Understanding Photosystem II Water Splitting Mechanism by Photoassembly of PSII Crystals and In Vivo

Natural photosynthesis combines carbon dioxide (CO2), nutrients and water using solar energy to make the majority of biomass on Earth. The net annual primary productivity is approximately 105 Gt C yr−1, which is modestly greater than the 75 Gt C yr−1 emitted annually into the atmosphere as carbon dioxide by human activities. This process typically has a solar to biomass conversion efficiency below 1%, based on the energy content of the biomass divided by the energy absorbed by the sun in the photosynthetically active spectral region. Such low efficiency limits the applications in both agriculture (food) and bioenergy (fuel). The main solar energy conversion center that powers this process in all oxygenic photosynthetic organisms on Earth is called Photosystem II (PSII) and its associated water oxidation complex (WOC).The proposed studies will investigate the fundamental mechanisms controlling operation of PSII and the water oxidation complex, its solar conversion to chemical products using the isolated enzyme (in vitro). The Aim is to understand the mechanism of PSII-WOC operation in isolated, ultrapure, microcrystals. PSII microcrystals contain the most highly purified form of this enzyme and the most (catalytically) active forms yet obtained. They are unique in revealing for the first time the occurrence of distinct catalytic cycles.This project brings together two well established laboratories with expertise in PSII research: in PSII biochemistry, structural analysis by Xray crystallography, electron paramagnetic resonance spectrometry, oximetry, and fluorescence spectroscopy. PSII core complexes isolated from the cyanobacterium Thermosynechococcus elongatus will be provided by Dr Fromme. The knowledge gained will provide the scientific basis for how to improve the low intrinsic solar to biomass efficiency of natural photosynthesis and extend its useful lifetime by suppression of photodamage.