Unique Conformer Selection of Human Growth-Regulatory Lectin Galectin-1 for Ganglioside GM₁ versus Bacterial Toxins

Endogenous lectins induce effects on cell growth by binding to antennae of natural glycoconjugates. These complex carbohydrates often present more than one potential lectin-binding site in a single chain. Using the growth-regulatory interaction of the pentasaccharide of ganglioside GM ₁ with homodimeric galectin-1 on neuroblastoma cell surfaces as a model, we present a suitable strategy for addressing this issue. The approach combines NMR spectroscopic and computational methods and does not require isotope-labeled glycans. It involves conformational analysis of the two building blocks of the GM₁ glycan, i.e., the disaccharide Galβ1-3GalNAc and the trisaccharide Neu5Acα2-3Galβ1-4Glc. Their bound-state conformations were determined by transferred nuclear Overhauser enhancement spectroscopy. Next, measurements on the lectin-pentasaccharide complex revealed differential conformer selection regarding the sialylgalactose linkage in the tri- versus pentasaccharide (Φ and Ψ value of -70° and 15° vs 70° and 15°, respectively). To proceed in the structural analysis, the characteristic experimentally detected spatial vicinity of a galactose unit and Trp68 in the galectin's binding site offered a means, exploiting saturation transfer from protein to carbohydrate protons. Indeed, we detected two signals unambiguously assigned to the terminal Gal and the GalNAc residues. Computational docking and interaction energy analyses of the entire set of ligands supported and added to experimental results. The finding that the ganglioside's carbohydrate chain is subject to differential conformer selection at the sialylgalactose linkage by galectin-1 and GM ₁-binding cholera toxin (Φ and Ψ values of -172° and -26°, respectively) is relevant for toxin-directed drug design. In principle, our methodology can be applied in studies aimed at blocking galectin functionality in malignancy and beyond glycosciences.