08.08.2015   Advances in Biochemical Engineering/Biotechnology

T. reesei Cellulases: The Current Industry Standard

The most widespread, commercial enzyme products currently available for biomass hydrolysis are produced by submerged fermentation of the sapro­phytic mesophilic fungus T. reesei [30]. This organism, first isolated over 60 years ago from decaying cotton tents during World War II [31] is a prolific producer of secreted cellulases. Since its initial isolation, numerous mutants have been isolated that increase the productivity of the strain by over 20­fold [28,32,33]. Three enzymes classes form the core of the T. reesei cellulase system: exoglucanases comprised of two primary cellobiohydrolases, a num­ber of endoglucanases, and в-glucosidases (Fig. 5). There are two types of

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Fig. 5 Schematic of the primary T. reesei enzymes involved in hydrolysis of cellulose. Cel­lulose is represented as stacked chains of black circles with reducing (R) and non-reducing (NR) ends indicated. There are two major cellobiohydrolases that attack the cellulose chain ends processively from the reducing (CBH I) and non-reducing (CBHII) ends of the chain, releasing the glucose disaccharide cellobiose. In addition, there are three major en — doglucanases depicted (EGI, II, and III) that attack the cellulose chain randomly, and two в-glucosidases (BG) that hydrolyze cellobiose released by the CBHs to glucose. Triangles represent cellulose binding motifs, and the arrow represents an additional hypothetical protein components that may assist in cellulase action by disrupting the cellulose crystal structure

cellobiohydrolases, CBH I and CBH II, that constitute roughly 60% and 20% of the secreted protein mix and are critical to the efficient hydrolysis of cel­lulose [34]. The CBH I and II hydrolyze the cellulose chain processively from the reducing and non-reducing ends of cellulose chains, respectively, releas­ing the glucose disaccharide cellobiose. Endoglucanases (EG I-IV) constitute roughly 15% of the secreted protein and hydrolyze в-1,4 linkages within the cellulose chains, creating new reducing and non-reducing ends that can then be attacked by the CBHs. в-Glucosidases (BGL I and II), constituting roughly 0.5% of the secreted protein mix, and hydrolyze cellobiose and some other short-chain cellodextrins into glucose.

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