Calorimetric study of carbohydrate binding to concanavalin A

Flow microcalorimetry has been used to examine the delta H of binding of two types of saccharides, a series of simple monosaccharides and a series of alpha-(1----4)-linked glucosides, to the lectin Concanavalin A. It has been found that the delta H decreases with any change in the stereochemistry of a hydroxyl group relative to methyl alpha-D-mannopyranoside. The data have allowed the calculation of the relative contribution of two of the hydroxyl groups. The delta H's of binding for the alpha-(1----4)-linked glucosides are approximately 31 kJ/mol, and the apparent association constants vary insignificantly with increasing length. This result indicates that only one glucose residue binds to concanavalin A by hydrogen bonds, and that the additional glucose residues have no interaction either by hydrogen bonds or by nonspecific hydrophobic interactions. This result confirms the absence of an extended binding site for alpha-(1----4)-linked glucopyranosides, in contrast to that proposed for alpha-(1----2)-linked mannopyranosides which show an increase in apparent association constants with increasing length.