The term hemicellulase is often used to refer to a mix of enzyme activities that act upon the non-cellulose, non-pectin polysaccharides in biomass. The diversity and complexity of plant cell wall structural hemicelluloses preclude a more precise definition. In general terms, the enzymes involved in hemicellulose degradation can be divided into two major categories: depolymerizing and debranching. Tables 10.3-10.6 have been compiled from several online enzyme databases, the Carbohydrate Active eZyme database (CAZy, http://www. cazy. org), the Expert Protein Analysis System (ExPASy, http://www. expasy. org), and BRENDA, the Comprehensive Enzyme Information System (http://www. brenda. uni-koeln. de) (14-16). These tables further categorize the enzymes involved in cellulose and hemicellulose degra­dation and demonstrate the complexity of the problem. It is important to understand that the classification of biomass-degrading enzymes spans several nomenclatures, with some, but nowhere near complete, cross-referencing. One common system is the IUMB system, with the enzymes being classified by four numbers, designating the enzyme type and activity, i. e., З.2.1.4. The second system, which has gained much popularity over the last decade or so, is the so-called CAZy database, which, in contrast to the IUMB system, classifies glyco — syl hydrolases (and other carbohydrate-active enzymes) into families based on structural and evolutionary similarities, GH7 for example. As there is no comprehensive and simple cross-reference available between these two systems, enzyme designations here are given in either the original connotation from the referenced material, or in the current most common usage.

Depolymerizing enzymes act on the backbone sugar chain and are usually classified as either endo-acting, which cut the chain in the midst of a long polymer, or exo-acting, which work from the end of the chain. Several, however, have been reported to be associated with both types of activity. In addition, there are a series of enzymes that act on the oligomers

Table 10.3 Cell wall polysaccharide depolymerizing |3-glucanases IUPAC Name Families Dominant substrates Dominant products Dominant linkages 3.2.1.4 Cellulase 5, 6, 7, 8, 9, 10, 12, 26, 44, 45, 48, 51,61, 74 Cellulose, |3-glucans, mixed linkage |3-glucans Glucan oligomers (1 —>4)-P-d glucoside 3.2.1.6 Endo-1,3(4)-|3-glucanase 16 Laminarin, lichenin, mixed |3-glucans Glucan oligomers (1 —>3,4)-P-d glucoside 3.2.1.21 |3-glucosidase 1, 3, 9 Cellobiose, other oligoglycans Glucose (1 —>4)-P-d glucoside 3.2.1.39 Glucan endo-1,3- |3-D-glucosidase 15, 17, 55, 64, 81 Laminarin Glucan oligomers (1 —>3)-P-d glucoside 3.2.1.58 Glucan 1,3-fi-glucosidase 3, 5, 17, 55 Laminarin Glucose Non-reducing end (1 v3)-|3-D-glucoside 3.2.1.73 Licheninase 5, 8, 11, 12, 16, 17 Lichenin, mixed P-(1 —>3,4) glucans Mixed |3-(1 -^3,4)-glucans (1 —>4)-P-d glucoside 3.2.1.74 Glucan 1,4-|3-glucosidase 3 Cello-oligomers Glucose exo (1 —>4)-P-d glucoside 3.2.1.91 Cellulose 1,4-|3-cellobiosidase 5, 6, 7, 9 Cellulose Cellobiose Non-reducing end |3-(1 v4) glucoside 3.2.1.120 Oligoxyloglucan |3-glycosidase Xyloglucan oligomers Isoprimeverose (xyl-p-a- (1 ^ 6)-|3-D-glc-p) (1 —>4)-P-d glucoside 3.2.1.150 Oligoxyloglucan reducing-end-specific cellobiohydrolase 74 Xyloglucan oligomers Cellobiose — non-reducing glc may be substituted xyl-p-a-(1 ^6) Reducing end (1 —>4)-P-d glucoside 3.2.1.151 Xyloglucan-specific endo-|3-1,4-glucanase 5, 12, 16, 26, 44, 74 Xyloglucan Xyloglucan oligomers (1 -^4)-|3-D-glucoside 3.2.1.155 Xyloglucan-specific exo-|3-1,4-glucanase Xyloglucan Xyloglucan oligomers (1 -^4)-|3-D-glucoside, mixed endo/exo modes

generated by the combination of endo — and exo-activity. These include p-glucosidase (3.2.1.21), p-xylosidase (3.2.1.37), and p-mannosidases (3.2.1.25) among others. Due to the low degree of polymerization of their substrates, distinction between endo — and exo­hydrolysis modes of action is difficult.

Debranching enzymes, often referred to as accessory enzymes, can be further subdivided into those acting on glycosidic linkages and those acting on ester-linkages. The dominant enzymes of the former include a-L-arabinofuranosidases (3.2.1.55) and a-glucuronidases (3.2.1.139), both of which remove glycosidic side chains from xylan (2, 13). The dominant esterases include acetyl xylan esterase (3.1.1.72) and feruloyl esterase (3.1.1.73), both of which also act on xylan (11). Enzymes acting on the major hemicelluloses are described in Figure 10.1. There are also reports of other esterases acting on other acetylated polysaccha­rides, including glucomannan, galactomannan, and even cellulose (12).

Some hemicellulases and accessory enzymes exhibit cross reactivity across the different hemicelluloses, while others may be very specific for a particular oligomers sequence or conformation. p-xylosidase, for example, may preferentially hydrolyze xylobiose, but may also act on xylotriose and higher xylo-oligomers, gentibiose, and cellobiose. Feruloyl esterase, though normally most active on ferulic acid ester-linked to arabinose, may also be active on coumaroyl esters. p-glucosidases have a wide range of specificities, with certain enzymes acting across a broad range of p-(1—>4), p-(1 —>3), and mixed p-(1—>3,4) linkages and other versions of the same enzymes requiring a specific linkage sequence or side chain pattern. Due to the high degree of heterogeneity in hemicellulose structure, the number of permutations involved in the conformation of sugars, non-sugars, and their linkages

precludes a comprehensive, or even in-depth, discussion of the specific enzymes and their activities. Instead, we will outline the basic activities of a variety of hemicellulase enzymes and indicate their potential use in the depolymerization of the key bioenergy feedstock hemicelluloses.