Zhou Huan-Xiang
Department of Physics, Drexel University, Philadelphia, PA 19104, USA.
Proc Natl Acad Sci U S A. 2002 Mar 19;99(6):3569-74. doi: 10.1073/pnas.052030599. Epub 2002 Mar 12.
Characterization of the unfolded state is essential for understanding the protein folding problem. In the unfolded state, a protein molecule samples vastly different conformations. Here I present a simple theoretical method for treating residual charge-charge interactions in the unfolded state. The method is based on modeling an unfolded protein as a Gaussian chain. After sampling over all conformations, the electrostatic interaction energy between two charged residues (separated by l peptide bonds) is given by W = 332(6/pi)(1/2)[1 - pi(1/2)xexp(x(2))erfc(x)]/epsilond, where d = bl(1/2) + s and x = kappad/6(1/2). In unfolded barnase, the residual interactions lead to downward pK(a) shifts of approximately 0.33 unit, in agreement with experiment. pK(a) shifts in the unfolded state significantly affect pH dependence of protein folding stability, and the predicted effects agree very well with experimental results on barnase and four other proteins. For T4 lysozyme, the charge reversal mutation K147E is found to stabilize the unfolded state even more than the folded state (1.39 vs. 0.46 kcal/mol), leading to the experimentally observed result that the mutation is net destabilizing for the folding. The Gaussian-chain model provides a quantitative characterization of the unfolded state and may prove valuable for elucidating the energetic contributions to the stability of thermophilic proteins and the energy landscape of protein folding.
对未折叠状态的表征对于理解蛋白质折叠问题至关重要。在未折叠状态下，蛋白质分子会采样出极为不同的构象。在此，我提出一种简单的理论方法来处理未折叠状态下的残余电荷 - 电荷相互作用。该方法基于将未折叠的蛋白质建模为高斯链。在对所有构象进行采样后，两个带电残基（相隔l个肽键）之间的静电相互作用能由W = 332(6/π)(1/2)[1 - π(1/2)xexp(x(2))erfc(x)]/εd给出，其中d = bl(1/2) + s且x = κd/6(1/2)。在未折叠的核糖核酸酶抑制剂中，残余相互作用导致pK(a)向下移动约0.33个单位，与实验结果一致。未折叠状态下的pK(a)移动显著影响蛋白质折叠稳定性的pH依赖性，并且预测的效应与核糖核酸酶抑制剂和其他四种蛋白质的实验结果非常吻合。对于T4溶菌酶，发现电荷反转突变K147E使未折叠状态比折叠状态更稳定（1.39对0.46千卡/摩尔），导致实验观察到的结果是该突变对折叠是净不稳定的。高斯链模型提供了未折叠状态的定量表征，并且可能对于阐明对嗜热蛋白质稳定性的能量贡献以及蛋白质折叠的能量景观具有重要价值。
