The beautiful structures of single and multi-domain proteins are clearly ordered in some fashion but cannot be readily classified using group theory methods that are successfully used to describe periodic crystals. For this reason, protein structures are considered to be aperiodic, and may have evolved this way for functional purposes, especially in instances that require a combination of softness and rigidity within the same molecule. By analyzing the solved protein structures, we show that orientational symmetry is broken in the aperiodic arrangement of the secondary structural elements (SSEs), which we deduce by calculating the nematic order parameter, P₂. We find that the folded structures are nematic droplets with a broad distribution of P₂. We argue that non-zero values of P₂, leads to an arrangement of the SSEs that can resist mechanical forces, which is a requirement for allosteric proteins. Such proteins, which resist mechanical forces in some regions while being flexible in others, transmit signals from one region of the protein to another (action at a distance) in response to binding of ligands (oxygen, ATP or other small molecules).

中文翻译：

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关于折叠蛋白中定向顺序的出现，对变构有影响

单域和多域蛋白的美丽结构以某种方式清晰地排列有序，但不能使用成功用于描述周期晶体的族理论方法进行分类。由于这个原因，蛋白质结构被认为是非周期性的，并且可能已经出于功能目的以这种方式进化，特别是在需要在同一分子内兼具柔软性和刚性的情况下。通过分析所解析的蛋白质结构，我们表明在二级结构元素（SSE）的非周期性排列中破坏了方向对称性，这是通过计算向列顺序参数P ₂得出的。我们发现折叠结构是向列液滴，具有广泛的P ₂分布。我们认为P的非零值_{如图2所示}，导致SSE的排列可以抵抗机械力，这是变构蛋白的要求。这样的蛋白质在某些区域抵抗机械力，而在其他区域具有柔韧性，响应配体（氧气，ATP或其他小分子）的结合，将信号从蛋白质的一个区域传递到另一区域（远距离作用）。