Abstract
Galectins are involved in many biological processes, generally functioning by interacting with various cell surface glycoconjugates, targeting β-galactoside epitopes. The β-galactoside binding feature is attributed to the evolutionally conserved carbohydrate recognition domain (CRD) where the glycan binding takes place. Currently, 16 galectins have been found in mammals out of which 12 are expressed in humans. Human galectins can be divided into three subgroups according to their CRD distribution: the prototype galectins, which contain one CRD (galectins 1, 2, 7, 10, 13, 14, and 16), the tandem-repeat galectins which consist of two CRDs (galectins 4, 8, 9 and 12) and the chimera-type galectins (galectin-3). The only member of chimeric galectins has a single CRD at the C-terminus and a short non-lectin peptide motif at the N-terminus. Its CRD consists of 130-140 residues which fold into two antiparallel β-sheets of 6 (S1-S6) and 5 β-strands (F1- F5), respectively. Galectins have a role ...
Galectins are involved in many biological processes, generally functioning by interacting with various cell surface glycoconjugates, targeting β-galactoside epitopes. The β-galactoside binding feature is attributed to the evolutionally conserved carbohydrate recognition domain (CRD) where the glycan binding takes place. Currently, 16 galectins have been found in mammals out of which 12 are expressed in humans. Human galectins can be divided into three subgroups according to their CRD distribution: the prototype galectins, which contain one CRD (galectins 1, 2, 7, 10, 13, 14, and 16), the tandem-repeat galectins which consist of two CRDs (galectins 4, 8, 9 and 12) and the chimera-type galectins (galectin-3). The only member of chimeric galectins has a single CRD at the C-terminus and a short non-lectin peptide motif at the N-terminus. Its CRD consists of 130-140 residues which fold into two antiparallel β-sheets of 6 (S1-S6) and 5 β-strands (F1- F5), respectively. Galectins have a role in many pathophysiological processes such as cardiovascular diseases, cancer development and progression, autoimmune diseases, and metabolic disorders. The unique chimera-type galectin-3 is one of the most well-studied members of this family. Galectin-3 is involved in a plethora of diseases, with emphasis on those associated with chronic inflammation, and it is currently a validated pharmaceutical target. Pharmaceutical industry is employing various strategies, for the design and development of potent synthetic small molecule antagonists or large biologics from natural sources to target galectin-3 for therapeutic intervention. The most potent antagonists thus far, are thiodigalactoside derivatives carrying aryl-triazolyl moieties attached to C-3 of both galactose units with affinities for galectin-3 in the low nanomolar range. One compound of this class, GB0139, is currently in phase 2b clinical trials against lung fibrosis (NCT03832946). Acknowledging the potential of galectin-3 for the development of potent drugs, we studied the binding of 46 compounds to galectin-3 by X-ray crystallography and ITC. We studied two groups of compounds. The first group were thiodisaccharides, where the two sugar rings are linked via sulfur by 1→2 bonds instead of 1→1 which was present in the thiodigalactosides studied thus far. The second group were compounds that had a β-D-galactopyranoside core decorated with various substituents such as phenyl, pyrimidine, triazole, tetrazole and carbonyl groups. Due to the high conservation of galectins’ CRD their binding mode was also studied in galectins 7, 8N and 10. The best antagonist of the first group displayed a KD value of 8 μM for galectin-3. Analysis of the thermodynamic binding parameters revealed that the binding Gibbs free energy (ΔG) of the compounds in the first group was dominated by enthalpy (ΔH). The structural mode of binding of 1,2-thiodisaccharides to galectin-3 was studied by X-ray crystallography that uncovered the unique role of water-mediated hydrogen bonds in conferring enthalpy-driven affinity enhancement for the new antagonists. This 1,2-thiodisaccharide-type scaffold represents a new lead for galectin-3 inhibitor discovery and offers several possibilities for further development. From 41 compounds in the second group, 14 antagonists were found to be more selective for galectin-3, over galectins 7, 8N, and 10. These antagonists can serve as lead compounds for the development of selective galectin-3 compounds, providing new perspectives for the therapeutic intervention of various diseases. In parallel, we determined the structural mode of binding of a potent antagonist (KD = 2.4 μM) of galectin-7 to highlight the structural elements that confer selectivity for this galectin.
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