Kataoka M, Kuwajima K, Tokunaga F, Goto Y
Department of Earth and Space Science, Graduate School of Science, Osaka University, Toyonaka, Japan.
Protein Sci. 1997 Feb;6(2):422-30. doi: 10.1002/pro.5560060219.
A compact denatured state is often observed under a mild denaturation condition for various proteins. A typical example is the alpha-lactalbumin molten globule. Although the molecular compactness and shape are the essential properties for defining the molten globule, there have been ambiguities of these properties for the molten globule of alpha-lactalbumin. Using solution X-ray scattering, we have examined the structural properties of two types of molten globule of alpha-lactalbumin, the apo-protein at neutral pH and the acid molten globule. The radius of gyration for the native holo-protein was 15.7 A, but the two different molten globules both had a radius of gyration of 17.2 A. The maximum dimension of the molecule was also increased from 50 A for the native state to 60 A for the molten globule. These values clearly indicate that the molten globule is not as compact as the native state. The increment in the radius of gyration was less than 10% for the alpha-lactalbumin molten globule, compared with up to 30% for the molten globules of other globular proteins. Intramolecular disulfide bonds restrict the molecular expansion of the molten globule. The distance distribution function of the alpha-lactalbumin molten globule is composed of a single peak suggesting a globular shape, which is simply swollen from the native state. The scattering profile in the high Q region of the molten globule indicates the presence of a significant amount of tertiary fold. Based on the structural properties obtained by solution X-ray scattering, general and conceptual structural images for the molten globules of various proteins are described and compared with the individual, detailed structural model obtained by nuclear magnetic resonance.
在温和变性条件下，常常能观察到各种蛋白质呈现出一种紧密的变性状态。一个典型的例子就是α-乳白蛋白熔球态。尽管分子紧密程度和形状是定义熔球态的关键性质，但对于α-乳白蛋白熔球态的这些性质一直存在模糊之处。利用溶液X射线散射技术，我们研究了α-乳白蛋白的两种熔球态的结构性质，即中性pH下的脱辅基蛋白熔球态和酸性熔球态。天然全蛋白的回转半径为15.7 Å，但这两种不同的熔球态的回转半径均为17.2 Å。分子的最大尺寸也从天然态的50 Å增加到了熔球态的60 Å。这些数值清楚地表明，熔球态不像天然态那样紧密。与其他球状蛋白熔球态高达30%的增加幅度相比，α-乳白蛋白熔球态的回转半径增加幅度小于10%。分子内二硫键限制了熔球态分子的扩张。α-乳白蛋白熔球态的距离分布函数由一个单一峰组成，表明其呈球状，只是相对于天然态有所膨胀。熔球态在高Q区域的散射图谱表明存在大量的三级结构折叠。基于溶液X射线散射获得的结构性质，描述了各种蛋白质熔球态的一般概念性结构图像，并与通过核磁共振获得的单个详细结构模型进行了比较。
