2.5.1 A Multiplicity of Hemicellulases

Mirroring the variety of polysaccharides containing pentoses, hexoses, or both (with or without sugar hydroxyl group modifications) collectively described as hemicelluloses (figure 1.23), hemicellulolytic organisms are known across many species and genera:15 [16]

Table 2.8 summarizes major classes of hemicellulases, their general sites of action, and the released products. Microorganisms capable of degrading hemicelluloses have, however, multiple genes encoding many individual hemicellulases; for exam­ple, Bacillus subtilis has in its completely sequenced genome at least 16 separate genes for enzymes involved in hemicellulose degradation.127

TABLE 2.8

Major Hemicellulases, Their Enzymic Sites of Action and Their Products

Hemicellulase EC number Site(s) of action Released products

Hemicelluloses comprise both linear and branched heteropolysaccharides. Endoxylanases fragment the xylan backbone, and xylosidases cleave the resulting xylan oligosaccharides into xylose; removal of the side chains is catalyzed by glucuron­idases, arabinofuranosidases, and acetylesterases — the action of these enzymes can limit the overall rate of hemicellulose saccharification because endoacting enzymes cannot bind to and cleave xylan polymers close to sites of chain attachment.128

Much of the fine detail of hemicellulase catalytic action is beginning to emerge and will be vital for directed molecular evolution of improved hemicellulase biocatalysts.129,130 Already, however, a thermostable arabinofuranosidase has been identified and shown to have a unique selectivity in being able to degrade both branched and debranched arabinans.131 Synergistic interactions among different microbial arabinofuranosidases have also been demonstrated to result in a more extensive degradation of wheat arabinoxylan than found with individual enzymes.132 The activity of biotech companies in patenting novel hemicellulase activities is evi­dent (table 2.9) in exploring hemicellulases from unconventional microbial sources, and deep-sea thermophilic bacteria from the Pacific have been shown to synthesize thermotolerant xylanases, to be active over a wide pH range, and to degrade cereal hemicelluloses.133