A carbohydrate binding module-5 is essential for oxidative cleavage of chitin by a multi-modular lytic polysaccharide monooxygenase from Bacillus thuringiensis serovar kurstaki

Abstract

Conversion of crystalline chitin to soluble sugar molecules, using lytic polysaccharide mono-oxygenases (LPMOs) has emerged as a new avenue for the production of biofuels. The present study describes the role of accessory domains in a multi-modular LPMO from Bacillus thuringiensis serovar kurstaki (BtLPMO10A). The full length BtLPMO10A (BtLPMO10A-FL) possesses an N-terminal LPMO of AA10 family (BtAA10) and a C-terminal CBM5 (BtCBM5) connected via two fibronectin (Fn) III domains (aligned as AA10-FnIII-FnIII-CBM5 from N- to C-terminus). To determine the role of individual domains, we generated truncation mutants of BtLPMO10A-FL. Substrate binding and kinetic studies revealed that BtCBM5 was involved in increasing binding efficiency of BtAA10 which otherwise has feeble binding towards β-chitin and could not bind to α-chitin. Furthermore, binding assays also indicated that the presence of CBM5 increases the binding efficiency of BtLPMO10A-FL under extreme pH conditions. FnIII domains neither bind nor assist BtLPMO10A-FL in chitin binding and serve as linkers in BtLPMO10A-FL. BtLPMO10A-FL and BtAA10 generated oxidized chito-oligosaccharides from the insoluble β-chitin substrate. It is concluded that BtCBM5 is responsible for increasing binding efficiency of BtLPMO10A-FL, whereas; BtAA10 domain is accountable for oxidative cleavage of recalcitrant chitin.