Comparative studies of structural properties and conformational changes of proteins by analytical ultracentrifugation and other techniques

Analytical ultracentrifugation is a powerful tool for investigating the size of proteins in solution, especially by measuring sedimentation and diffusion coefficients and molar masses. Several further molecular parameters such as frictional ratios, axial ratios of hydrodynamic models, and Stokes radii allow a rough estimate of the protein overall structure. Sedimentation analysis may also be applied efficaciously for monitoring conformational changes of proteins occurring upon ligand binding or denaturation. For the determination of very small changes in shape, however, great care and a series of precautions are required. We investigated the enzymes citrate synthase and malate synthase in the absence and in the presence of ligands, in order to study the structural properties of the proteins and their ligand complexes. We also compared the results of the ultracentrifugal analysis with the results of other solution techniques such as UV absorption, fluorescence spectroscopy, circular dichroism, and small-angle x-ray scattering on the one hand, and the crystallographic 3D structure of citrate synthase on the other. The spectroscopic methods may be used as efficient and rapid tools for screening the occurrence of conformational changes caused by alterations of chromophores and fluorophores. The structural information provided by small-angle scattering (e.g., radii of gyration, maximum particle diameters, volumes and surface areas) can be used to establish quantitative correlations between solution scattering and hydrodynamic data. In this context, however, knowledge or qualified assumptions of partial specific volumes and hydration are additionally required. Good agreement was reached between small-angle scattering and ultracentrifugal data, and also with crystallographic data if protein hydration was considered properly. The given approaches may be used to predict hydrodynamic properties if x-ray data are available, and for many verifications of other structural data, e.g., Stokes radii, diffusion coefficients, axial and frictional ratios determined by independent methods.