Glycosidic side groups connected to xylan and glucomannan main chains are primarily removed by a-glucuronidase, a-arabinofuranosidase, and a-D-galactosidase. Acetyl and hydroxycinnamic acid substituents bound to hemicellulose are removed by acetyl xylan esterases and ferulic/coumaric acid esterases (Figure 10.1). These are clearly different types of side-group cleaving enzymes. Some of them are able to hydrolyze only substituted short- chain oligomers, which first must be produced by the backbone depolymerizing endoen — zymes (xylanases and mannanases). Others are capable of debranching intact polymeric substrates. Most accessory enzymes of the latter type, however, prefer oligomeric substrates. The synergism between different hemicellulolytic enzymes is also observed by the acceler­ated action of endoglycanases in the presence of accessory enzymes. Side groups that are still attached to oligosaccharides after the hydrolysis of xylans and mannans by xylanase or mannanase, respectively, restrict the action of p-xylosidase and p-mannosidase.

10.4.1 a — glucuronidase

a-glucuronidase (3.2.1.139) catalyzes the release of glucuronic acid or 4- O — methylglucuronic acid from xylan. Reports of specific substrate specificities vary from activity on long-chain xylans to a requirement for attachment to a terminal non-reducing endxylose (64,65). At least one membrane-bound bacterial enzyme exhibited activity specif­ically on soluble xylan-derived oligomers (66). However, synergy between a-glucuronidase and endoxylanase on wheat xylan has been shown to be significant with simultaneous activity yielding the highest release of 4- O-methylglucuronic acid (67).

10.4.2 a — arabinofuranosidase

a-arabinofuranosidase cleaves arabinose side chains from the xylan main chain. Many en­zymes in this class have demonstrated activity on pectin, arabinan, and arabinoxylan with the preferred chain length and side-group assignments also varied (13, 68, 69). A com­prehensive review can be found here (69). Synergy with xylanases has been reported, as has synergy with ferulic acid and acetyl xylan esterase (2, 13, 65, 67, 70). As ferulic acid is linked to the arabinose, which in turn is linked to the xylan chain, these synergies are not unexpected. The substrate specificity of this enzyme class is still somewhat muddled (65).