Structural aspects of binding of -linked digalactosides to human galectin-1

By definition, adhesion/growth-regulatory galectins are known for their ability to bind -galactosides such as Gal (1 → 4)Glc (lactose). Indications for affinity of human galectin-1 to -linked digalactosides pose questions on the interaction profile with such bound ligands and selection of the galactose moiety for CH-π stacking. These issues are resolved by a combination of 15N-1H heteronuclear single quantum coherence (HSQC) chemical shift and saturation transfer difference nuclear magnetic resonance (STD NMR) epitope mappings with docking analysis, using the (1 → 3/4)-linked digalactosides and also Gal (1 → 6)Glc (melibiose) as test compounds. The experimental part revealed interaction with the canonical lectin site, and this preferentially via the non-reducing-end galactose moiety. Low-energy conformers appear to be selected without notable distortion, as shown by molecular dynamics simulations. With the (1 → 4) disaccharide, however, the typical CH-π interaction is significantly diminished, yet binding appears to be partially compensated for by hydrogen bonding. Overall, these findings reveal that the type of -linkage in digalactosides has an impact on maintaining CH-π interactions and the pattern of hydrogen bonding, explaining preference for the (1 → 3) linkage. Thus, this lectin is able to accommodate both - and -linked galactosides at the same site, with major contacts to the non-reducing-end sugar unit.