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MetaCyc Compound Class: hemicellulose

Superclasses: all carbohydratesa carbohydratea glycana polysaccharide

Please note that hemicellulose is a not a precise biological term.

The plant cell wall is composed of cellulose and many additional complex polymers. Unlike cellulose, which is crystalline and resistant to hydrolysis, the other polymers are amorphous and are easily hydrolyzed by dilute acid or base.

Hemicellulose is a heterogeneous term that is used for all the plant cell wall components that are soluble in dilute alkali solutions, or that go into solution quite readily in hot dilute mineral acids with the formation of simple sugars. The term is often used to describe all the polysaccharide components of the cell wall other than cellulose.

Hemicelluloses constitute about one-fourth of perennial plants and about one-third of annual plants. Since the term does not refer to a particular chemical entity, hemicelluloses extracted from different plant sources are rarely identical. However, D-xylose is the dominant building unit of the hemicelluloses of most woods and annual plants. D-xylose occurs as the polymer xylan, as derivatives of xylan, such as glucuronoxylan and arabinoxylan, and as side chains on other types of backbones, such as xyloglucan and xylogalacturonan. D-Mannose is also very abundant, and occurs as the polymer mannan or in combination with D-glucose or D-galactose as glucomannans, galactomannan, or galactoglucomannan. Arabinose is common as well, either in the form of L-arabinan or in the form of side chains on other polysaccharides.

Other sugars common in hemicellulose include galactose, rhamnose, galacturonate and mannuronate.

Hemicellulose consists of shorter chains than cellulose, typically 500 - 3,000 sugar units, and is usually branched.

Child Classes: a xyloglucan (1), xylan (3)

Unification Links: Wikipedia:Hemicellulose

Reactions known to produce the compound:

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

Revised 05-Apr-2011 by Caspi R, SRI International


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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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