The thermodynamics of carbohydrate binding to the 14 kDa dimeric β- galactoside-binding lectin galectin-1 (Gal-1) from Chinese hamster ovary cells and four galactose-specific plant lectins were investigated by isothermal titration microcalorimetry. Recombinant Gal-1 from Escherichia coli, a Cys→Ser mutant with enhanced stability (C2S-Gal-1), and a monomeric mutant of the lectin (N-Gal-1) were studied along with the soybean agglutinin and the lectins from Erythrina indica, Erythrina crystagalli, and Erythrina corollodendrum. Although the pattern of association constants of the Erythrina lectins was similar for mono- and disaccharides, variations exist in their enthalpy of binding (-ΔH) values for individual carbohydrates. While the Erythrina lectins show greater affinities and -ΔH values for lactose and N-acetyllactosamine, the soybean agglutinin possesses similar affinities for methyl β-galactopyranoside, lactose, and N-acetyllactosamine and a greater -ΔH value for the monosaccharide. Gal-1 and the plant lectins possess essentially the same affinities for N-acetyllactosamine; however, the animal lectin shows a lower -ΔH value and more favorable binding entropy for the disaccharide. While Gal-1, C2S-Gal-1, and N-Gal-1 all possess essentially the same affinities for N-acetyllactosamine, the two mutants possess much lower -ΔH values, even though the mutation site(s) are far removed from the carbohydrate binding site. These results indicate that there are different energetic mechanisms of carbohydrate binding between galectin-1, its two mutants, and the Gal-specific plant lectins.
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