In general, xylanases are specific for the internal P-1,4 linkages of polymeric xylan and are designated as endoxylanases. Based on their action on different polysaccharides, endoxylanases have been classified as either specific or non-specific (Coughlan, 1992; Coughlan et al., 1993). The specific endoxylanases are active on xylans with only P-1,4 linkages, whereas non-specific endoxylanases hydrolyse P-1,4-linked xylans, P-1,4 linkages of mixed xylans and other P-1,4-linked polymers such as CM-cellulose. In fact, the determination of the kinetic constants ratio kcat/Km for xylan and CM-cellulose, for a particular non-specific xylanase, should confirm whether the enzyme in question is a xylanase or a cellulase. Generally, endoxylanase activity and affinity for xylo-oligosaccharides decrease with decreasing DP (Coughlan et al., 1993). Most endoxylanases are specific for unsubstituted xylosidic linkages of xylans and release both substituted and unsubstituted xylo-oligosaccharides. In contrast, some endoxylanases are specific for xylosidic linkages adjacent to substituted groups in the main chain xylan. For example, two endoxylanases from Aspergillus niger (pI 8.0 and 9.6) showed little or no action on xylo-oligomers or xylans from which the arabinose substituents were removed (Frederick et al., 1985). Furthermore, the endoxylanase with pI 9.6 cleaved the xylan chain only in the vicinity of 4-O-methylglucuronic acid, and this substitution was reported to be an absolute requirement for the action of the enzyme (Nishitani and Nevins, 1991). Nevertheless, both enzymes failed to release any substituent as a free product, which indicated that these substitutions are necessary for the proper orientation of the substrate in the active site.

Most endoxylanases, active on mixed xylan (rhodymenan with P-1,3 and P-1,4 linkages), are specific for the P-1,4 linkages (Coughlan, 1992). Also, endoxylanases have been grouped either as debranching or non-debranching based on their ability to release arabinose in addition to hydrolysing the main chain (Coughlan et al., 1993). Although some endoxylanases purified to homogeneity hydrolysed both main-chain xylan and arabinose side-chains (Matte and Forsberg, 1992), there is always a question as to whether the release of arabinose is due to an intrinsic property of the enzyme or is due to the presence of a trace contaminant.

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