10.3.1 Xylanases

Xylans from different plant sources are categorized by the substituents of their side chains (i. e., arabinoxylan, glucuronxylan, etc.); however, all xylans are built upon apoly-p-(1^4)- xylopyranose backbone. Xylanases depolymerize the (3-(1^4)-xylopyranose backbone of
a variety of xylans, with the endoxylanases classified in 3.2.1.8 [P-(1—>4) hydrolysis] or 3.2.1.32 [p-(1—4) or (3-(1—— 3) hydrolysis]. In both groups, the anomeric configuration of the reducing-end xylan is retained. Most xylanases belong to the two structurally different glycosyl hydrolase groups (families 10 and 11) and differ from each other with respect to their catalytic properties (17). Xylanases with high Mr/low pI (family 10) seem to exhibit greater catalytic versatility than the low Mr/high pI-xylanases (family 11) and thus they are, for example, able to more efficiently hydrolyze highly substituted xylans. Few xylanases have been classified in family 5, which is a very versatile family containing various types of glycanases. Some xylanases have been reported to contain either a xylan-binding domain (18) or a cellulose-binding domain (19). Some of the binding domains have been found to increase the degree of hydrolysis of fiber-bound xylan, whereas others have no effect thereon.

Most of the xylanases characterized are able to hydrolyze different types of xylans, show­ing only differences in the spectrum of end products. However, no systematic studies on the substrate specificity of xylanases belonging to different families on fiber-bound sub­strates have been carried out. The majority of the endoxylanases produce mainly xylobiose, xylotriose, and substituted oligomers of two to four xylosyl residues as products upon ex­tended incubation. Xylose, xylopentaose, and higher oligomers may also be produced, with specific product patterns being dependent upon the individual enzyme and the substrate. Most of the endoxylanases hydrolyze unsubstituted xylose polymers, with the tolerance of side chains again being dependent on the specific xylanase.

The exoxylanases are found in 3.2.1.37,3.2.1.72, and 3.2.1.156, though enzymes in 3.2.1.72 are active on (3-(1—— 3)-linkedxylosides and have limited activity on the majority of xylans. The other two categories, though similar in activity, are distinguished by their difference in preferred substrate degree of polymerization, with the p-xylosidase (1,4-p-D-xyloside xylohydrolase, EC 3.2.1.37) acting more effectively on polymeric xylan, while enzymes in 3.2.1.156 have a greater affinity for xylo-oligomers. Both groups act at the reducing end of the chain, yielding xylose as their product. Exoxylanases are less abundant and consequently less well known and understood than the endoxylanases, as only a few examples of the former have been characterized. Taking into account the high degree of substitution of native xylans, the exo-mode of action maybe less dominant among hemicellulases than cellulases. Exoglycanases are generally larger proteins than endoglycanases, with molecular weights above 100 kDa and they are often built up by two or more subunits. The p-xylosidases, which have a few well-studied examples and though active primarily on xylobiose, may have a processive activity on xylo-oligomers.