General Microbiology - Schlegel, H. 1987

Decomposition of Natural Substances
Xylan

Among CARBOHYDRATES, xylan ranks second to Cellulose in natural Abundance: straw and bast fibers contain up to 30% xylan, sugarcane bagasse up to 30%, softwood 7-12%, and hardwood 20-25% (by weight).

Xylan belongs to the group of carbohydrates also known as hemicelluloses. They are structurally and compositionally unrelated to cellulose and are soluble (at least partially) in Water and alkalis. Hemicelluloses consist of pentoses (xylose, arabinose) or hexoses (glucose, mannose, galactose), as well as uronic acids. In plants, they function as reserve or structural substances. The term "hemicellulose" is now generally avoided because many analogous Polysaccharides have been discovered in Fungi and Bacteria.

Xylan molecules consist of ß-D-xylose residues linked by 1,4-glycosidic bonds. Thus, they differ from cellulose molecules by the replacement of —СН₂ОН groups with H atoms. However, the degree of polymerization in xylans is significantly lower (30-100). Some xylans also contain arabinose, glucose, galactose, and glucuronic acid.

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Xylan is degraded faster and by a wider range of microorganisms than cellulose. Many cellulose-degrading microorganisms produce xylanase alongside cellulase. It is even secreted by Sporocytophaga myxococcoides, an Organism capable of breaking down cellulose only upon direct contact with cellulose fibers. Which organisms will primarily attack xylan introduced into the soil depends on environmental conditions. Fungi predominate in acidic soils, whereas bacilli, Sporocytophaga, and other bacteria prevail in neutral and alkaline soils. For fungi, The ability to utilize xylan is more the rule than the exception. Xylan even serves as an excellent substrate for cultivated mushrooms.

In some bacteria (Clostridium), xylanase is a constitutive enzyme, whereas in others it is produced via xylan induction. Action of extracellular xylanase on xylan yields xylobiose and longer fragments alongside xylose. Evidently, this enzyme attacks multiple sites of the xylan molecule simultaneously.