Thermodynamic analysis of saccharide binding to snake gourd (Trichosanthes anguina) seed lectin. Fluorescence and absorption spectroscopic studies.

The interaction of different saccharides with the snake gourd (Trichosanthes anguina) seed lectin (SGSL) was investigated by fluorescence spectroscopy. Binding of 4-methylumbelliferyl-beta-D-galactopyranoside (MeUmb beta Gal) 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 degrees C to 80.2% at 40 degrees C. At 20 degrees C the association constant, K(a), for the MeUmb beta Gal-SGSL interaction was found by fluorescence titration to be 5.8 x 10(4) M(-1). From the temperature dependence of the association constants, the changes in enthalpy (Delta H) and entropy (Delta S) associated with binding of MeUmb beta Gal 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 MeUmb beta Gal. The Ka values for different sugars were determined at several temperatures, and Delta H and Delta S 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.