Katherine Davis - Using X-rays to Probe Protein Dynamics

Speaker: Katherine Davis, Princeton University

Abstract: Metalloenzymes are metal-containing proteins that catalyze a broad range of biological transformations important for life. A detailed understanding of the mechanisms underlying their reactivity requires a confluence of methods to elucidate both local electronic changes to the metal center as well as global structural motions. Although x-ray spectroscopic methods are incredibly powerful, they are nonetheless limited in scope. X-ray crystallography, in contrast, can provide key insights into global structure. Here, I will explore the application of these x-ray techniques to diverse enzymatic systems. For example, to investigate the mechanism by which Nature splits water during photosynthesis, we performed time-resolved laser pump, x-ray probe experiments to characterize sub-millisecond reactive intermediates of photosystem II, the metalloenzyme complex responsible for the oxidation of water in plants and cyanobacteria. This approach probed the catalytic Mn4CaO5 cluster and yielded the first ever x-ray emission data on the water splitting step. Intriguingly, no additional oxidation past the S3-intermediate state was observed, and evidence of significant Mn reduction within 50 μs of the final laser flash led to the proposal of a new mechanistic model of natural water oxidation.