Alonso D O, Dill K A
Department of Pharmaceutical Chemistry, University of California, San Francisco 94143.
Biochemistry. 1991 Jun 18;30(24):5974-85. doi: 10.1021/bi00238a023.
Statistical thermodynamic theory has recently been developed to account for the stabilities of globular proteins. Here we extend that work to predict the effects of solvents on protein stability. Folding is assumed to be driven by solvophobic interactions and opposed by conformational entropy. The solvent dependence of the solvophobic interactions is taken from transfer experiments of Nozaki and Tanford on amino acids into aqueous solutions of urea or guanidine hydrochloride (GuHCl). On the basis of the assumption of two pathways involving collapse and formation of a core, the theory predicts that increasing denaturant should lead to a two-state denaturation transition (i.e., there is a stable state along each path separated by a free energy barrier). The denaturation midpoint is predicted to occur at higher concentrations of urea than of GuHCl. At neutral pH, the radius of the solvent-denatured state should be much smaller than for a random-flight chain and increase with either denaturant concentration or number of polar residues in the chain. A question of interest is whether free energies of folding should depend linearly on denaturant, as is often assumed. The free energy is predicted to be linear for urea but to have some small curvature for GuHCl. Predicted slopes and exposed areas of the unfolded states are found to be in generally good agreement with experiments. We also discuss stabilizing solvents and compare thermal with solvent denaturation.
统计热力学理论最近已得到发展，用于解释球状蛋白质的稳定性。在此，我们拓展这项工作，以预测溶剂对蛋白质稳定性的影响。折叠被假定由疏溶剂相互作用驱动，并受到构象熵的阻碍。疏溶剂相互作用对溶剂的依赖性取自野崎和坦福德关于氨基酸转移到尿素或盐酸胍（GuHCl）水溶液中的实验。基于涉及塌缩和核心形成的两条途径的假设，该理论预测，变性剂浓度增加应导致两态变性转变（即，每条路径上存在一个由自由能垒分隔的稳定状态）。预计变性中点在尿素浓度高于GuHCl时出现。在中性pH值下，溶剂变性状态的半径应比无规飞行链的半径小得多，并随变性剂浓度或链中极性残基数量的增加而增大。一个有趣的问题是，折叠自由能是否应如通常所假设的那样线性依赖于变性剂。预计尿素的自由能是线性的，但GuHCl的自由能有一些小的曲率。发现预测的斜率和未折叠状态的暴露面积与实验总体上吻合良好。我们还讨论了稳定化溶剂，并比较了热变性和溶剂变性。
