Peptide agonists acting on the glucagon-like peptide 1 receptor (GLP-1R) promote glucose-dependent insulin release and therefore represent important therapeutic agents for type 2 diabetes (T2D). Previous data indicated that an N-terminal type II β-turn motif might be an important feature for agonists acting on the GLP-1R. In contrast, recent publications reporting the structure of the full-length GLP-1R have shown the N-terminus of receptor-bound agonists in an α-helical conformation. To reconcile these conflicting results, we prepared N-terminally constrained analogues of glucagon-like peptide 1 (GLP-1) and exendin-4 and evaluated their receptor affinity and functionality in vitro; we then examined their crystal structures in complex with the extracellular domain of the GLP-1R and used molecular modeling and molecular dynamics simulations for further investigations. We report that the peptides’ N-termini in all determined crystal structures adopted a type II β-turn conformation, but in vitro potency varied several thousand-fold across the series. Potency correlated better with α-helicity in our computational model, although we have found that the energy barrier between the two mentioned conformations is low in our most potent analogues and the flexibility of the N-terminus is highlighted by the dynamics simulations.

中文翻译：

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α-螺旋或β-转弯？胰高血糖素样肽1（GLP-1）和Exendin-4的N末端约束类似物的研究。

作用于胰高血糖素样肽1受体（GLP-1R）的肽激动剂可促进葡萄糖依赖性胰岛素的释放，因此代表了2型糖尿病（T2D）的重要治疗剂。先前的数据表明，对于作用于GLP-1R的激动剂，N端II型β-转角基序可能是一个重要特征。相反，报道全长GLP-1R的结构的最新出版物显示了受体结合激动剂的N-末端呈α-螺旋构象。为了调和这些矛盾的结果，我们制备了胰高血糖素样肽1（GLP-1）和exendin-4的N末端受限类似物，并在体外评估了它们的受体亲和力和功能; 然后，我们检查了其晶体结构与GLP-1R胞外结构域的复合情况，并使用分子模型和分子动力学模拟进行了进一步的研究。我们报告说，在所有确定的晶体结构中，肽的N末端均采用II型β-转角构象，但在整个系列中，体外效价变化了数千倍。在我们的计算模型中，效价与α螺旋度的相关性更好，尽管我们已经发现，在我们最有效的类似物中，上述两个构象之间的能垒很低，并且动力学模拟突出了N末端的灵活性。