Chou K C, Némethy G, Pottle M, Scheraga H A
Computational Chemistry Unit, The Upjohn Company, Kalamazoo, MI 49001.
J Mol Biol. 1989 Jan 5;205(1):241-9. doi: 10.1016/0022-2836(89)90378-1.
An explanation in terms of conformational energies is provided for the observed nearly exclusive preference of the beta alpha beta structure for forming a right-handed, rather than a left-handed, crossover connection. Conformational energy computations have been carried out on a model beta alpha beta structure, consisting of two six-residue Val beta-strands and of a 12-residue Ala alpha-helix, connected by two flexible four-residue Ala links to the strands. The energy of the most favorable right-handed crossover is 15.51 kcal/mol lower than that of the corresponding left-handed cross-over. The right-handed crossover is a strain-free structure. Its energy of stabilization arises largely from the interactions of the two beta-strands with one another and with the alpha-helix. On the other hand, the left-handed crossover is either disrupted after energy minimization or it remains conformationally strained, as indicated by an energetically unfavorable left twisting of the beta-sheet and by the presence of high-energy local residue conformations. In the energetically most favorable right-handed crossover, the right twisting of the beta-sheet and its manner of interacting with the alpha-helix are identical with those computed earlier for isolated beta-sheets and for packed alpha/beta structures. This result supports a proposed principle that it is possible to account for the main features of frequently occurring structural arrangements in globular proteins in terms of the properties of their component structural elements.
本文对观察到的β-α-β结构几乎完全倾向于形成右手交叉连接而非左手交叉连接的现象，从构象能量的角度给出了解释。我们对一个β-α-β结构模型进行了构象能量计算，该模型由两条含六个残基的缬氨酸β链和一条含十二个残基的丙氨酸α螺旋组成，通过两条含四个残基的柔性丙氨酸连接链与β链相连。最有利的右手交叉结构的能量比相应左手交叉结构的能量低15.51千卡/摩尔。右手交叉结构是一种无应变结构。其稳定能量主要源于两条β链之间以及它们与α螺旋之间的相互作用。另一方面，左手交叉结构在能量最小化后要么被破坏，要么仍然存在构象应变，这表现为β折叠片层的能量不利左旋以及高能局部残基构象的存在。在能量最有利的右手交叉结构中，β折叠片层的右旋及其与α螺旋的相互作用方式，与之前对孤立β折叠片层和堆积的α/β结构计算得到的结果相同。这一结果支持了这样一个提出的原则，即根据球状蛋白质中组成结构元件的性质，可以解释其频繁出现的结构排列的主要特征。
