Comparative studies of two AA10 family lytic polysaccharide monooxygenases from Bacillus thuringiensis

Lytic polysaccharide monooxygenases (LPMOs) are a group of copper-dependent enzymes that depolymerize recalcitrant polysaccharides via an oxidative mechanism. However, the biological reasons that could be accounted for this diversity remains largely unknown. In previous reports, two chitin-active LPMOs from Bacillus thuringiensis , Bt LPMO10A and Bt LPMO10B, have been preliminarily characterized. However, some important biochemical features and substrate preference, as well as their potential applications in chitin degradation, still deserve further investigation. Results from present study showed that both Bt LPMO10A and Bt LPMO10B exhibit similar catalytic domains as well as highly conserved substrate-binding planes. However, unlike Bt LPMO10A, which has comparable binding ability to both crystalline and amorphous form of chitins, Bt LPMO10B exhibited much stronger binding ability to colloidal chitin, which mainly attribute to its carbohydrate-binding module-5 (CBM5). Interestingly, the relative high binding ability of Bt LPMO10B to colloidal chitin does not lead to high catalytic activity of the enzyme. In contrast, the enzyme exhibited higher activity on β-chitin. Further experiments showed that the binding of Bt LPMO10B to colloidal chitin was mainly non-productive, indicating a complicated role for CBM5 in LPMO activity. Furthermore, synergistic experiments demonstrated that both LPMOs boosted the activity of the chitinase, and the higher efficiency of Bt LPMO10A can be override by Bt LPMO10B.