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.