Lytic Polysaccharide Monooxygenases-Driven Degradation of Biorefinery Lignocellulose

Abstract

Bacterial and fungal lytic polysaccharide monooxygenase (LPMO) play an important role for cleavage and conversion of recalcitrant polysaccharides. Notwithstanding, their modes of action has persisted largely obscure till the high-resolution structure was revealed. Alongside, modality of binding of LPMO active site for insoluble substrates and redox site were revealed. The LPMOs disrupt the crystalline packing of cellulose microfibrils to create new chain ends for hydrolase enzymes. LPMOs contain a mononuclear Cu(II) center which is being responsible for C-H hydroxylation. In LPMOs action mechanism, the role of co-substrates (O2 or H2O2) is to oxidize polysaccharide with core of cellobiohydrgenase, endoglucanases, and β-glucosidases. Therefore, LPMOs reduces O2 in the coupled hydroxylation of substrate and utilizes H2O2 generates from the coupled reduction of O2. The co-substrates and copper-active site dependent mechanism are linked with a Cu reduction to Cu(I) with non-specific oxidation by H2O2 and regiospecific substrate oxidation. This article focuses on structural and reactivity of LPMOs for wood biomass degradation by the reduction of active site copper prior to the O2 activation. The major areas that are focused: (1) spectroscopic analysis of three-dimensional structures of LPMOs (AA9 to AA13), (2) conformation of polysaccharides, (3) catalytic domains, (4) role in cellulose accessibility, (5) penetration and movement on cellulose ribbons. Additionally, structural evolution of the LPMOs, their role in oxidative mechanisms and redox site determination are reviewed. The choice of redox partner and evolution is linked with biorefinery applications for starch, cellulose, and chitin that possess tight organization of polymers with low hydration levels. Mysterious substrate dependence of the LPMO reactivity with H2O2 was reviewed to emphasize how the power of LPMO improved further. Prospects of current biorefinery applications of LPMOs were discussed with an emphasis of fungal species.