Fersht A R
Department of Chemistry, University of Cambridge, United Kingdom.
Proc Natl Acad Sci U S A. 1995 Nov 21;92(24):10869-73. doi: 10.1073/pnas.92.24.10869.
Small, single-module proteins that fold in a single cooperative step may be paradigms for understanding early events in protein-folding pathways generally. Recent experimental studies of the 64-residue chymotrypsin inhibitor 2 (CI2) support a nucleation mechanism for folding, as do some computer stimulations. CI2 has a nucleation site that develops only in the transition state for folding. The nucleus is composed of a set of adjacent residues (an alpha-helix), stabilized by long-range interactions that are formed as the rest of the protein collapses around it. A simple analysis of the optimization of the rate of protein folding predicts that rates are highest when the denatured state has little residual structure under physiological conditions and no intermediates accumulate. This implies that any potential nucleation site that is composed mainly of adjacent residues should be just weakly populated in the denatured state and become structured only in a high-energy intermediate or transition state when it is stabilized by interactions elsewhere in the protein. Hierarchical mechanisms of folding in which stable elements of structure accrete are unfavorable. The nucleation-condensation mechanism of CI2 fulfills the criteria for fast folding. On the other hand, stable intermediates do form in the folding of more complex proteins, and this may be an unavoidable consequence of increasing size and nucleation at more than one site.
以单一协同步骤折叠的小单模块蛋白可能是理解蛋白质折叠途径早期事件的范例。最近对64个残基的胰凝乳蛋白酶抑制剂2（CI2）的实验研究支持折叠的成核机制，一些计算机模拟也是如此。CI2有一个仅在折叠过渡态才形成的成核位点。该核心由一组相邻残基（一个α螺旋）组成，通过在蛋白质其余部分围绕其折叠时形成的长程相互作用而稳定。对蛋白质折叠速率优化的简单分析预测，当变性态在生理条件下几乎没有残余结构且没有中间体积累时，折叠速率最高。这意味着任何主要由相邻残基组成的潜在成核位点在变性态中应该只有很少的占据，并且只有在被蛋白质其他部位的相互作用稳定时，才会在高能中间体或过渡态中形成结构。结构稳定元件逐步积累的分层折叠机制是不利的。CI2的成核-凝聚机制符合快速折叠的标准。另一方面，在更复杂蛋白质的折叠过程中确实会形成稳定中间体，这可能是蛋白质大小增加和多个位点成核导致的不可避免的结果。
