16.3.1 Native cellulolytic microorganisms

C. thermocellum, an anaerobic, thermophilic, Gram-positive bacterium, exhibits one of the highest rates of cellulose utilization among described microorganisms (15). C. thermocellum produces a cellulase complex, or “cellulosome,” a substantial fraction of which is bound to the cell surface under most culture conditions (30-33). Because the turnover number of cellulases on insoluble substrates is much lower than most catalytic enzymes on soluble substrates, substantial amounts of cellulase (~2-20% of cellular proteins, weight based) are required to support cellulolytic cells growing on cellulose. Because cellulose is an insoluble
substrate, cellulases must be secreted across the cell membrane so that they can access and hydrolyze insoluble cellulose to soluble sugars that cells can assimilate.