The cellulases and hemicellulases belong to a large group of enzymes called glycoside hydro­lases, which hydrolyze the glycosidic bond between two or more carbohydrates or between a carbohydrate and a noncarbohydrate moiety. Previous classification schemes have been based usually on the substrate specificities of the enzyme, but such classification is largely inappropriate for the glycoside hydrolases, because single protein folds are known to harbor a diversity of substrate specificities. A better classification scheme has been instituted over a decade ago by Bernard Henrissat and colleagues (Coutinho and Henrissat 1999; Henrissat and Davies 2000) Cantarel et al. 2009) , which is based on the amino acid sequence and consequent fold of the protein. The various glycoside hydrolases are thus divided into fami­lies, which currently number 114. This scheme serves to provide comparative structural features of the enzymes within a family, their evolutionary relationships, and their mechanism of action. A compendium of the glycoside hydrolases and related carbohydrate-active enzymes (CAZymes) can be found on the CAZy website (http://www. cazy. org/).

The members of most of the glycoside hydrolase families, relevant to this chapter, exhibit multiple types of activities on either cellulosic and/or hemicellulosic substrates—independent of the fold, although some of the families are restricted to a certain type of activity. The specificity of these enzymes is thus a function of the architecture of the active site, the car­bohydrate binding modules(s), and the linker peptide(s); not necessarily dictated by the overall structure of the enzyme.

The enzymes of some families occur mainly or exclusively in fungi, for example, GH7, GH45, and GH61. Conversely, members of some other families occur mainly or exclusively in bacteria, for example, GH8, GH44, and GH48. The major glycoside hydrolases and their key substrate activities are listed in Table 5.1.

Indeed, the glycoside hydrolase is usually only part of the story, albeit the definitive “busi­ness” part of the protein where the actual bond cleavage of the target carbohydrate is per­formed. Nevertheless, hydrolysis is generally modulated by the action of additional ancillary components of the enzymes, usually in the form of modules—an independently folding sequence within the intact polypeptide. The glycoside hydrolase itself forms the major cata­lytic module of the enzyme. Various other modules may often be present. Some of these ancillary modules may have enzymatic activity, such as the carbohydrate esterases, notably the feruloyl and coumaroyl esterases and the acetyl xylan esterases, all of which are commonly found in association with xylanase catalytic modules. Moreover, some glycoside hydrolases comprise two or more catalytic modules (multiple GHs from one or more fami­lies). In view of the multi-modular nature of these enzymes, they sometimes reach extremely large molecular proportions.