EFFECT OF SUBSTITUENTS ON THE THERMODYNAMICS OF D-GALACTOPYRANOSIDE BINDING TO WINGED BEAN (PSOPHOCARPUS-TETRAGONOLOBUS) BASIC LECTIN

Isothermal titration calorimetric measurements of the binding of deoxy , fluorodeoxy, and methoxy derivatives of D-galactopyranoside (alpha-D -Gal) to the basic lectin from winged bean Psophocarpus tetragonolobus , WBA I, have been carried out. Each of the ligands binding to WBA I d isplayed the same stoichiometry of one per subunit (29 kDa) of WBA I. The binding enthalpies for various derivatives are essentially indepen dent of temperature and show complementary changes with respect to bin ding entropies. Replacement of the hydroxyl group by fluorine or hydro gen on C3 and C4 of the galactopyranoside eliminates binding to the le ctin, consistent with C3-OH and C4-OH acting as hydrogen bond donors. The affinity for C2 derivatives of galactose decreases in the order Ga lNAc > 2MeOGal > 2FGal congruent to Gal > 2HGal, which suggests that b oth polar and nonpolar residues surround the C2 locus of galactose, co nsistent with the observed high affinity of WBA I toward GalNAc where the acetamido group at C2 position is probably stabilized by both nonp olar interactions with the methyl group and polar interactions with th e carbonyl group. The binding of C6 derivatives follows the order Gal > 6FGal > D-Fuc much greater than 6MeOGal congruent to L-Ara, indicati ng the presence of favourable polar interactions with a hydrogen bond donor in the vicinity. On the basis of these results the hydrogen bond donor-acceptor relationship of the complexation of methyl-alpha-D-gal actopyranoside with the primary combining site of WBA I is proposed.