The interaction of carbohydrate-binding modules with insoluble non-crystalline cellulose is enthalpically driven

Natural cellulose exists as a composite of cellulose forms, which can be broadly characterized as crystalline or non-crystalline. The recognition of both of these forms of cellulose by the CBMs (carbohydrate-binding modules) of microbial glycoside hydrolases is important for the efficient natural and biotechnological conversion of cellulosic biomass. The category of CBM that binds insoluble non-crystalline cellulose does so with an affinity approx. 10–20-fold greater than their affinity for cello-oligosaccharides and/or soluble polysaccharides. This phenomenon has been assumed to originate from the effects of changes in configurational entropy upon binding. The loss of configurational entropy is thought to be less profound upon binding to conformationally restrained insoluble non-crystalline cellulose, resulting in larger free energies of binding. However, using isothermal titration calorimetry, it is shown that this is not the case for the high-affinity interactions of CcCBM17 (the family 17 CBM from EngF of Clostridium cellulovorans) and BspCBM28 (the family 28 CBM from Cel5A of Bacillus species 1139) with regenerated cellulose, an insoluble preparation of primarily non-crystalline cellulose. The enhanced free energy of binding of non-crystalline cellulose relative to cello-oligosaccharides is by virtue of improved enthalpy, not entropy.