Photosystem II Water Oxidation: Mechanism, Efficiency and Flux in Diverse Oxygenic Phototrophs

Virtually all life on Earth depends on the process of oxygenic photosynthesis for removal of CO2 from the atmosphere, biomass formation, and oxygen production. The majority of solar energy converted by photosynthesis occurs within a highly conserved protein complex, photosystem II (PSII). We study both natural PSIIs variants across the tree of life and from subcellular fractions. While isolated PSII complexes are commonly studied in vitro, our research focuses on their in vivo regulation and system level functioning in living cells. Under this grant, we developed the VZAD model, the most accurate mathematical model to date for fitting experimental O2 flash yields from PSII water-oxidizing complexes (WOCs). In the final grant year we plan four aims: 1) extend the VZAD model to allow analysis of PSII chlorophyll fluorescence emission as modulated by interaction with the WOC; 2) compare the solar energy conversion efficiencies of PSII-WOCs from intact cells, isolated thylakoid membranes and PSII core complexes and crystals from cyanobacterium Thermosynechococcus elongatus (collaboration with Lawrence Livermore National Laboratory); 3) determine whether PSIIs can store light energy by pumping protons across the thylakoid membrane (PSII-cyclic electron flow) and how it is regulated within the green alga Chlorella ohadii (collaboration with the Hebrew University of Jerusalem); and 4) genetically replace the native PSII-D1 protein subunit from a higher plant with two cyanobacterial D1 isoforms to test whether their functional advantages in growth and photoprotection can be transferred (collaboration with Rutgers University). Collectively, this research will improve understanding of photosynthesis for applications to biotechnology, agriculture, and energy.