Binding of polysaccharides to human galectin-3 at a noncanonical site in its carbohydrate recognition domain

Galectin-3 (Gal-3) is a multifunctional lectin, unique to galectins by the presence of a long N-terminal tail (NT) off of its carbohydrate recognition domain (CRD). Many previous studies have investigated binding of small carbohydrates to its CRD. Here, we used nuclear magnetic resonance spectroscopy (¹⁵N-¹H heteronuclear single quantum coherence data) to assess binding of ¹⁵N-Gal-3 (and truncated ¹⁵N-Gal-3 CRD) to several, relatively large polysaccharides, including eight varieties of galactomannans (GMs), as well as a β(1 → 4)-polymannan and an α-branched mannan. Overall, we found that these polysaccharides with a larger carbohydrate footprint interact primarily with a noncanonical carbohydrate-binding site on the F-face of the Gal-3 CRD β-sandwich, and to a less extent, if at all, with the canonical carbohydrate-binding site on the S-face. While there is no evidence for interaction with the NT itself, it does appear that the NT somehow mediates stronger interactions between the Gal-3 CRD and the GMs. Significant Gal-3 resonance broadening observed during polysaccharide titrations indicates that interactions occur in the intermediate exchange regime, and analysis of these data allows estimation of affinities and stoichiometries that range from 4 × 10⁴ to 12 × 10⁴ M^-1 per site and multiple sites per polysaccharide, respectively. We also found that lactose can still bind to the CRD S-face of GM-bound Gal-3, with the binding of one ligand attenuating affinity of the other. These data are compared with previous results on Gal-1, revealing differences and similarities. They also provide research direction to the development of these polysaccharides as galectin-targeting therapeutics in the clinic.