Engineering Photosystem I for Enhanced Electron Transport Rates In Vitro for Applied Photosynthesis

Photosystem I (PSI) has been shown to be a robust photoactive nanoparticle capable of generating both hydrogen and electricity in vitro. These processes both derive their electrons from soluble metalloproteins, such as plastocyanin and cytochrome c, which re-reduce the oxidized special pair (P700) of PSI. Much of what we know about this process has been via the studying of model single cell organisms such as cyanobacteria. Cyanobacteria are important for carbon fixation and oxygen production and also offer a powerful and facile genetic system. Moreover, as oxygenic organisms they utilize the Z-scheme employing both PSII and PSI resembling that found in plants. Currently, the thermophilic cyanobacterium, Thermosynechococcus elongates BP-1 is extensively studied due to its sequenced genome, genetic transformability, and the recent success in crystallizing many protein complexes, including PSI and PSII. My project is to engineer the PSI’s surface chemistry. For example, we are making complementary changes on both PSI and cytochrome c6 surfaces so that they interact quicker in solution. Specifically, we have made a psaF modification to enhance docking of cytochrome c6 to the lumenal surface of PSI. These mutations will be functionally characterized through electrochemical and photochemical assays. The electron transfer rate can be tested using flash photolysis and analyzing the re-reduction profile of P700. The kinetics of this electron transport step is being investigated as a function of cytochrome content, temperature, pH, ionic strength, and post selective mutagenesis. These results will be discussed in light of our understanding of how Cyt c6 and PSI interact.