Structures of exoglucanase from Clostridium cellulovorans: cellotetraose binding and cleavage

Exoglucanase/cellobiohydrolase (EC 3.2.1.176) hydrolyzes a β-1,4-glycosidic bond from the reducing end of cellulose and releases cellobiose as the major product. Three complex crystal structures of the glycosyl hydrolase 48 (GH48) cellobiohydrolase S (ExgS) from Clostridium cellulovorans with cellobiose, cellotetraose and triethylene glycol molecules were solved. The product cellobiose occupies subsites +1 and +2 in the open active-site cleft of the enzyme–cellotetraose complex structure, indicating an enzymatic hydrolysis function. Moreover, three triethylene glycol molecules and one pentaethylene glycol molecule are located at active-site subsites −2 to −6 in the structure of the ExgS–triethylene glycol complex shown here. Modelling of glucose into subsite −1 in the active site of the ExgS–cellobiose structure revealed that Glu50 acts as a proton donor and Asp222 plays a nucleophilic role.