Cellobiohydrolase A (CbhA) from the cellulolytic bacterium Cellulomonas fimi is a beta-1,4-exocellobiohydrolase analogous to Trichoderma reesei CBH II

Substrate specificity of endoglucanase A from Cellulomonas fimi: fundamental differences between endoglucanases and exoglucanases from family 6

Values of kcat. and Km for the hydrolysis of cellotetraose, cellotriose, beta-cellobiosyl fluoride and various beta-aryl cellobiosides by endoglucanase A (CenA) from Cellulomonas fimi indicate that specific binding interactions between the reducing-end glucose residues of cellotetraose and cellotriose and the enzyme at the transition state provide enormous stabilization, endowing glucose with the "effective leaving group ability' of 2,4-dinitrophenol. As has been seen with several other inverting glycosidases, CenA hydrolyses the "wrong' anomer of its glycosyl fluoride substrate, alpha-cellobiosyl fluoride, according to non-Michaelian kinetics. This indicates that CenA carries out this hydrolysis by a mechanism involving binding of two substrate molecules in the active site (Hehre, Brewer and Genghof (1979) J. Biol. Chem. 254, 5942-5950] in contrast with that reported for cellobiohydrolase II, another family-6 enzyme [Konstantinidis, Marsden and Sinnott (1993) Biochem. J. 291, 833-838]. The pH profiles for wild-type CenA indicate that kcat. for CenA depends on the presence of both a protonated group and a deprotonated group for full activity, consistent with the presence of an acid and a base catalyst at the active site. By contrast, the profile for the Asp252Ala mutant of CenA shows a dependence only on a base-catalytic group, thereby confirming the role of Asp-252 as an acid catalyst. These results show that hydrolysis by CenA occurs by a typical inverting mechanism involving both acid and base catalysis, as first proposed by Koshland. It also suggests that endoglucanases from family 6 may function by fundamentally different mechanisms for exoglucanases in this family.

Cellobiohydrolase B, a second exo-cellobiohydrolase from the cellulolytic bacterium Cellulomonas fimi

The gene cbhB from the cellulolytic bacterium Cellulomonas fimi encodes a polypeptide of 1090 amino acids. Cellobiohydrolase B (CbhB) is 1037 amino acids long, with a calculated molecular mass of 109765 Da. The enzyme comprises five domains: an N-terminal catalytic domain of 643 amino acids, three fibronectin type III repeats of 97 amino acids each, and a C-terminal cellulose-binding domain of 104 amino acids. The catalytic domain belongs to family 48 of glycosyl hydrolases. CbhB has a very low activity on CM-cellulose. Viscometric analysis of CM-cellulose hydrolysis indicates that the enzyme is an exoglucanase. Cellobiose is the major product of hydrolysis of cellulose. In common with two other exoglycanases from C. fimi, CbhB has low but detectable endoglucanase activity. CbhB is the second exo-cellobiohydrolase found in C. fimi. Therefore, the cellulase system of C. fimi resembles those of fungi in comprising multiple endoglucanases and cellobiohydrolases.

Enhancement of the endo-beta-1,4-glucanase activity of an exocellobiohydrolase by deletion of a surface loop

In the commonly accepted mechanism for enzymatic hydrolysis of cellulose, endo-beta-1,4-glucanases randomly cleave glucosidic bonds within glucan polymers, providing sites for attack by exo-cellobiohydrolases (EC 3.2.1.91). It has been proposed that hydrolysis by Trichoderma reesei cellobiohydrolase II is restricted to the ends of cellulose polymers because two surface loops cover its active site to form a tunnel. In a closely related endoglucanase, E2 from Thermomonospora fusca, access to the substrate appears to be relatively unhindered because the carboxyl-proximal loop is shortened, and the amino-proximal loop is displaced. The hypothesis was examined by deletion of a region in Cellulomonas fimi cellobiohydrolase A corresponding to part of the carboxyl-proximal loop of T. reesei cellobiohydrolase II. The mutation enhanced the endoglucanase activity of the enzyme on soluble O-(carboxymethyl)cellulose and altered its activities on 2',4'-dinitrophenyl-beta-D-cellobioside, insoluble cellulose, and cellotetraose.

Studies of Streptomyces reticuli cel-1 (cellulase) gene expression in Streptomyces strains, Escherichia coli, and Bacillus subtilis

Various streptomyces strains [Streptomyces lividans 66, Streptomyces vinaceus, and Strepotmyces coelicolor A3 (2)] acquired the ability to utilize crystalline cellulose (Avicel) after transformation with a multicopy vector containing the cel-1 gene from Streptomyces reticuli. The expression level in these hosts was two to three times lower than in S. reticuli, indicating the absence of positive regulatory elements. Like S. reticuli, they processed the Avicelase to its catalytic domain and to an enzymatically inactive part. The cel-1 gene with its original upstream region was not expressed within Escherichia coli. When cel-1 had been fused in phase with the lacZ gene, large quantities of the fusion protein were produced in E. coli. However, this protein was enzymatically inactive and proteolytically degraded to a series of truncated forms. As the cellulase (Avicelase) synthesized by S. reticuli is not cleaved by the E. coli proteases, its posttranslational modification is proposed. With Bacillus subtilis as host, the cel-1 gene was expressed neither under its own promoter nor under the control of a strong Bacillus promoter.