On the meaning of affinity: Cluster glycoside effects and concanavalin A
On the meaning of affinity: Cluster glycoside effects and concanavalin A
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DOI：
10.1021/ja991729e
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发表时间：
1999-11-10
影响因子：
15
通讯作者：
Toone, EJ
中科院分区：
文献类型：
--
作者：
Dimick, SM;Powell, SC;Toone, EJ
蛋白质-碳水化合物相互作用的抑制为治疗无数人类疾病提供了强有力的治疗策略。到目前为止，这些方法的应用已经受到碳水化合物配体对其蛋白质受体的固有低亲和力的阻碍。由于凝集素通常存在于多聚体组装体中，因此已经制备了多种多价糖配体以寻求高亲和力。簇糖苷效应，或来自寡糖配体中多价性的高亲和力的观察，显然代表了克服“弱结合”问题的最佳策略。在这里，我们报告了一系列多价树突状的合成和生物物理评价其与植物凝集素伴刀豆球蛋白A的相互作用。虽然通过凝集试验观察到在经价态校正的基础上亲和力增强30倍，但用一系列多价配体对可溶性蛋白质进行量热滴定显示结合自由能没有增强。相反，来自凝集测量的IC 50值与结合熵很好地相关。这一观察结果表明，血凝测量的是一种不同于结合的现象，其典型特征是大的有利熵和不利焓：该过程几乎可以肯定是聚集。支持这一论断，我们报告的多价配体交联伴刀豆球蛋白A二聚体的晶体结构。据我们所知，这些结构是首次报道的。我们的研究结果表明，血凝试验评价的能力，配体抑制交联晶格的形成，一个过程只与可逆的配体结合相切。因此，必须谨慎看待凝集试验中观察到的簇糖苷效应。这种效果可能与治疗上有用的药物的设计有关，也可能无关。
The inhibition of protein-carbohydrate interaction provides a powerful therapeutic strategy for the treatment of myriad human diseases. To date, application of such approaches have been frustrated by the inherent low affinity of carbohydrate ligands for their protein receptors. Because lectins typically exist in multimeric assemblies, a variety of polyvalent saccharide ligands have been prepared in the search for high affinity. The cluster glycoside effect, or the observation of high affinity derived from multivalency in oligosaccharide Ligands, apparently represents the best strategy for overcoming the "weak binding" problem. Here we report the synthesis of a series of multivalent dendritic saccharides and a biophysical evaluation of their interaction with the plant lectin concanavalin A. Although a 30-fold enhancement in affinity on a valence-corrected basis is observed by agglutination assay, calorimetric titration of soluble protein with a range of multivalent ligands reveals no enhancement in binding free energies. Rather, IC50 values from agglutination measurements correlate well with entropies of binding. This observation suggests that hemagglutination measures a phenomenon distinct from binding that is typified by a large favorable entropy and an unfavorable enthalpy: this process is almost certainly aggregation. Supporting this assertion, we report crystal structures of multivalent ligands cross-linking concanavalin A dimers. To the best of our knowledge, these structures are the first reported of their kind. Our results indicate that hemagglutination assays evaluate the ability of ligands to inhibit the formation of cross-linked lattices, a process only tangentially related to reversible ligand binding. Cluster glycoside effects observed in agglutination assays must, therefore, be viewed with caution. Such effects may or may not be relevant to the design of therapeutically useful saccharides.