Solution structure of halophilic malate dehydrogenase from small-angle neutron and X-ray scattering and ultracentrifugation.

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.