OBJECTIVE The αIIbβ3 antagonist antiplatelet drug abciximab is the chimeric antigen-binding fragment comprising the variable regions of murine monoclonal antibody 7E3 and the constant domains of human IgG1 and light chain κ. Previous mutagenesis studies suggested that abciximab binds to the β3 C177-C184 specificity-determining loop (SDL) and Trp129 on the adjacent β1-α1 helix. These studies could not, however, assess whether 7E3 or abciximab prevents fibrinogen binding by steric interference, disruption of either the αIIbβ3-binding pocket for fibrinogen or the β3 SDL (which is not part of the binding pocket but affects fibrinogen binding), or some combination of these effects. To address this gap, we used cryo-electron microscopy to determine the structure of the αIIbβ3-abciximab complex at 2.8 Å resolution. Approach and Results: The interacting surface of abciximab is comprised of residues from all 3 complementarity-determining regions of both the light and heavy chains, with high representation of aromatic residues. Binding is primarily to the β3 SDL and neighboring residues, the β1-α1 helix, and β3 residues Ser211, Val212 and Met335. Unexpectedly, the structure also indicated several interactions with αIIb. As judged by the cryo-electron microscopy model, molecular-dynamics simulations, and mutagenesis, the binding of abciximab does not appear to rely on the interaction with the αIIb residues and does not result in disruption of the fibrinogen-binding pocket; it does, however, compress and reduce the flexibility of the SDL. CONCLUSIONS We deduce that abciximab prevents ligand binding by steric interference, with a potential contribution via displacement of the SDL and limitation of the flexibility of the SDL residues.

中文翻译：

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αIIbβ3-Abciximab 复合物的冷冻电子显微镜结构。


目的αIIbβ3拮抗剂抗血小板药物阿昔单抗是包含鼠单克隆抗体7E3可变区和人IgG1和轻链κ恒定区的嵌合抗原结合片段。先前的诱变研究表明，abciximab 与 β3 C177-C184 特异性决定环 (SDL) 和相邻 β1-α1 螺旋上的 Trp129 结合。然而，这些研究无法评估 7E3 或 abciximab 是否通过空间干扰、破坏纤维蛋白原的 αIIbβ3 结合袋或 β3 SDL（不是结合袋的一部分，但影响纤维蛋白原结合）或某些因素来阻止纤维蛋白原结合。这些效果的组合。为了解决这一差距，我们使用冷冻电子显微镜以 2.8 Å 的分辨率确定了 αIIbβ3-abciximab 复合物的结构。方法和结果：阿昔单抗的相互作用表面由轻链和重链所有 3 个互补决定区的残基组成，其中芳香族残基具有较高的代表性。主要与 β3 SDL 和邻近残基、β1-α1 螺旋以及 β3 残基 Ser211、Val212 和 Met335 结合。出乎意料的是，该结构还表明与 αIIb 存在多种相互作用。根据冷冻电子显微镜模型、分子动力学模拟和诱变判断，阿昔单抗的结合似乎并不依赖于与 αIIb 残基的相互作用，并且不会导致纤维蛋白原结合袋的破坏；然而，它确实压缩并降低了 SDL 的灵活性。结论 我们推断阿昔单抗通过空间干扰阻止配体结合，可能通过 SDL 的置换和 SDL 残基灵活性的限制而发挥作用。