Coupled oxidation-reduction (Redox) reactions are required for life. Photosynthetic organisms utilize light driven pigment-metal-protein complexes to oxidize water and reduce carbon dioxide to carbohydrates. A necessary byproduct of these reactions is the formation of O2, a dangerous gas prone to form reactive oxygen species (ROS) with the capacity to damage cellular components. Large concentrations of transition metals, necessary components of photosynthetic reaction centers, within photosynthetic cells can react with and generate new ROS. Photosynthetic organisms have developed extensive antioxidant networks and redox buffering systems in order to prevent cellular oxidation and to maintain redox homeostasis during normal metabolism. In addition, photosynthetic organisms have evolved many mechanisms for storage and transport of transition metals to prevent unwanted interactions with cellular components.
The redox buffering system is comprised of cytoplasmic and membrane soluble components. Glutathione, a small tripeptide thiol, is a major component of the soluble redox buffering system and contributes significantly to the redox environment of the cell. We are using genetic methods to dissect the roles of glutathione in the model photosynthetic organism Synechocystis 6803. In addition to soluble, non-protein antioxidants, Synechocystis 6803 contains enzymatic antioxidants. One class, the peroxiredoxins, are of significant interest because a member of this family is localized to the lumen, or intermembrane space of the thylakoid. We are interested in the function of this protein, PrxQ, and especially the potential electron donors because little is known regarding the lumenal environment.