Cellulase is any of several enzymes produced chiefly by fungi, bacteria, and protozoans that catalyze cellulolysis, the decomposition of cellulose and of some related polysaccharides; specifically, the hydrolysis of the 1,4-beta-D-glycosidic linkages in cellulose, hemicellulose, lichenin, and cereal beta-D-glucans. Cellulases break down the cellulose molecule into monosaccharides such as beta-glucose, or shorter polysaccharides and oligosaccharides. The name is also used for any naturally occurring mixture or complex of various such enzymes, that act serially or synergistically to decompose cellulosic material.
Most mammals have only very limited ability to digest dietary fibres such as cellulose, by themselves. Important example are, the cellulase produced mainly by symbiotic bacteria in ruminants like cattle and sheep and in hindgut fermenters like horses that allows them to digest the cellulose from their grass diet. Cellulases are also produced by a few other types of organisms, such as some termites.

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