10.3 Breakdown of polysaccharide: hemicellulose
Hemicellulose is a name which covers a variety of branched-chain polymers containing a mixture of various hexose and pentose sugars, which might also be substituted with uronic and acetic acids. The main hemicelluloses found in plants are xylans (1→4-linked polymers of the pentose sugar xylose), but arabans (polyarabinose), galactans (polygalactose), mannans and copolymers (e.g. glucomannans and galactoglucomannans) are also encountered. The major angiosperm hemicellulose is a xylan with up to 35% of the xylose residues acetylated, and it is also substituted with 4-O-methylglucuronic acid in dicotyledonous plants. Enzymes responsible for hemicellulose degradation are named according to their substrate specificity; for example, mannanases degrade mannans, xylanases degrade xylans, etc. As xylans predominate in plant walls, more is known about xylanases.
Xylanases can be induced by their substrate, the response being for the fungus to produce a complex of enzymes rather than a single one. The complex consists of at least two endoxylanases and a β-xylosidase. The endoxylanases degrade xylan to xylobiose and other oligosaccharides while the xylosidase degrades these smaller sugars to xylose. Some arabinose is also formed, showing that the xylanase complex is able to hydrolyse the branch points in xylan.
Updated December 17, 2016