Direct coordination of phenol reductants to copper enables the Cu(II) reduction in Lytic polysaccharide monooxygenases

Copper-dependent lytic polysaccharide monooxygenases (LPMOs) are key enzymes involved in the breakdown of recalcitrant polysaccharides such as cellulose and chitin. LPMOs require external electrons for the activations of either O2 or H2O2, which can be supplied by enzymatic electron donors or small molecule reductants. As quite abundant reductants in nature, phenolic compounds can serve as efficient reducing agents for reactions of LPMOs. Despite extensive studies, how phenolic compounds fuel the reactions of LPMOs is enigmatic. In this study, we report a novel mechanism for the reduction of LPMO-Cu(II) by the phenol reductants. Among various mechanisms investigated, we found the most favorable one involves the coordination re-placement of water by the phenol reductant. The coordination of pyrogallol (Pyr) to LPMO-Cu(II) can remarkably facilitate both the electron transfer from Pyr to Cu(II) and the proton transfer from the ligated OH group to the adjacent Glu148, thereby enhancing proton-coupled electron transfer process for the reduction of LPMO-Cu(II). Detailed comparisons and analysis have shown that the different ligand effects between LPMOs and the copper-dependent pMMO can result in the divergent mecha-nisms for Cu(II) reduction in two enzymes. These insights have greatly expanded our understanding on the interaction machin-ery of copper-dependent enzymes with phenol compounds in nature.