CAZyme Characterization and Engineering for Biofuels Applications

Lignocellulosic biomass is composed primarily of cellulose (homopolysaccharide of glucose), hemicellulose (heteropolysaccharide of glucose, xylose, and/or mannose), and lignin (phenolic organic polymer). Biochemical deconstruction of biomass to fermentable sugar monomers for biofuel production requires application of cocktails of synergistic enzymes belonging to several different families of Carbohydrate Active enZymes (CAZymes). Among these, cellulolytic enzymes are well-known to catalyze hydrolysis of glycosidic linkages within cellulose and can be categorized as exocellulases (or cellobiohydrolases) or endocellulases (or endoglucanases). These cellulases differ in substrate binding site preference, hydrolysis products formed, and mechanistic mode of action and often contain linked substrate-specific carbohydrate binding modules (CBMs). Other, accessory CAZymes such as xylanases and Lytic Polysaccharide Monooxygenases (LPMOs) act synergistically along with cellulases to break down cellulose and hemicellulose into fermentable sugars. Here we discuss the structure, catalytic mechanisms, rate-limitations, and relevant enzyme engineering studies that have been completed in recent years for several different CAZyme families with a focus on the most well-studied enzymes from industrially relevant microbes such as Trichoderma reesei and Thermobifida fusca.