How structural dynamics influence the substrate oxidation energetics in lytic polysaccharide monooxygenases

Lytic polysaccharide monooxygenases (LPMOs) are copper-dependent enzymes that have fueled the hope for sustainable biofuel production since they enhance the breakdown of recalcitrant polysaccharides like cellulose. In the consensus mechanism, their catalytic activity relies on forming an ‘oxyl’, [CuO˙⁻]⁺, species at the active site, followed by subsequent hydrogen atom abstraction (HAA) from the substrate. Some studies report rather high barriers for this reaction, identifying it as the rate-limiting step in the oxidation process, whereas other investigations have reported significantly lower barriers. In this study, we have constructed a force field for the active site and show through extensive sampling from molecular dynamics simulations that the QM/MM reaction barrier depends critically on the underlying structural conformations of the enzyme–substrate complex. The results support low-energy barriers for the HAA step and help to explain previous discrepancies in the literature, which may be attributed to insufficient conformational sampling.