Lytic Polysaccharide Monooxygenase Os LPMO10A Redefines Substrate Scope: Chitosan Oxidation

The emergence of lytic polysaccharide monooxygenases (LPMOs) has fundamentally advanced the landscape of enzymatic biomass conversion. However, while LPMOs targeting chitin, cellulose, and starch are well-characterized, enzymes capable of effectively modifying chitosan have remained elusive. Herein, a chitin-active AA10 LPMO (OsLPMO10A) from Oceanobacillus sp. J11TS1 exhibits a surprisingly high affinity for chitosan. The promotional effect of OsLPMO10A on chitosan is associated with its redox-active catalytic cycle, operating via oxidative modification that alters the substrate surface. Notably, the enzyme demonstrates a specialized substrate preference, processing (GlcN)4 while remaining inactive toward (GlcNAc)4, consistent with the distinct geometry of its elongated catalytic cavity. Structural analyses suggest that a specialized Asp152–Asp153–Arg125 motif situated at the cavity periphery may contribute to substrate positioning through electrostatic interactions. Through targeted micro-etching, OsLPMO10A reduces substrate crystallinity while largely preserving the overall chitosan framework, thereby pre-activating recalcitrant chitosan for enhanced subsequent hydrolysis by chitosanases. These findings provide new insights into LPMO-mediated chitosan modification, offering structural insights into AA10 functional versatility and a potential platform for the enzymatic valorization of marine-derived resources.