Thermodynamic analysis of saccharide binding to snake gourd (Trichosanthesanguina) seed lectin - Fluorescence and absorption spectroscopic studies

The interaction of different saccharides with the snake gourd (Trichosanthe s 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 i ntensity of the sugar at 376 nm, and this change was used to estimate the a ssociation constants for the binding interaction. Interestingly, the increa se in emission intensity changed with a change in temperature, increasing f rom 19.2% at 20 degreesC to 80.2% at 40 degreesC, At 20 degreesC the associ ation constant, K-a, for the MeUmb beta Gal-SGSL interaction was found by f luorescence titration to be 5.8 x 10(4) M-1. From the temperature dependenc e of the association constants, the changes in enthalpy (DeltaH) and entrop y (DeltaS) 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 fluorescen ce intensity when they were added to a mixture of SGSL and MeUmb beta Gal. The K-a values for different sugars were determined at several temperatures , and DeltaH and DeltaS were determined from the van't Hoff plots. Enthalpy -entropy compensation was noticed in all cases. The results indicate that s accharide binding to SGSL is enthalpy-driven and the negative contribution from entropy is, in general, quite high.