Unraveling Cyanobacterial Primary Reaction Center- Photosystem I and Enhancing Electron Transfer by Bioengineering the Photosystem I-Ferredoxin Interface

The most crucial step in oxygenic Photosynthesis, which maintains our biosphere, is the light-driven charge separation catalyzed by Photosystem I (PSI) and Photosystem II (PSII). Cyanobacterial PSI exists in monomeric, trimeric, and tetrameric forms, as opposed to the monomeric form of PSI in plants and algae. We have reported the first ~3.72 Å resolution cryo-EM structure of tetrameric PSI from the thermophilic cyanobacterium Chroococcidiopsis TS-821. We conclude that the tetramer is arranged via two different interfaces resulting from a dimer-of-dimers organization. Tetrameric PSI may function as a key evolutionary step between the trimeric and monomeric forms of PSI organization in photosynthetic organisms. The tetrameric organization raises questions about its structural, physiological, and evolutional significance. We also highlight the investigation of the structural changes that takes place within microseconds involved in the electron transfer from the lumenal side of cyanobacterial trimeric PSI to stromal end and between PSI to Fd, its primary electron acceptor. We attempted to investigate their subsequent dissociation, using pump-probe Serial Femtosecond Crystallography at X-Ray Free-Electron Laser facilities. We have established conditions to grow crystals of PSI-Fd co-complex. Understanding the highly conserved molecular mechanisms that enable electron transfer from PSI to Fd, and Fd unbinding will be broadly applicable across photosynthetic organisms and serve as a model system to study inter-protein electron transfer. This interaction is also crucial to understand the assembly of our model bio-hybrid solar cells for the purpose of applied photosynthesis. We have computationally and biochemically generated single and double mutants (combination of both) of Fd from thermophilic cyanobacteria Thermosynechococcus elongatus BP-1 with potentially enhanced or ‘sticky’ binding affinity to PSI. We also describe a novel spectroscopic method using Joliot Type Spectrophotometer (JTS-100) for studying the affinity of Fd and potential high-affinity Fd mutants for PSI.