Redox Potentials of Iron-Sulfur Clusters in Type I Photosynthetic Reaction Centers

The electron transfer pathways in type I photosynthetic reaction centers, such as photosystem I (PSI) and reaction centers from green sulfur bacteria (GsbRC), are terminated by two Fe₄S₄ clusters, F_A and F_B. The protein structures are the basis of understanding how the protein electrostatic environment interacts with the Fe₄S₄ clusters and facilitates electron transfer. Using the protein structures, we calculated the redox potential (E_m) values for F_A and F_B in PSI and GsbRC, solving the linear Poisson-Boltzmann equation. The F_A-to-F_B electron transfer is energetically downhill in the cyanobacterial PSI structure, while it is isoenergetic in the plant PSI structure. The discrepancy arises from differences in the electrostatic influences of conserved residues, including PsaC-Lys51 and PsaC-Arg52, located near F_A. The F_A-to-F_B electron transfer is slightly downhill in the GsbRC structure. E_m(F_A) and E_m(F_B) exhibit similar levels upon isolation of the membrane-extrinsic PsaC and PscB subunits from the PSI and GsbRC reaction centers, respectively. The binding of the membrane-extrinsic subunit at the heterodimeric/homodimeric reaction center plays a key role in tuning E_m(F_A) and E_m(F_B).