Capturing the Sequence of Events During the Catalytic Reaction using XFELs

The water oxidation reaction in the Photosystem II (PS II) enzyme produces molecular oxygen, which sustains life on Earth, and releases four electrons and protons that are used downstream in other photosynthetic processes for generating ATP. Thus, understanding how this reaction is catalyzed will help in development of energy conversion technologies. The catalytic center is a Manganese complex (Mn4CaO5) which gets oxidized progressively upon absorption of light before releasing the stored energy and oxidizing water. Structural studies of PS II have been limited at traditional synchrotron sources due to radiation damage of this complex. The emergence of X-ray free-electron lasers (XFEL) with intense femtosecond X-ray pulses have opened opportunities to collect structural data on PS II under physiological temperature. The time-resolved electron density changes at the Manganese complex reveal notable structural changes, including the insertion of a new water OX, which disappears upon completion of the reaction [1-3]. We are also able to follow the structural dynamics of the protein coordinating with the complex, revealing well- orchestrated conformational changes in response to electronic changes at the Manganese cluster. The method described here can be applicable to other (metallo)enzymes by initiating reaction with different reaction-triggering methods.