Characterization of family 17 and family 28 carbohydrate-binding modules from Clostridium josui Cel5A

The endoglucanase Cel5A from an anaerobic celluloltyic bacterium, Clostridium josui, contains family 17 CBM (CjCBM17) and family 28 CBM (CjCBM28) in tandem. Individual CjCBM17 and CjCBM28 and tandem CjCBM17/28 were constructed to determine the binding characteristics of CjCBM17/28 and to compare the binding affinity of the three CBMs. CjCBM17/28 bound to non-crystalline cellulose, soluble cellulose derivatives, and oat-spelt xylan, but not to birchwood xylan or starch. The thermodynamic parameters for the binding of the CBMs with cellooligosaccharides were determined by isothermal titration calorimetry. The binding of CjCBM28 to cellotetraose and cellopentaose was enthalpically driven (large negative DeltaH value), while that of CjCBM17 was entropically driven (positive DeltaS value). These two CBMs had different mechanisms for binding to cellooligosaccharides. They showed similar binding constants (K(a)) for cellopentaose, but in the case of insoluble polysaccharides, the K(a) of CjCBM17/28 was approximately 3-7 times higher than that of individual CBMs.