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The concept of the sugar code ascribes functionality to glycan epitopes via protein (lectin)-carbohydrate interactions (1). Galectins, a family of endogenous lectins, are involved in tumor progression and metastasis, making them promising targets for rational drug design. Toward this aim analysis of the bound-state conformation of a complex natural ligand in solution will provide essential input. As alternative to the use of - often prohibitively expensive - isotope-labeled oligosaccharides we have successfully established a new strategy combining three different NMR methods and computer-assisted modeling. Using the interaction of galectin-1 and the pentasaccharide chain of ganglioside GM1 as example, we first determined spatial vicinity of a galactose unit to Trp68 in the lectin’s binding site by laser photo CIDNP spectra. Despite signal overlap trNOE monitoring delineated conformational details, i.e. the Φ, Ψ-angle combination of the Neu5Acα2-3Gal linkage. It represents exclusively one of the three low-energy conformations of the free state and a notable difference to the conformer selection by cholera toxin. Docking analysis with this input came up with only one conformation of the complete chain accomodated in the binding site. MD simulations confirmed expected restriction of ligand flexibility. The reliability of the model, which yielded a detailed interaction energy distribution for each residue showing remarkable energy contributions by GalNAc and Neu5Ac residues in addition to Gal, was challenged by information of STD spectra. The unambigious assignment of two signals predicted vicinity for the Galβ1-3GalNAc part of the ligand to the binding site. Indeed, the structural model from our knowledge-based docking was in accord with these data. Thus, we present the first complete analysis of a galectin-bound conformation of a complex natural ligand in solution revealing the unique conformer selection. The developed strategy is suggested to be of general relevance. (1) Gabius et al., Biochim. Biophys. Acta 1572, 165-177 (2002)

[Proc Amer Assoc Cancer Res, Volume 45, 2004]