Saccharide binding to three Gal/GalNAc specific lectins: fluorescence, spectroscopic and stopped-flow kinetic studies

Abstract

Fluorescence and stopped-flow spectrophotometric studies on three plant lectins fromPsophocarpus tetragonolobus (winged bean),Glycine max (soybean) andArtocarpus integrifolia (jack fruit) have been studied usingN-dansylgalactosamine as a fluorescent ligand. The best monosaccharide for the winged bean agglutinin I (WBA I) and soybean (SBA) is Me-αGalNAc and for jack fruit agglutinin (JFA) is Me-αGal. Examination of the percentage enhancement and association constants (1.51×10⁶, 6.56×10⁶ and 4.17×10⁵ M⁻¹ for SBA, WBA I and JFA, respectively) suggests that the combining regions of the lectins SBA and WBA I are apolar whereas that of JFA is polar. Thermodynamic parameters obtained for the binding of several monosaccharides to these lectins are enthalpically favourable. The binding of monosaccharides to these lectins suggests that the-OH groups at C-1, C-2, C-4 and C-6 in thed-galactose configuration are important loci for interaction with these lectins. An important finding is that the JFA binds specifically to Galβ1-3GaINAc with much higher affinity than the other disaccharides which are structurally and topographically similar. The results of stopped-flow spectrometry on the binding of N-dansylgalactosamine to these lectins are consistent with a bimolecular single step mechanism. The association rate constants (2.4×10⁵, 1.3×10⁴, and 11.7×10⁵ M⁻¹ sec⁻¹ for SBA, WBA I and JFA, respectively) obtained are several orders of magnitude slower than the ones expected for diffusion controlled reactions. The dissociation rate constants (0.2, 3.2×10⁻², 83.3 sec⁻¹ for SBA, WBA I and JFA, respectively) obtained for the dissociation of N-dansylgalactosamine from its lectin complex are slowest for SBA and WBA I when compared with any other lectin-ligand dissociation process.