Thermodynamic analysis of ligand binding to winged bean (Psophocarpus tetragonolobus) acidic agglutinin reveals its specificity for terminally monofucosylated H-reactive sugars

The sugar-specific binding of N-dansylgalactosamine to $WBA II (n = 2; Ka = 5.6 \times 10^3 M^{-1}; \Delta H = -21 kJ.mol^{-1}; \Delta S = -21.3 J.mol^{-1}.K^{-1})$ was utilized in substitution titrations for evaluating the association constants for the interaction of sugars with the lectin. An axial hydroxyl at C-4 and equatorial hydroxyls at C-3 and C-6 as in D-galacto configuration are crucial for binding. Both axial and equatorial hydroxyls are tolerated at C-2. Conformationally akin disaccharides such as lactose, N-acetyllactosamine, Gal $\beta$ 1-3GlcNAc, and Gal $\beta$ 1-3 GalNAc show similar affinities. $2^\prime$-Fucosyllactose and H-disaccharide display 146 and 13 times stronger affinity over lactose and galactose, yet fucose by itself is devoid of activity. An interesting feature, noted for the first time, in protein-sugar interactions is the positive entropy change for the binding of $2^\prime$-fucosyllactose, suggesting that nonpolar interactions play an important role in stabilization of the lectin-sugar complex. 3-Fucosyllactose, lactodifucotetraose, lacto-N-fucopentaose II and III are inactive, whereas lacto-N-fucopentaose I has 14-fold lower affinity as compared with $2^\prime$-fucosyllactose. Conformational analysis indicates that the substitution at subterminal glucose or GlcNAc by L-fucose in either $\alpha$ 1-3 or $\alpha$ 1-4 linkage leads to its projection so as to sterically hinder the access of $3^\prime$-fucosyllactose, lactodifucotetraose, and lacto-N-fucopentaose II and III to the binding site of winged bean agglutinin II. Similarly the projection of $\alpha$ 1-3 linked Gal/GalNAc also leads to steric hindrance and hence prevents the binding of blood group A and B reactive sugars. Considering its unique specificity winged bean agglutinin II should be useful in the isolation and characterization of terminally monofucosylated H-reactive oligosaccharides from those that are difucosylated or internally fucosylated