Isothermal Titration Calorimetric Studies on the Binding of a Family 6 Carbohydrate-binding Module of Clostridium thermocellumXynA with Xlylooligosaccharides

The family 6 carbohydrate-binding module (CBM) of Clostridium thermocellumXynA was expressed, and the binding equilibria of the CBM with xylooligosaccharides (degree of polymerization DP=2−8) were observed by isothermal titration calorimetry (ITC) at pH 8. The association constant, Ka, increased with increasing DP from 5×103M−1(DP=2) to approximately 5×105M−1(DP=5-8) at 20°C. The Kavalues at 60°C were about 1/10 of those at 20°C. The binding was found to be an enthalpy-driven reaction. The DP dependence of the thermodynamic parameters of the binding reaction suggested the size of the ligand-binding site to be 5 xylose units long.