A small lytic polysaccharide monooxygenase from Streptomyces griseus targeting α- and β-chitin

The lytic polysaccharide monooxygenases (LPMOs) have received considerable attention subsequent to their discovery because of their ability to boost the enzymatic conversion of recalcitrant polysaccharides. In the present study, we describe the enzymatic properties of SgLPMO10F, a small (15 kDa) auxilliary activity (AA) family 10 LPMO from Streptomyces griseus belonging to a clade of the phylogenetic tree without any characterized representative. The protein was expressed using a Brevibacillus-based expression system that had not been used previously for LPMO expression and that also ensures correct processing of the N-terminus crucial for LPMO activity. The enzyme was active towards both α- and β-chitin and showed stronger binding and a greater release of soluble oxidized products for the latter allomorph. In chitinase synergy assays, however, SgLPMO10F worked slightly better for α-chitin, increasing chitin solubilization yields by up to 30-fold and 20-fold for α- and β-chitin, respectively. Synergy experiments with various chitinases showed that the addition of SgLPMO10F leads to a substantial increase in the (GlcNAc)2 :GlcNAc product ratio, in reactions with α-chitin only. This underpins the structural differences between the substrates and also shows that, on α-chitin, SgLPMO10F affects the binding mode and/or degree of processivity of the chitinases tested. Variation in the only exposed aromatic residue in the substrate-binding surface of LPMO10s has previously been linked to preferential binding for α-chitin (exposed Trp) or β-chitin (exposed Tyr). Mutation of this residue, Tyr56, in SgLPMO10F to Trp had no detectable effect on substrate-binding preferences but, in synergy experiments, the mutant appeared to be more efficient on α-chitin.