Multipoint Precision Binding of Substrate Protects Lytic Polysaccharide Monooxygenases from Self-Destructive Off-Pathway Processes

Lytic polysaccharide monooxygenases (LPMOs) play a crucial role in the degradation of polysaccharides in biomass by catalyzing powerful oxidative chemistry using only a single copper ion as a cofactor. Despite the natural abundance and importance of these powerful monocopper enzymes, the structural determinants of their functionality have remained largely unknown. We have used site-directed mutagenesis to probe the roles of 13 conserved amino acids located on the flat substrate-binding surface of CBP21, a chitin-active family AA10 LPMO from Serratia marcescens, also known as SmLPM010A. Single mutations of residues that do not interact with the catalytic copper site, but rather are involved in substrate binding had remarkably strong effects on overall enzyme performance. Analysis of product formation over time showed that these mutations primarily affected enzyme stability. Investigation of protein integrity using proteomics technologies showed that loss of activity was caused by oxidation of essential residues in the enzyme active site. For most enzyme variants, reduced enzyme stability correlated with a reduced level of binding to chitin, suggesting that adhesion to the substrate prevents oxidative off-pathway processes that lead to enzyme inactivation. Thus, the extended and highly evolvable surfaces of LPMOs are tailored for precise multipoint substrate binding, which provides the confinement that is needed to harness and control the remarkable oxidative power of these enzymes. These findings are important for the optimized industrial use of LPMOs as well as the design of LPMO-inspired catalysts.