Andrews P R, Jeffrey P D
Biophys Chem. 1976 Jan;4(1):93-102. doi: 10.1016/0301-4622(76)80010-5.
The sedimentation coefficients of proteins are dependent on their sizes, shapes and densities and on the density and viscosity of the solvent. However, when the sedimentation coefficients of an oligomeric protein and its protomer are measured under the same experimental conditions, the ratio of the two coefficients depends only on the protomer shape and the mode of aggregation. This property, which we shall call the sedimentation ratio, therefore provides a way of distinguishing between models for oligomeric proteins. To allow examination of the behaviour of the sedimentation ratio, sedimentation coefficients are calculated for a comprehensive range of protomer shapes and modes of aggregation in hexameric systems using equations derived by Kirkwood. As illustrations of the method the resulting sedimentation ratios are compared with experimental values for insulin and arthroped hemocyanin, which eliminates many of the possible structures for these proteins. When experimental estimates of degree of hydration and molecular dimensions are also considered, all but a group of virtually identical structures are eliminated for the insulin hexamer and a single most likely structure remains for arthropod hemocyanin. The insulin structure is in good agreement with that determined by X-ray crystallography while the hemocyanin hexameric structure is a hexagonal prism formed by the cyclic aggregation of prolate ellipsoids of axial ratio about 2.5 : 1.
蛋白质的沉降系数取决于其大小、形状和密度，以及溶剂的密度和粘度。然而，在相同实验条件下测量寡聚蛋白及其原体的沉降系数时，这两个系数的比值仅取决于原体形状和聚集模式。因此，我们将这种特性称为沉降比，它为区分寡聚蛋白的模型提供了一种方法。为了研究沉降比的行为，使用柯克伍德推导的方程，计算了六聚体系统中各种原体形状和聚集模式的沉降系数。作为该方法的示例，将得到的沉降比与胰岛素和节肢动物血蓝蛋白的实验值进行比较，这排除了这些蛋白质的许多可能结构。当还考虑水合程度和分子尺寸的实验估计时，胰岛素六聚体除了一组几乎相同的结构外都被排除，而节肢动物血蓝蛋白只剩下一种最可能的结构。胰岛素的结构与通过X射线晶体学确定的结构非常吻合，而血蓝蛋白的六聚体结构是由轴比约为2.5:1的长椭球体循环聚集形成的六棱柱体。
