The Ustilago maydis AA10 LPMO is active on fungal cell wall chitin

The upsurge of lytic polysaccharide monooxygenases in biomass deconstruction: characteristic functions and sustainable applications

Lytic polysaccharide monooxygenases (LPMOs) are one of the emerging classes of copper metalloenzymes that have received considerable attention due to their ability to boost the enzymatic conversion of intractable polysaccharides such as plant cell walls and chitin polymers. LPMOs catalyze the oxidative cleavage of β-1,4-glycosidic bonds using molecular O2 or H2O2 in the presence of an external electron donor. LPMOs have been classified as an auxiliary active (AA) class of enzymes and, further based on substrate specificity, divided into eight families. Until now, multiple LPMOs from AA9 and AA10 families, mostly from microbial sources, have been investigated; the exact mechanism and structure-function are elusive to date, and recently discovered AA families of LPMOs are just scratched. This review highlights the origin and discovery of the enzyme, nomenclature, three-dimensional protein structure, substrate specificity, copper-dependent reaction mechanism, and different techniques used to determine the product formation through analytical and biochemical methods. Moreover, the diverse functions of proteins in various biological activities such as plant-pathogen/pest interactions, cell wall remodeling, antibiotic sensitivity of biofilms, and production of nanocellulose along with certain obstacles in deconstructing the complex polysaccharides have also been summarized, while highlighting the innovative and creative ways to overcome the limitations of LPMOs in hydrolyzing the biomass.

Impact of the Copper Second Coordination Sphere on Catalytic Performance and Substrate Specificity of a Bacterial Lytic Polysaccharide Monooxygenase

Lytic polysaccharide monooxygenases (LPMOs) catalyze the oxidative cleavage of glycosidic bonds in recalcitrant polysaccharides, such as cellulose and chitin, using a single copper cofactor bound in a conserved histidine brace with a more variable second coordination sphere. Cellulose-active LPMOs in the fungal AA9 family and in a subset of bacterial AA10 enzymes contain a His-Gln-Tyr second sphere motif, whereas other cellulose-active AA10s have an Arg-Glu-Phe motif. To shine a light on the impact of this variation, we generated single, double, and triple mutations changing the His216-Gln219-Tyr221 motif in cellulose- and chitin-oxidizing MaAA10B toward Arg-Glu-Phe. These mutations generally reduced enzyme performance due to rapid inactivation under turnover conditions, showing that catalytic fine-tuning of the histidine brace is complex and that the roles of these second sphere residues are strongly interconnected. Studies of copper reactivity showed remarkable effects, such as an increase in oxidase activity following the Q219E mutation and a strong dependence of this effect on the presence of Tyr at position 221. In reductant-driven reactions, differences in oxidase activity, which lead to different levels of in situ generated H2O2, correlated with differences in polysaccharide-degrading ability. The single Q219E mutant displayed a marked increase in activity on chitin in both reductant-driven reactions and reactions fueled by exogenously added H2O2. Thus, it seems that the evolution of substrate specificity in LPMOs involves both the extended substrate-binding surface and the second coordination sphere.

The disordered C-terminal tail of fungal LPMOs from phytopathogens mediates protein dimerization and impacts plant penetration

Lytic polysaccharide monooxygenases (LPMOs) are monocopper enzymes that oxidatively degrade various polysaccharides, such as cellulose. Despite extensive research on this class of enzymes, the role played by their C-terminal regions predicted to be intrinsically disordered (dCTR) has been overlooked. Here, we investigated the function of the dCTR of an LPMO, called CoAA9A, up-regulated during plant infection by Colletotrichum orbiculare, the causative agent of anthracnose. After recombinant production of the full-length protein, we found that the dCTR mediates CoAA9A dimerization in vitro, via a disulfide bridge, a hitherto-never-reported property that positively affects both binding and activity on cellulose. Using SAXS experiments, we show that the homodimer is in an extended conformation. In vivo, we demonstrate that gene deletion impairs formation of the infection-specialized cell called appressorium and delays penetration of the plant. Using immunochemistry, we show that the protein is a dimer not only in vitro but also in vivo when secreted by the appressorium. As these peculiar LPMOs are also found in other plant pathogens, our findings open up broad avenues for crop protection.