Topsy-turvy binding of negatively charged homogalacturonan oligosaccharides to galectin-3

Abstract

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 non-reducing 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.

The Use of Endo-Cellulase and Endo-Xylanase for the Extraction of Apple Pectins as Factors Modifying Their Anticancer Properties and Affecting Their Synergy with the Active Form of Irinotecan

“…Since the first observation of Nangia-Makker et al [56]-that modified citrus pectin can bind to Gal-3 and act as its competitive inhibitor, many studies have reported the direct interaction between these molecules. Binding between various types of pectins or polysaccharides with Gal-3 has been detected via nuclear magnetic resonance [57,58], surface plasmon resonance [59,60], and atomic force microscopy [61], as well as fluorescence microscopy and flow cytometry [60]. The inhibition of the Gal-3-mediated hemagglutination of erythrocytes by pectins has also been observed [29,62,63].…”

Heterologous Interactions with Galectins and Chemokines and Their Functional Consequences

“…Polysaccharides may also interact with the N-terminal tail (NT) of Gal-3, a process that is likely modulated by NT proline isomerization [ 109 ], and macromolecular assemblies of complex polysaccharides can interact with Gal-3 with synergestic effects on function [ 110 ]. In addition, Zheng et al [ 111 ] showed that negatively-charged homogalacturonan-derived oligosaccharides bind to the Gal-3 CRD canonical site in a “topsy-turvy” fashion with their reducing end residue (as opposed to the non-reducing end residues as in lactose) interacting with the conserved tryptophan. This scenario may well be how galectins bind to heparan sulfate-based glycoconjugates on the surfaces of cells, as further exemplified by the binding of keratan sulfate and related saccharides to Gal-1, -3, -7 and -9N [ 112 ].…”

Heterologous Interactions with Galectins and Chemokines and Their Functional Consequences

“…Polysaccharides may also interact with the N-terminal tail (NT) of Gal-3, a process that is likely modulated by NT proline isomerization [109], and macromolecular assemblies of complex polysaccharides can interact with Gal-3 with synergestic effects on function [110]. In addition, Zheng et al [111] showed that negativelycharged homogalacturonan-derived oligosaccharides bind to the Gal-3 CRD canonical site in a "topsy-turvy" fashion with their reducing end residue (as opposed to the non-reducing end residues as in lactose) interacting with the conserved tryptophan. This scenario may well be how galectins bind to heparan sulfate-based glycoconjugates on the surfaces of cells, as further exemplified by the binding of keratan sulfate and related saccharides to Gal-1, -3, -7 and -9N [112].…”