Qu Y, Bolen C L, Bolen D W
Department of Human Biological Chemistry and Genetics, University of Texas Medical Branch, Galveston, TX 77555-1052, USA.
Proc Natl Acad Sci U S A. 1998 Aug 4;95(16):9268-73. doi: 10.1073/pnas.95.16.9268.
The Stokes radius characteristics of reduced and carboxamidated ribonuclease A (RCAM RNase) were determined for transfer of this "random coil" protein from water to 1 M concentrations of the naturally occurring protecting osmolytes trimethylamine N-oxide, sarcosine, sucrose, and proline and the nonprotecting osmolyte urea. The denatured ensemble of RCAM RNase expands in urea and contracts in protecting osmolytes to extents proportional to the transfer Gibbs energy of the protein from water to osmolyte. This proportionality suggests that the sum of the transfer Gibbs energies of individual parts of the protein is responsible for the dimensional changes in the denatured ensemble. The dominant term in the transfer Gibbs energy of RCAM RNase from water to protecting osmolytes is the unfavorable interaction of the osmolyte with the peptide backbone, whereas the favorable interaction of urea with the backbone dominates in RCAM RNase transfer to urea. The side chains collectively favor transfer to the osmolytes, with some protecting osmolytes solubilizing hydrophobic side chains as well as urea does, a result suggesting there is nothing special about the ability of urea to solubilize hydrophobic groups. Protecting osmolytes stabilize proteins by raising the chemical potential of the denatured ensemble, and the uniform thermodynamic force acting on the peptide backbone causes the collateral effect of contracting the denatured ensemble. The contraction decreases the conformational entropy of the denatured state while increasing the density of hydrophobic groups, two effects that also contribute to the ability of protecting osmolytes to force proteins to fold.
测定了还原型和氨甲酰化核糖核酸酶A（RCAM核糖核酸酶）的斯托克斯半径特征，以研究这种“无规卷曲”蛋白从水转移至1M浓度的天然存在的保护性渗透剂三甲胺N-氧化物、肌氨酸、蔗糖和脯氨酸以及非保护性渗透剂尿素中的情况。RCAM核糖核酸酶的变性整体在尿素中膨胀，而在保护性渗透剂中收缩，其程度与蛋白从水转移至渗透剂的转移吉布斯自由能成正比。这种比例关系表明，蛋白各个部分的转移吉布斯自由能之和导致了变性整体的尺寸变化。RCAM核糖核酸酶从水转移至保护性渗透剂时，转移吉布斯自由能的主要项是渗透剂与肽主链的不利相互作用，而尿素与主链的有利相互作用在RCAM核糖核酸酶转移至尿素的过程中占主导。侧链总体上有利于转移至渗透剂，一些保护性渗透剂溶解疏水侧链的能力与尿素相当，这一结果表明尿素溶解疏水基团的能力并无特殊之处。保护性渗透剂通过提高变性整体的化学势来稳定蛋白质，作用于肽主链的均匀热力学力导致了变性整体收缩的附带效应。收缩降低了变性态的构象熵，同时增加了疏水基团的密度，这两种效应也有助于保护性渗透剂促使蛋白质折叠。
