6KXB image
Entry Detail
PDB ID:
6KXB
Title:
Galectin-3 CRD binds to GalA trimer
Biological Source:
Source Organism:
Host Organism:
PDB Version:
Deposition Date:
2019-09-10
Release Date:
2020-08-26
Method Details:
Experimental Method:
Resolution:
1.50 Å
R-Value Free:
0.20
R-Value Work:
0.16
R-Value Observed:
0.16
Space Group:
P 21 21 21
Macromolecular Entities
Polymer Type:polypeptide(L)
Description:Galectin-3
Chain IDs:A
Chain Length:138
Number of Molecules:1
Biological Source:Homo sapiens
Primary Citation
Topsy-turvy binding of negatively charged homogalacturonan oligosaccharides to galectin-3.
Glycobiology 31 341 350 (2021)
PMID: 32909036 DOI: 10.1093/glycob/cwaa080

Abstact

Galectin-3 is crucial to many physiological and pathological processes. The generally accepted dogma is that galectins function extracellularly by binding specifically to β(1→4)-galactoside epitopes on cell surface glycoconjugates. Here, we used crystallography and NMR spectroscopy to demonstrate that negatively charged homogalacturonans (HG, linear polysaccharides of α(1→4)-linked-D-galacturonate (GalA)) bind to the galectin-3 carbohydrate recognition domain. The HG carboxylates at the C6 positions in GalA rings mandate that this saccharide bind galectin-3 in an unconventional, "topsy-turvy" orientation that is flipped by about 180o relative to that of the canonical β-galactoside lactose. In this binding mode, the reducing end GalA β-anomer of HGs takes the position of the nonreducing end galactose residue in lactose. This novel orientation maintains interactions with the conserved tryptophan and seven of the most crucial lactose-binding residues, albeit with different H-bonding interactions. Nevertheless, the HG molecular orientation and new interactions have essentially the same thermodynamic binding parameters as lactose. Overall, our study provides structural details for a new type of galectin-sugar interaction that broadens glycospace for ligand binding to Gal-3 and suggests how the lectin may recognize other negatively charged polysaccharides like glycoaminoglycans (e.g. heparan sulfate) on the cell surface. This discovery impacts on our understanding of galectin-mediated biological function.

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