Protein conformations explored by difference high-angle solution X-ray scattering: oxidation state and temperature dependent changes in cytochrome C.

We demonstrate the use of high-angle X-ray scattering to explore protein conformational states in solution by resolving oxidation state- and temperature-dependent changes in the conformation of horse heart cytochrome c. Several detailed models exist for oxidation-dependent changes in mitochondrial class I c cytochromes determined by X-ray crystallography and solution NMR techniques. These models differ in the magnitude and locations of structural change. Our scattering measurements show that high-angle X-ray scattering can discriminate between these models, and that the experimental scattering data for horse cytochrome c can be best reconciled with selected NMR models for the same protein. These results demonstrate the ability to use high-angle X-ray scattering to resolve conformational states of proteins in solution, and to relate these measurements to detailed structural models. Furthermore, temperature-dependent changes are found in the high angle scattering patterns for horse cytochrome c, illustrating the sensitivity of these measurements to dynamic aspects of protein structure. These results demonstrate the ability to use difference high angle scattering as a quantitative monitor of reaction-linked changes in protein conformation and structural dynamics. Synchrotron-based high-angle scattering holds promise as a widely applicable, high throughput technique for exploring conformational states linked to physiological protein function, for resolving configurational differences between protein structures in solution and crystalline states, and for bridging the gap between solution NMR and crystallographic structure techniques.