Binding of Disaccharides by Peanut Agglutinin as Studied by Ultraviolet Difference Spectroscopy

Abstract

The binding of the disaccharides methyl β-d-lactoside and 2-acetamido-2-deoxy-3-O-(β-d-galactopyranosyl)- β-dD-galactopyranose [β-d-Gal-(l→3)-d-GalNAc] to peanut agglutinin was studied by ultraviolet difference spectroscopy. The magnitude of the difference spectra varied with the concentration of the carbohydrates; association constants and thermodynamic parameters were determined from titration experiments at different temperatures. The enthalpy and entropy changes for binding of methyl β-d-lactoside were found to be ΔH°= -65 ± 4 kJ mol⁻¹, ΔSdeg; =−156 ± 14 J mol⁻¹ K⁻¹. For β-dD-Gal- (1→3)-d-Gal NAc the observed thermodynamic parameters were ΔH°=−78 ± kJ mol⁻¹, ΔS°=−177 ± 16 J mol⁻¹ K⁻¹. For both disaccharides, the enthalpy change upon binding to the lectin is much larger than that found for the binding of methyl α- and β-d-galactopyranosides, strongly suggesting the existence of an extended binding site on peanut agglutinin. The observed parameters are compared with those found for the binding of monosaccharides and oligosaccharides to other lectins and to lysozymc. Molecular models of the minimum energy conformers of β-GaI-(1→3)-d-GalNAc and methyl β-d-Iactoside are used to interpret the interaction of these, and structurally related ligands, with the peanut agglutinin binding site.