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collectively as cellobiases). The cellulase enzymes have distinct roles in cleaving the various bonds within the microfibril structure. This causes disruption of the crystalline structure followed by depolymerization into short glucose chains. Endoglucanases act randomly on both soluble and insoluble glucose chains by cleaving the ,5(1-4) linkages, yielding glucose and cellooligosaccharides. Following this, exoglucanases, including glucanhydrolase, act on the nonreducing ends of the cellulose chains, yielding glucose and cellobiose (glucose dimers) and cellotriose (glucose trimers). In the final step of decomposition, ,-glucosidase hydrolyzes the glucose chain fragments to glucose. Breakdown of the glucose subunits occurs rapidly and the products can inhibit the activity of the cellulase system.

A wide range of organisms degrade cellulose, but only a few have demonstrated the complete depolymerization and hydrolysis of the crystalline microfibril structure in vitro. The cellulase system of the fungus genus Trichoderma has been extensively studied and shows a large production of endo-, -glucanases and exo-,-glucanases but low levels of ,-glucocidases. In contrast, Aspergillus produces large amounts of endo-, -glucanase and , -glucocidases but low levels of exo-,-glucanases. Chaetoium, an ascomycetous fungus, is found on a wide variety of cellulose materials from paper to composts, especially in moist environments. It produces thermotolerant cellulases that may be commercially viable for converting cellulose to simple sugars from a variety of sources suitable as sources of biofuels. Other fungi with extensively studied cellulase systems include Cremonium cellu-loyticus, Penicillium, Fusarium, and the edible mushroom Agaricus.

Bacteria have less extensive cellulase systems compared to fungi. Bacterial cellulases are organized into a globular scaffolding protein called cellusomes, bound to their cell walls. These structures coordinate the cellulase system for direct attack of crystalline microfibrils, increasing the activity or efficiency of the individual enzymes and allowing the bacteria a better chance to ingest the freed glucose. This is in contrast to fungi that secrete cellulases, resulting in the degradation of cellulytic substances away from the hyphae and thus not guaranteeing that the soluble glucose products are ingested. Common aerobic soil bacteria that depolymerize cellulose are Cellulomas, Cellovibrio, Pseudomonas, and Bacillus. Anaerobic bacteria include Acetobacter, Bacteriodes, Clostridium, Fibrobacter, and Rummococcus.

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