Enzymes for Modification of Chitin and Chitosan

Nature is abundant in carbohydrates in monomeric, oligomeric and polymeric forms, and different enzymes have evolved to synthesize, modify or degrade complex carbohydrate structures. Carbohydrate-active enzymes (CAZymes) are organised and classified in the CAZy database, where they are divided into different classes, according to function, and families, according to amino acid sequence similarity [1]. Currently, the CAZy database contains five enzyme classes and one class of non-catalytic modules that are associated with CAZymes. The glycoside hydrolase (GH) class contains enzymes that hydrolyse glycosidic linkages. Some GHs also have transglycosylating activity, where a sugar (instead of water, as in hydrolysis) acts as an acceptor resulting in the formation of a new glycosidic bond [2]. Glycosyl transferases (GT) synthesize glycosidic linkages using activated sugars, while polysaccharide lyases (PL) perform non-hydrolytic cleavage of glycosidic bonds. Carbohydrate esterases (CE) remove ester modifications on carbohydrates [2]. The fifth class is referred to as auxiliary activities (AA) and contains a variety of redox enzymes acting in conjunction with other CAZymes [3]. The AA class includes the so-called lytic polysaccharide monoxygenases (LPMOs; see Section 8.1.5) that were discovered in 2010 [4] and play a pivotal role in polysaccharide degradation. The AA class also contains redox enzymes acting on lignin, since lignin is found together with polysaccharides in plant cell walls. Next to these five classes of catalytic domains, the carbohydrate-binding module (CBM) class contains proteins with no enzymatic activity. CBMs are normally covalently attached to enzymes and their primary function is to promote substrate binding [2].