Small Angle X-Ray Scattering: A Powerful Tool to Analyze Protein Conformation in Solution

In the field of molecular biology and biochemistry in which structural genomics comes as a complement to genome sequencing, Small-Angle X-ray Scattering (SAXS) is an experimental technique that, though not widely known and applied, represents a very powerful tool in the framework of post-genome structural studies. The aim of this review is to present a synthetic description of the basic principles of the theory of SAXS and to discuss in more detail its applications to the study of different biological molecules, focusing the attention to the recent advances in the structural analysis of proteins. These studies are presently undergoing a spectacular expansion associated with the development of powerful data analysis software, with the improvement of the quality of data recorded with synchrotron radiation, and finally with the increasing availability of high resolution three-dimensional structures which can constitute a starting point for the analysis of protein conformations in solution. The advantage of SAXS with respect to other techniques in the structural studies of proteins resides in the possibility of performing the measurements in any desired solvent and in the ability to follow changes of the protein structure which may occur as a response to a variety of stimuli: pH or temperature changes, interaction with small ligands, influence of substrate analogues, chemical or genetic modifications, etc. In this review we describe the principles of SAXS and the most recent methods for data analysis in the field of structural biology and, finally, we report some examples of the use of this technique as a powerful tool to the structural study of proteins in solution.