We have shown recently that the notion of poking pairwise interactions along a chain provides a unifying framework for understanding the formation of both secondary and the tertiary protein structure based on symmetry and geometry. α-helices and β-sheets are found to be special geometries that have systematic poking contacts in a repetitive manner with the contacts being local along the α-helix and non-local along a pair of adjacent strands within a β-sheet. Pairwise poking interactions also govern tertiary structure formation, but they are weaker and there are no special geometrical constraints as in secondary structure formation. Here we demonstrate that protein turns, the most prevalent non-repetitive structural element in proteins, are instances of local (as in α-helices) and isolated (non-repetitive) poking pairwise contacts for which the geometrical constraints are partially relaxed. This simple and purely geometrical definition of protein turns (also sometimes known as reverse turns, β-turns, β-bends, hairpin bends, 3₁₀ bends, kinks, widgets, etc.) provides a simple framework for unifying them. We present the results of a systematic analysis and identify their structural classes as well as their respective amino acid preferences.

中文翻译：

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三．思考球状蛋白质的几何框架：蛋白质的转折

我们最近表明，沿着链戳两两相互作用的概念为理解基于对称性和几何形状的二级和三级蛋白质结构的形成提供了一个统一的框架。α-螺旋和β-折叠被发现是特殊的几何形状，以重复的方式具有系统的戳接触，接触沿着α-螺旋是局部的，而沿着β-折叠内的一对相邻链是非局部的。成对戳相互作用也控制三级结构的形成，但它们较弱，并且不存在二级结构形成中的特殊几何约束。在这里，我们证明蛋白质转角是蛋白质中最普遍的非重复结构元素，是局部（如α-螺旋）和孤立（非重复）戳成对接触的实例，其几何约束部分放松。蛋白质转角（有时也称为反向转角、β 转角、β 弯曲、发夹弯曲、3 ₁₀弯曲、扭结、小部件等）的这种简单且纯粹的几何定义为统一它们提供了一个简单的框架。我们展示了系统分析的结果，并确定了它们的结构类别以及它们各自的氨基酸偏好。