CELLOBIOHYDROLASE-B, A 2ND EXO-CELLOBIOHYDROLASE FROM THE CELLULOLYTIC BACTERIUM CELLULOMONAS-FIMI

The gene cbhB from the cellulolytic bacterium Cellulomonas fimi encode s a polypeptide of 1090 amino acids. Cellobiohydrolase B (CbhB) is 103 7 amino acids long, with a calculated molecular mass of 109 765 Da. Th e enzyme comprises five domains: an N-terminal catalytic domain of 643 amino acids, three fibronectin type III repeats of 97 amino acids eac h, and a C-terminal cellulose-binding domain of 104 amino acids. The c atalytic domain belongs to family 48 of glycosyl hydrolases. CbhB has a very low activity on CM-cellulose. Viscometric analysis of CM-cellul ose hydrolysis indicates that the enzyme is an exoglucanase. Cellobios e is the major product of hydrolysis of cellulose. In common with two other exoglycanases from C. fimi, CbhB has low but detectable endogluc anase activity. CbhB is the second exo-cellobiohydrolase found in C. f imi. Therefore, the cellulase system of C. fimi resembles those of fun gi in comprising multiple endoglucanases and cellobiohydrolases.