Bacterial cellulase

Bacterial cellulases are known to adopt different mechanisms for the hydrolysis of cellulose. For example, the aerobes Cellulomonas, Pseudomonas, Thermoactinomycetes, T. fusca and Microbispora and the anaerobe C. stercorarium produce a cellulase system similar to that of aerobic fungi, and degrade cellulose by the cooperative interaction of the different cellulase components (Beguin et al., 1992; Wood, 1992b; Gilbert and Hazlewood, 1993). In contrast, the anaerobic thermophilic bacterium C. thermocellum degrades crystalline cellulose very effectively by means of a high molecular mass multienzyme complex called the cellulosome (Lamed and Bayer, 1988; Beguin and Lemaire, 1996). The cellulosome of C. thermocellum has been extensively studied and used as a model system for understanding how anaerobic bacteria, including rumen bacteria, degrade cellulose (Wood, 1992a). The production of cellulosomes is not restricted to C. thermocellum alone. Various other anaerobic bacteria, such as R. albus, R. flavefaciens, F. succinogenes, A. cellulolyticus and C. cellulovorans also produce cellulosomes on their cell surface which possess similar properties to the cellulosome from C. thermocellum (Groleau and Forsberg, 1981; Lamed et al., 1987; Beguin and Lemaire, 1996).

Cellulosomes of C. thermocellum degrade crystalline cellulose extensively in the presence of Ca2+ and DTT (Lamed and Bayer, 1988). According to current understanding, the cellulosome contains a large molecular mass, non-catalytic subunit termed scaffoldin (CipA, S1 or SL) which possesses a cellulose-binding domain (CBD) and numerous duplicated attachment sites called cohesins (Bayer et al, 1994; Beguin and Lemaire, 1996; Beguin and Alzari, 1998; Bhat and Bhat, 1998). In addition to catalytic domains, the enzymatic subunits of the cellulosome each contain a highly conserved duplicated docking domain termed a dockerin, which interacts with the cohesins of the scaffoldin polypeptide (Bayer et al., 1994). Some of the enzymatic subunits also contain a CBD. The exact mechanism by which the cellulosome achieves cellulolysis is unclear, but it is evident that the efficiency of the multienzyme complex is a function of its quaternary structure, and is dependent on both endoglucanase/CBH synergism and clustering of the complex on to the substrate surface (Beguin and Lemaire, 1996).

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