Thermodynamic and kinetic studies on the mechanism of binding of methylumbelliferyl glycosides to jacalin

The binding of Artocarpus integrifolia lectin (jacalin) to 4-methylumbelliferyl (Meumb)-glycosides, GalαMeumb, GalβMeumb, GalNAcαMeumb, GalNAcβ-Meumb and Galβ3GalNAcβMeumb was examined by extrinsic fluorescence quenching titration and stopped flow spectrofluorimetry. The binding was characterized by 100% quenching of fluorescence of Meumb-glycosides. Their association constants range from 2.0 × 10⁴ to 1.58 × 10⁶ M^-1 at 15°C. Entropic contribution is the major stabilizing force for avid binding of Meumb-glycosides indicating the existence of at hydrophobic site that is complementary to their methylumbelliferyl group. The second order association rate constants for interaction of these sugars with lectin at 15°C vary from 8.8 ×10⁵ to 3.24 × 10⁶ M^-1 s^-1, at pH 7.2. The first order dissociation rate constants range from 2.30 to 43.0 s^-1 at 15°C. Despite the differences in their association rate constants, the overall values of association constants for these saccharides are determined by their dissociation rate constants. The second order rate constant for the association of Meumb-glycosides follows a pattern consistent with the magnitude of the activation energies involved therein. Activation parameters for association of all ligands illustrate that the origin of the barrier between binding of jacalin to Meumb-glycosides is entropic, and the en-thalpic contribution is small. A correlation between these parameters and the structure of the ligands on the association rates underscores the importance of steric factors in determining protein saccharide recognitions.