Schmid F X, Buonocore M H, Baldwin R L
Biochemistry. 1984 Jul 17;23(15):3389-94. doi: 10.1021/bi00310a002.
L.-N. Lin and J.F. Brandts recently proposed a simple model for the folding kinetics of ribonuclease A in which folding intermediates are not detectable. We have tested the basic assumption of the simple model for the major unfolded species, which is produced by a slow isomerization (the "X in equilibrium Y reaction" according to Lin and Brandts) after unfolding. The simple model assumes that in refolding the slow Y----X reaction must occur before any folding can take place. We have measured the Y----X reaction during folding. Tyrosine-detected folding occurs before the Y----X reaction; the difference in rate between the Y----X reaction and folding monitored by tyrosine absorbance becomes large when the stabilizing salt 0.56 M (NH4)2SO4 is added. The simple model predicts that the kinetic properties of the X in equilibrium Y reaction in unfolded ribonuclease are the same as those of tyrosine-detected folding. We find, however, that the kinetics of the X in equilibrium Y reaction in unfolded ribonuclease are independent of urea concentration, whereas the rate of tyrosine-detected folding decreases almost 100-fold between 0.3 and 5 M urea, as reported by Lin and Brandts. We point out that the kinetic properties of the X in equilibrium Y reaction in unfolded ribonuclease are characteristic of proline isomerization.
林 - N. 林和J.F. 布兰德茨最近提出了一个关于核糖核酸酶A折叠动力学的简单模型，在该模型中折叠中间体无法被检测到。我们已经测试了这个简单模型对于主要未折叠物种的基本假设，该未折叠物种是在展开后通过缓慢的异构化反应（按照林和布兰德茨的说法是“X在平衡Y反应”）产生的。这个简单模型假设在重折叠过程中，缓慢的Y→X反应必须在任何折叠发生之前发生。我们已经测量了折叠过程中的Y→X反应。酪氨酸检测到的折叠发生在Y→X反应之前；当加入稳定盐0.56 M硫酸铵时，Y→X反应与通过酪氨酸吸光度监测的折叠之间的速率差异变得很大。这个简单模型预测未折叠核糖核酸酶中X在平衡Y反应的动力学性质与酪氨酸检测到的折叠的动力学性质相同。然而，我们发现未折叠核糖核酸酶中X在平衡Y反应的动力学与尿素浓度无关，而正如林和布兰德茨所报道的，酪氨酸检测到的折叠速率在0.3 M至5 M尿素之间几乎降低了100倍。我们指出，未折叠核糖核酸酶中X在平衡Y反应的动力学性质是脯氨酸异构化的特征。
