Thermodynamics of binding between saccharides and Helix pomatia A hemagglutinin

Binding constants for the interactions between Helix pomatia A hemagglutinin and the following saccharides were estimated at pH 7.0 and 25 degree C, using the circular dichroism method: N-acetyl-D-galactosamine, 5700 M-1; N-acetyl-D-glucosamine, 1000 M-1; melibiose, 86 M-1; raffinose, 350 M-1. The binding of N-acetyl-D-galactosamine to Helix pomatia A hemagglutinin was investigated in detail, using the circular dichroism and fluorescence methods, over the temperature range 5-50 degree C and pH range 7.0-2.5. The thermodynamic parameters, delta H degree (kcal . mol-1), delta G degree (kcal . mol-1), and delta S degree (e.u.), for this binding reaction at 25 degrees C were estimated as follows: -11.3, -5.24, -20.3 at pH 7.0; -9.1, -5.71, -11.2 at pH 4.5; -38.3, -3.19, -118 at pH 2.5. The negative values of delta H degrees and delta S degrees at pH 2.5 were especially large. This may be related to the restoration of the Helix pomatia A hemagglutinin molecule or its binding site from an unstable configuration caused by lowering the pH of the reaction medium to the original configuration of the presence of N-acetyl-D-galactosamine.