To overcome limited germline combinatorial diversity, bovines have evolved a subset of antibodies with ultra-long CDRH3 regions that harbour cysteine-rich knob domains. To produce affinity-maturated peptides, we previously isolated autonomous 3-6 kDa knob domains from bovine antibodies. Here, we show that binding of four knob domain peptides elicits a range of effects on the clinically validated drug target complement C5. Allosteric mechanisms predominated, with one peptide selectively inhibiting C5 cleavage by the alternative pathway C5 convertase, revealing a targetable mechanistic difference between the classical and alternative pathway C5 convertases. Taking a hybrid biophysical approach, we present C5-knob domain co-crystal structures and, by solution methods, observed allosteric effects propagating >50 Å from the binding sites. This study expands the therapeutic scope of C5, presents new inhibitors and introduces knob domains as new, low molecular weight antibody fragments, with therapeutic potential.

中文翻译：

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球形结构域肽对补体C5的变构调节

为了克服有限的种系组合多样性，牛进化出了具有超长CDRH3区域的抗体子集，该区域带有富含半胱氨酸的旋钮结构域。为了产生亲和力成熟的肽，我们先前从牛抗体中分离出了自主的3-6 kDa旋钮域。在这里，我们表明四个旋钮域肽的绑定引起对临床验证的药物靶标补体C5的一系列影响。变构机制占主导地位，一种肽选择性抑制旁路途径C5转化酶裂解C5，揭示了经典途径和旁路途径C5转化酶之间的可靶向机制差异。采用一种混合生物物理方法，我们提出了C5-旋钮结构域共晶体结构，并通过溶液方法观察到了从结合位点传播> 50Å的变构效应。