Zaccai G, Wachtel E, Eisenberg H
J Mol Biol. 1986 Jul 5;190(1):97-106. doi: 10.1016/0022-2836(86)90078-1.
Data from small-angle X-ray and neutron scattering and ultracentrifugation experiments on solutions of malate dehydrogenase from Halobacterium maris mortui are analysed together to yield a model for the enzyme particle formed by the protein and its interactions with water and salt in the solvent. The halophilic enzyme is stable only in high concentrations of salt and the model has structural features that are absent from non-halophilic malate dehydrogenase. The complementarity of the information derived from the three experimental methods is discussed extensively and quantitatively. It derives from the fact that mass density (ultracentrifugation), electron density (X-rays) and neutron scattering density are independent of each other. Each method gives a different "view" of the same particle, and an analysis of the combined data provided thermodynamic and structural parameters with, apart from the chemical composition of the solutions, only one other assumption: a constant partial specific volume for water equal to 1.00 cm3 g-1. Both the insights gained by this novel approach and its limitations are carefully pointed out. In solvents between 1 M and 5 M-NaCl, the enzyme forms a particle of invariant volume, consisting of a protein dimer (87,000 g mol-1) with which are associated 0.87 g of water and 0.35 g of salt per gram of protein. The partial specific volume of the protein calculated from the combined experimental data is 0.753(+/- 0.030) cm3 g-1, in good agreement with the value calculated from the amino acid composition. The particle has a radius of gyration of 32 A and an equivalent Stokes radius of 43 A. By combining the data from the X-ray and neutron scattering studies, the radii of gyration of the protein moiety alone and of the associated water and salt distribution were calculated. They are 28 A and about 40 A, respectively. The large-angle scattering curves show that the shapes of the particle and of the protein moiety alone are similar. At very low resolution they can be approximated by an ellipsoid of axial ratio 1:1:0.6 (or 1:1:1.5). At higher resolution, it becomes apparent that the particle has a significantly larger interface with solvent than an homogeneous ellipsoid or globular protein. The model has a globular protein core similar to non-halophilic malate dehydrogenase, with about 20% of the protein extending loosely out of the core, forming the large interface with solvent. The main interactions with water and salt take place on this outer part.
对来自死海嗜盐杆菌的苹果酸脱氢酶溶液进行小角X射线散射、中子散射和超速离心实验的数据进行综合分析，以建立该酶颗粒的模型，该模型包括蛋白质及其与溶剂中水和盐的相互作用。这种嗜盐酶仅在高浓度盐中稳定，该模型具有非嗜盐苹果酸脱氢酶所没有的结构特征。广泛且定量地讨论了从这三种实验方法获得的信息的互补性。这种互补性源于质量密度（超速离心）、电子密度（X射线）和中子散射密度相互独立这一事实。每种方法都给出了同一颗粒的不同“视图”，对综合数据的分析除了溶液的化学成分外，仅在一个假设下提供了热力学和结构参数：水的恒定偏比容等于1.00 cm³ g⁻¹。仔细指出了这种新方法所获得的见解及其局限性。在1 M至5 M - NaCl的溶剂中，该酶形成体积不变的颗粒，由蛋白质二聚体（87,000 g mol⁻¹）组成，每克蛋白质与0.87 g水和0.35 g盐结合。根据综合实验数据计算出的蛋白质偏比容为0.753（±0.030）cm³ g⁻¹，与根据氨基酸组成计算的值高度一致。该颗粒的回转半径为32 Å，等效斯托克斯半径为43 Å。通过结合X射线和中子散射研究的数据，计算出了仅蛋白质部分以及相关水和盐分布的回转半径。它们分别为28 Å和约40 Å。大角散射曲线表明颗粒和仅蛋白质部分的形状相似。在非常低的分辨率下，它们可以近似为轴比为1:1:0.6（或1:1:1.5）的椭球体。在更高分辨率下，很明显该颗粒与溶剂的界面比均匀椭球体或球状蛋白质大得多。该模型具有与非嗜盐苹果酸脱氢酶相似的球状蛋白质核心，约20%的蛋白质从核心松散地伸出，形成与溶剂的大界面。与水和盐的主要相互作用发生在这个外部部分。
