Energetics of carbohydrate binding to Momordica charantia (bitter gourd) lectin: an isothermal titration calorimetric study

Abstract

Physico-chemical and carbohydrate binding studies have been carried out on the Momordica charantia (bitter gourd) seed lectin (MCL). The lectin activity is maximal in the pH range 7.4-11.0, but decreases steeply below pH 7.0. The lectin activity is mostly unaffected in the temperature range 4-50 °C, but a sharp decrease is seen between 50 and 60 °C, which could be correlated to changes in the structure of the protein as seen by circular dichroism and fluorescence spectroscopy. Isothermal titration calorimetric studies show that the tetrameric MCL binds two sugar molecules and the binding constants (K_b), determined at 288.15 K, for various saccharides were found to vary between 7.3 × 10³ and 1.52 × 10⁴ M⁻¹. The binding reactions for all the saccharides investigated were essentially enthalpy driven, with the binding enthalpies (ΔH_b) at 288.15 K being in the range of −50.99 and −43.39 kJ mol⁻¹, whereas the contribution to the binding reaction from the entropy of binding was negative, with values of binding entropy (ΔS_b) ranging between −99.2 and −72.0 J mol⁻¹ K⁻¹ at 288.15 K. Changes in heat capacity (ΔC_p) for the binding of disaccharides, lactose and lactulose, were significantly larger in magnitude than those obtained for the monosaccharides, methyl-β-D-galactopyranoside, and methyl-α-D-galactopyranoside, and could be correlated reasonably well with the surface areas of these ligands. Enthalpy-entropy compensation was observed for all the sugars studied, suggesting that water structure plays an important role in the overall binding reaction. CD spectroscopy indicates that carbohydrate binding does not lead to significant changes in the secondary and tertiary structures of MCL, suggesting that the carbohydrate binding sites on this lectin are mostly preformed.