BPS2026 – Unravelling the mechanism of water oxidation in photosystem II with time-resolved X-ray crystallography

The water oxidation complex in photosystem II is the catalyst for the transformation of two water molecules to molecular oxygen (O 2 ), releasing four electrons and four protons in the process. Starting from the ground state, the complex gets progressively oxidized and moves along the Kok cycle (S 0 -S 4 intermediate states) upon absorption of a photon in each step. In the last state (S 4 ), the complex releases its stored oxidative power to catalyze the reaction and return to the ground state, thereby releasing O 2 . Here we present our time-resolved structural studies done on photosystem II using X-ray free-electron laser (XFEL) crystallography during the final S 3 ⋄[S 4 ]⋄S 0 transition. We collected data for 7 time points for the transition as well as the S 3 and S 0 states. The data show the sequence of events leading to the disappearance of water molecule (O X ) bridging Ca and Mn1 atom. The water disappearance is accompanied by electron density changes within the cluster as well as side-chain coordination with the complex. In addition, we observed critical structural changes in different water and proton channels that are coordinated with the O X disappearance and tell us about their functional role in this transition.