A Molecular Mechanism for the Reduction of the Cu Site in Lytic Polysaccharide Monooxygenases by Phenol Reductants

Lytic polysaccharide monooxygenases (LPMOs) play a pivotal role in the degradation of recalcitrant polysaccharides. As abundant reductants in nature, phenolic compounds may serve as sustainable reducing agents for LPMO reactions. However, the mechanism by which phenolic compounds drive the LPMO reactions remains elusive. In this study, we propose a molecular mechanism for the reduction of LPMO-Cu-(II) by phenolic reductants. Among the mechanisms that we investigated, the most favorable one involves the coordination replacement of water by the phenolic reductant. The coordination of phenols to LPMO-Cu-(II) significantly enhances the proton-coupled electron transfer process for the LPMO-Cu-(II) reduction. The proposed mechanism has been cross-validated by MD, QM/MM and QM/MM-MD studies, EPR spectroscopy, and phenol reductant oxidation experiments. Further analysis reveals that the different ligand effects between LPMOs and copper-dependent particulate methane monooxygenase (pMMO) can lead to divergent mechanisms for Cu-(II) reduction. These investigations underscore how differences in copper coordination environments dictate distinct reduction mechanisms. Collectively, our findings provide profound insights into phenol-mediated copper reduction in nature, advancing a broader understanding of copper enzyme reactivity and redox regulation.