Carbohydrate binding specificity of the T_n-antigen binding lectin from Vicia villosa seeds (VVLB₄)

Abstract

2-Dansylamino-2-deoxy-d-galactose (GalNDns) is a useful fluorescent probe to study the interaction of non-fluorescent sugars with the B₄ lectin from Vicia villosa seed; (VVLB₄). Binding of the lectin to GalNDns leads to a 5.2-fold increase in Dansyl fluorescence with a concomitant 10 nm blue shift in its emission maximum. The strong binding of GalNDns (K_u=7.33×10⁴ M⁻ at 20°C) is due to a favourable entropic contribution to the association process. Among the other sugars studied, GalNAcαl-O-Ser followed by MeαGalNAc are the best ligands. 2-Deoxygalactose, galactosamine and galactose are 2013, 469 and 130 times weaker ligands, respectively, as compared to GalNAc, whereas GalNDns is about 2.44 times more potent than GalNAc, indicating that substitutions at the C-2 position of GalNAc have a considerable influence on the binding affinities. Equatorial orientation of the hydroxyl group at C-3 and axial orientation at C-4 as in galactose are important for the interaction with VVLB₄. The C-6 hydroxyl group is not indispensable. The binding site of the lectin is directed exclusively towards monosaccharides alone. Interestingly enough, despite its preference for MeαGalNAc over MeβGalNAc, in oligosaccharides, the lectin prefers terminal β-linked GalNAc as compared to the α-linked one.