Thermodynamic analysis of saccharide binding to snake gourd (Trichosanthes anguina) seed lectin

Abstract

The interaction of different saccharides with the snake gourd (Trichosanthes anguina) seed lectin (SGSL) was investigated by fluorescence spectroscopy. Binding of 4-methylumbelliferyl-β-d-galactopyranoside (MeUmbβGal) to SGSL resulted in a significant increase in the fluorescence emission intensity of the sugar at 376 nm, and this change was used to estimate the association constants for the binding interaction. Interestingly, the increase in emission intensity changed with a change in temperature, increasing from 19.2% at 20 °C to 80.2% at 40 °C. At 20 °C the association constant, Ka, for the MeUmbβGal–SGSL interaction was found by fluorescence titration to be 5.8 × 10⁴ m⁻¹. From the temperature dependence of the association constants, the changes in enthalpy (ΔH) and entropy (ΔS) associated with binding of MeUmbβGal to SGSL were estimated to be −80.85 kJ·mol⁻¹ and −184.0 J·mol⁻¹·K⁻¹, respectively. Binding of unlabeled sugars was investigated by monitoring the decrease in fluorescence intensity when they were added to a mixture of SGSL and MeUmbβGal. The K_a values for different sugars were determined at several temperatures, and ΔH and ΔS were determined from the van’t Hoff plots. Enthalpy–entropy compensation was noticed in all cases. The results indicate that saccharide binding to SGSL is enthalpy-driven and the negative contribution from entropy is, in general, quite high.