Thermodynamics of wheat germ agglutinin-sialyloligosaccharide interactions by proton nuclear magnetic resonance

The thermodynamic parameters that characterize the binding of wheat germ agglutinin isolectin I (WGA I) to the alpha 2-3 isomer of (N-acetylneuraminyl)lactose have been determined by 360-MHz proton nuclear magnetic resonance spectroscopy. The chemical exchange of the ligand between the free and bound sites resulted in a broadening and upfield shifting of the N-acetyl methyl resonance [Kronis, K.A., & Carver, J.P. (1985) Biochemistry (preceding paper in this issue)] which has allowed the determination of the equilibrium constant, KD, and the dissociation rate constant, kD. In this paper, the analysis of the temperature dependence of the KD values between 25.4 and 51.6 degrees C yielded equilibrium parameters indicative of a large entropy barrier to binding: delta H degree = -13.3 +/- 1.0 kcal mol-1 and delta S degree = -31.9 +/- 2.4 cal mol-1 K-1. The Arrhenius plot of the effect of temperature on the dissociation rate (kD) and the plot of 1n (kD/T) vs. 1/T indicated that the transition complex represented an unfavorable energy state compared to the dissociated molecules with an activation energy (EA) of +18.0 kcal mol-1 and enthalpy and entropy of dissociation (delta HD not equal to and delta SD not equal to) values of +17.4 +/- 0.3 kcal mol-1 and +13.4 +/- 1.2 cal mol-1 K-1, respectively. The driving force for this binding reaction is the large negative delta H degree with a small enthalpic barrier to association (delta HA = +4.1 kcal mol-1).(ABSTRACT TRUNCATED AT 250 WORDS)