MetaCyc Compound Class: xylan

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.

Child Classes: a (1->3)-β-D-xylan (3) , a (1→4)-β-D-xylan (0) , a mix-linked (1→3/1→4)-β-D-xylan (0)

Unification Links: ChEBI:37166 , KEGG:C00707 , Wikipedia:Xylan

Reactions known to consume the compound:

Not in pathways:
acetylxylan + n H2O → xylan + n acetate
xylan + n H2O → n α-D-xylopyranose

Reactions known to produce the compound:

cellulose and hemicellulose degradation (cellulolosome) :
a feruloyl-polysaccharide + H2O → ferulate + a polysaccharide + H+

This compound has been characterized as an alternative substrate of the following enzymes: cellulase , cellulose 1,4-β-cellobiosidase , cellulase , cellulase

Created 06-Jan-2011 by Kothari A , SRI International
Revised 03-Oct-2011 by Weerasinghe D


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Please cite the following article in publications resulting from the use of MetaCyc: Caspi et al, Nucleic Acids Research 42:D459-D471 2014
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