Oxidizing side of the photosystem I
Photosynthesis converts solar energy into the biological sources of energy for the life on our planet. Photosystem I (PSI) is one of the two reaction centers of oxygenic photosynthesis. PSI is a multiheteromeric membrane-protein complex that catalyzes light-driven electron transfer from plastocyanin or cytochrome c6 to ferredoxin. The PsaA and PsaB subunits form the heterodimeric core that harbors the primary electron donor P700. On the oxidizing side of the PSI complex, plastocyanin or cytochrome c6 donates electrons to the P700 reaction center. The objective of this dissertation is to identify elements of molecular recognition on the oxidizing side of PSI;To identify PSI regions that are exposed on the luminal oxidizing side, the topography of the PSI complex from Synechocystis sp. PCC 6803 was examined by biochemical analyses. These studies indicated that the H and J luminal loops of the core proteins were the most logical candidates for hydrophobic interactions with the donor proteins. To test this hypothesis, site-directed mutations were generated in these loops. Biochemical characteristics of these mutants provided evidence for interactions between the electron donor proteins and a luminal surface helix in the J loop of the PsaB protein. Sections of the H loop of PsaB plays an important role in the structural integrity of the PsaB protein. The N-terminal lysine-rich region of PsaF in eukaryotic PSI complex has been proposed to provide a binding site for plastocyanin and cytochrome c6 through electrostatic interactions. Chimeric PsaF proteins containing the lysine-rich region of spinach PsaF were produced in Synechocystis sp. PCC 6803 cells through mutagenesis. Cross-linking and functional studies showed that the lysine-rich region of spinach PsaF protein is sufficient for binding of donor proteins, but not for the electron transfer within the intermolecular complex. The results in this dissertation revealed the structural basis for the molecular recognition on the oxidizing side of PSI.