Harvesting Photosynthetic Electrons with Exotic Cytochromes
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Authors
Ashton Trnka
Issue Date
2026
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Abstract
Photosynthesis captures the energy of sunlight in a stable form that is useful for biology: the carbon-carbon bonds of sugar molecules. Before being turned into sugar, however, that energy is very briefly used to generate an electrical current that runs through the thylakoid membranes within the chloroplast. In photosynthetic organisms like plants, Photosystem I is responsible for transferring the electrons from this current to an electron carrier called ferredoxin. Ferredoxin will, in turn, carry those electrons to be used for the synthesis of sugar and to power the molecular machinery of metabolism. If ferredoxin can be replaced with a controllable electron acceptor, then it is possible to harness these electrons from PSI. This has the potential, in other words, to channel the energy from photosynthesis directly into the production of electricity. Schwanella Oneidensis is a gram-negative electrogenic bacterium that contains Small Tetraheme Cytochrome (STC). These bacteria use STC to transfer electrons from inside the cell to the outside. This suggests that STC may be a good candidate for replacing ferredoxin, as it serves as an electron acceptor and donor. STC was cloned into an expression vector with an arabinose-inducible promoter. The STC sequence was fused with a MalE leader sequence for expression into the periplasm, which is necessary for the assembly of cytochromes in E. coli. In addition, a Strep II tag was added for later affinity column purification of the expressed protein using streptavidin resin. The purified protein will now be characterized by monitoring the electron transfer kinetics from thylakoid membranes to the purified cytochrome via spectroscopic techniques.