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Center for Nonlinear Studies |
To generate a deeper understanding into the relations between protein structure, dynamics, and function, we have developed time-resolved X-ray methods capable of probing changes in protein structure on time scales as short as ~100 ps. This infrastructure was first developed on the ID09B time-resolved beamline at the ESRF, and more recently at the ID14B BioCARS beamline at the APS. In these studies, a picosecond laser pulse first photoexcites a protein and triggers a structural change, then a suitably delayed picosecond X-ray pulse passes through the protein and the scattered X-rays are imaged on a 2D detector. When the sample is a protein crystal, this “pump- probe” approach recovers time-resolved diffraction “snapshots” whose corresponding electron density maps can be stitched together into movies that unveil the correlated motions that arise from the photoexcitation event. When the sample is a protein solution, we recover time-resolved small- and wide-angle X-ray scattering (SAXS/ WAXS) patterns that are sensitive to changes in the size, shape, and structure of the protein. The time-resolved x-ray scattering diffractometer developed at the APS is capable of simultaneously probing both SAXS and WAXS regions with 100 ps time resolution. Importantly, scattering studies of proteins in solution unveil structural dynamics without the constraints imposed by crystal contacts; thus, these scattering “fingerprints” complement results obtained from diffraction studies. Together, these studies provide stringent constraints for putative models of conformational states and structural transitions between them. Recent studies of heme proteins have unveiled the dynamics for tertiary and quaternary conformational changes as well as ligand migration and escape. Emerging from these studies are detailed insights into the mechanism of cooperative ligand binding in allosteric proteins. This research was supported in part by the Intramural Research Program of the NIH, NIDDK.
Host: Parthasarathi Ramakrishnan, T-6: THEORETICAL BIOLOGY AND BIOPHYSICS, rampartha@lanl.gov, 7-7812