CELLOBIOHYDROLASE-A (CBHA) FROM THE CELLULOLYTIC BACTERIUM CELLULOMONAS-FIMI IS A BETA-1,4-EXOCELLOBIOHYDROLASE ANALOGOUS TO TRICHODERMA-REESEI CBH-II

The gene cbhA from the cellulolytic bacterium Cellulomonas fimi encode s a protein of 872 amino acids designated cellobiohydrolase A (CbhA). Mature CbhA contains 832 amino acid residues and has a predicted molec ular mass of 85349 Da. It is composed of five domains: an N-terminal c atalytic domain, three repeated sequences of 95 amino acids, and a C-t erminal cellulose-binding domain typical of other C. fimi glycanases. The structure and enzymatic activities of the CbhA catalytic domain ar e closely related to those of CBH II, an exocellobiohydrolase in the g lycosyl hydrolase family B from the fungus Trichoderma reesei. CbhA is the first such enzyme to be characterized in bacteria. The data suppo rt the proposal that extended loops around the active site distinguish exohydrolases from endohydrolases in this enzyme family.