Theoretical study of the lytic polysaccharide monooxygenase catalytic mechanism

Lytic polysaccharide monooxygenases (LPMOs) represent a new paradigm in carbohydrate turnover and exhibit synergy with hydrolytic enzymes in biomass depolymerization. LPMO structures have revealed a mononuclear copper-containing active site and mechanistic studies have shown that they utilize dioxygen and a reducing agent to oxidatively cleave glycosidic linkages in polysaccharides. To date, several features of copper binding to LPMOs have been elucidated, but the identity of the reactive oxygen species (ROS) and the key steps in the oxidative mechanism have not been elucidated. We employed density functional theory (DFT) calculations with an enzyme active site model to identify the ROS and compare several hypothesized reaction pathways in LPMOs for hydrogen abstraction and polysaccharide hydroxylation. The N-terminal histidine methylation is also examined, which is thought to modify the structure and reactivity of the enzyme. Overall, this study suggests the steps in the LPMO mechanism for oxidative cleavage of glycosidic bonds.