Asmit Bhowmick, LBNL

Abstract

The water oxidation complex in Photosystem II is the catalyst for the transformation of two water molecules to molecular oxygen, releasing four electrons and four protons in the process. Starting from the ground state, the complex gets progressively oxidized upon absorption of a photon and moves along the Kok cycle (S0-S4 intermediate states). In the last state, the complex releases its stored oxidative power to catalyze the reaction and return to the ground state. Here we present our results on time-resolved structural studies done at room temperature on Photosystem II using X-ray free-electron laser (XFEL) crystallography of the various stable intermediates along the Kok cycle as well as timepoint in-between these intermediates. The data shows the sequence of events leading to the insertion of a new substrate water in the S2-->S3 transition. The water insertion is accompanied by distance changes within the cluster as well as side-chain coordination with the complex. In addition, we observed critical structural changes in the different water and proton channels that are coordinated with the substrate water insertion and tell us about their functional role in this transition. We also discuss preliminary results on the final transition that leads to formation of
molecular oxygen (i.e S3-->S0 transition).