Small-angle x-ray scattering study of halophilic malate dehydrogenase

Malate dehydrogenase from the organism Halobacterium marismortui was studied in solutions of varying salt concentration by using a small-angle X-ray system employing a linear position sensitive detector. Considerations pertaining to the study of absorbing multicomponent solutions are presented. The radius of gyration of halophilic malate dehydrogenase was found to be 31.8 +/- 0.6 A and the shape of the molecule spheroidal. The scattering from prolate ellipsoids of eccentricity between 1 and 2 best fitted the data while for oblate ellipsoids the scattering was best fitted for eccentricities between 1 and 0.5. No significant change in the radius of gyration or anisotropy of halophilic malate dehydrogenase was found in the range of NaCl concentrations studied (1-4 M). The contrast matching electron density was found to be 0.407 +/- 0.002 e/A3. A parallel study of bovine serum albumin yielded within experimental error a similar contrast matching electron density of 0.404 +/- 0.006 e/A3. This information combined with the diffusion coefficient and the amount of water and salt associated with halophilic malate dehydrogenase renders the existence of an outer hydration shell unlikely. The data are rather consistent at low resolution with a spheroidal particle of uniform electron density.