Milner-White E J
Department of Biochemistry, University of Glasgow, U.K.
J Mol Biol. 1988 Feb 5;199(3):503-11. doi: 10.1016/0022-2836(88)90621-3.
A common feature of alpha-helices in proteins is a loop at the C-terminal end, with a characteristic hydrogen bond pattern. It is noted that several loops with the same structural features occur independently of alpha-helices; two are even situated at the loop ends of beta-hairpins. The name paperclip is suggested for loops possessing the appropriate hydrogen bonds. A number of features of paperclips are described: they exist in two classes, depending on the number of residues at the loop end; one class is very much commoner than the other. Two paperclips are found that belong to the common class, except that the main-chain conformation of each is the mirror image of that normally found. The majority of paperclips are shown to have tightly clustered sets of main-chain dihedral angles. These are somewhat similar to, but distinct from, a subgroup of another common family of loops that have been called beta-bulge loops; in the latter, the dihedral angles are also tightly clustered. The high degree of clustering in both cases is likely to be a result of steric constraints associated with hydrogen bond patterns at the ends of loops.
蛋白质中α-螺旋的一个常见特征是在C末端有一个环，具有特定的氢键模式。值得注意的是，几个具有相同结构特征的环独立于α-螺旋出现；其中两个甚至位于β-发夹的环末端。对于具有适当氢键的环，建议使用“回形针”这一名称。文中描述了回形针的一些特征：根据环末端的残基数量，它们分为两类；一类比另一类常见得多。发现了两个属于常见类别的回形针，只是每个的主链构象都是通常所见的镜像。大多数回形针显示出主链二面角紧密聚集的情况。这些与另一个被称为β-凸起环的常见环家族的一个亚组有些相似，但又不同；在后者中，二面角也紧密聚集。两种情况下的高度聚集可能是与环末端氢键模式相关的空间位阻限制的结果。
