Chemical Physics Department, Weizmann Institute of Science, Rehovot 76100, Israel.
J Am Chem Soc. 2009 Mar 4;131(8):2942-7. doi: 10.1021/ja808305u.
The essential and nontrivial role of the denatured state of proteins in their folding reaction is being increasingly scrutinized in recent years. Single molecule FRET (smFRET) experiments show that the denatured state undergoes a continuous collapse (or coil-to-globule) transition as the concentration of a chemical denaturant is decreased, suggesting that conformational entropy of the denatured state is an important part of the free energy of folding. Such observations question the validity of the classical Tanford transfer model, which suggests that the folding free energy can be understood solely based on the difference in amino acid solvation between the folded state and a fixed unfolded state. An alternative to the transfer model is obtained here from a polymer theoretical analysis of a series of published smFRET data. The analysis shows that the free energy of denatured-state collapse has a linear dependence on denaturant concentration, an outcome of the interplay between enthalpic and entropic contributions. Surprisingly, the slope of the free energy of collapse agrees very well with the respective slope of the free energy of folding. This conformity of values obtained from two very different measurements shows that it is the collapse transition in the denatured state which mediates the effect of denaturants on folding. The energetics of folding are thus governed by the competition of solvation and conformational entropy in the denatured state.
近年来，蛋白质变性状态在其折叠反应中的重要且非平凡作用受到了越来越多的关注。单分子荧光共振能量转移（smFRET）实验表明，随着化学变性剂浓度的降低，变性状态会经历连续的坍缩（或卷曲到球团）转变，这表明变性状态的构象熵是折叠自由能的重要组成部分。这些观察结果对经典的 Tanford 转移模型的有效性提出了质疑，该模型表明折叠自由能可以仅基于折叠状态和固定未折叠状态之间的氨基酸溶剂化差异来理解。从一系列已发表的 smFRET 数据的聚合物理论分析中，得到了该转移模型的替代方案。分析表明，变性状态坍缩的自由能与变性剂浓度呈线性关系，这是焓和熵贡献相互作用的结果。令人惊讶的是，坍缩自由能的斜率与折叠自由能的斜率非常吻合。这两个非常不同的测量值得到的一致性表明，正是变性状态中的坍缩转变介导了变性剂对折叠的影响。因此，折叠的能量学由变性状态中的溶剂化和构象熵的竞争决定。
