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 180^o 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.

中文翻译：

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带负电荷的高半乳糖醛酸寡糖与半乳糖凝集素3的颠倒结合

Galectin-3 对许多生理和病理过程至关重要。普遍接受的教条是半乳凝素通过特异性结合细胞表面糖缀合物上的β(1→4)-半乳糖苷表位而在细胞外发挥作用。在这里，我们使用晶体学和 NMR 光谱来证明带负电荷的同型半乳糖醛酸（HG，α(1→4)-连接的-D-半乳糖醛酸 (GalA) 的线性多糖）与半乳糖凝集素 3 碳水化合物识别域结合。GalA 环中 C6 位置的 HG 羧酸盐要求这种糖以非常规的“颠倒”方向结合 galectin-3，该方向翻转约 180 ^o相对于经典的 β-半乳糖苷乳糖。在这种结合模式中，HG 的还原端 GalA β-异头物占据乳糖中非还原端半乳糖残基的位置。这种新的方向保持与保守的色氨酸和七个最关键的乳糖结合残基的相互作用，尽管具有不同的氢键相互作用。然而，HG 分子取向和新的相互作用具有与乳糖基本相同的热力学结合参数。总体而言，我们的研究提供了新型半乳凝素-糖相互作用的结构细节，该相互作用拓宽了配体与 Gal-3 结合的糖空间，并表明凝集素如何识别细胞表面上的其他带负电荷的多糖，如糖胺聚糖（例如硫酸乙酰肝素）。