the intramolecular C-C bond cleavage in salicylate bearing the OH and COOH groups with high regioselectivity in comparison with the extradiol and intradiol dioxygenases. The catalytic cleavage of a nonactivated substrate like salicylate that lacks an electron-donating group, also in the absence of a proton source, raises many puzzling questions about the oxy intermediates in the reaction pathway of dioxygenase enzymes in general. To answer these fundamental queries, we have investigated the full catalytic mechanism of SDO by a combination of quantum mechanics and molecular mechanics (QM/MM) calculations. Herein, our QM/MM study has several unexpected and interesting implications for the mechanistic pathway of SDO in comparison to the experimental observations. Importantly, it unravels the basis for the unexpected "intra"-cleavage regioselectivity in SDO. Ostensibly a similar alkylperoxo intermediate is formed in SDO much like in the extradiol and intradiol dioxygenases. In stark contrast to the two diol enzymes, the O-O bond breaking leads to an unprotonated gem-hydroxy carboxylate intermediate, a paradigm analogue of the elusive gem diol intermediate. This unprotonated gem-hydroxy carboxylate intermediate exclusively dictates the C-C cleavage regiospecificity in SDO, which is unprecedented in the family of dioxygenases. It forms a seven-membered lactone species, which eventually forms the ring-cleavage final product by incorporation of two oxygen atoms in the salicylate. Thus, our computational study unravels a detailed reaction pathway of the oxidative cleavage of salicylate without a proton source by identifying the hitherto elusive intermediates in the catalytic cycle of SDO with testable predictions. Moreover, we describe the atomistic origin of the new catalytic role of Arg127 in the catalysis and regioselectivity of SDO. It also rationalizes the formation of a side product without invoking a dioxetane intermediate. This study is important from a fundamental perspective and opens up a new window in the mechanism of the family of dioxygenase enzymes.