Picturing biological water oxidation by snapshot crystallography
Oxygenic photosynthesis is the primary means of fuelling life on Earth. This process, carried out by cyanobacteria, algae and plants, converts solar energy into chemical energy and provides us with the oxygen we breadth. The major ‘invention’ has been the ability to utilize the abundant and stable water as electron and proton source for carbon dioxide reduction to carbohydrates. The water oxidation reaction is carried out by Photosystem II, which thereby is the starting point of the photosynthetic electron transport chain of photosynthesis.
Oxidizing water to molecular oxygen by the energy of visible photons requires the coordination of four ultrafast (picoseconds) light-driven one-electron charge separations with the slow (milliseconds) four-electron and four-proton water oxidation chemistry. For interfacing these different time scales and stoichiometries Photosystem II utilizes a tetra-manganese calcium oxygen (Mn4CaO5) cluster that cycles to a set of five oxidation states.
In my presentation I will present recent progress in visualizing and deriving the mechanism of biological water oxidation. Special focus will be on the utilization of snapshot crystallography performed at x-ray free electron lasers (XFELs), which allowed deriving high-resolution structures of most key intermediates of this reaction cycle. In addition, experiments allowing the identification of one of the substrate waters and a tentative mechanism will be presented.