|Superclasses:||all carbohydrates → a carbohydrate → a glycan → a polysaccharide → hemicellulose|
Xylan is a general name given for polysaccharides composed of xylose residues. It is the principal type of hemicellulose in plant biomass representing the second most abundant polysaccharide in the biosphere after cellulose [Schadel10].
The most common form of xylan is a (1→4)-β-D-xylan, which is found in the cell walls of plants and some algae. Pure (1→4)-β-D-xylan is uncommon. In most cases it is decorated with additional sugars, in the form of arabinoxylan, glucuronoxylan, glucuronoarabinoxylan or arabinoglucuronoxylan.
The secondary hydroxyl groups on xylose carbons 2 and 3 can be substituted by a number of residues, the type and degree of substitution depending on xylan's botanical source. Arabinose and glucuronic acid are linked to the backbone by glycosidic bonds. Other substituents form ester bonds: acetate groups are bound directly to the backbone, while cinnamoyl (feruloyl and coumaroyl) residues are linked to the side chain arabinoses [Joseleau92].
Due to its heterogeneous structure, the biodegradation of xylan requires the concerted action of a complex set of enzymes [Gordillo06]. These enzymes are mainly extracellular and are secreted by numerous fungi and bacteria [Biely85]. The endoxylanases and β xylosidases hydrolyze the main chain; the former generating short chain oligosaccharides and the latter acting on these products to liberate xylose. The side chains are hydrolyzed by glycanases, α-glucuronidases, acetyl xylan esterases and feruloyl esterases [deVries01].
(1->3)-β-D-xylan is found only in cell walls of some red and green algae.
Reactions known to consume the compound:
Not in pathways:
xylan + n H2O → n α-D-xylopyranose
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