Energetics of the Electron Transfer Pathways in the Homodimeric Photosynthetic Reaction Center

[Image: see text] Photosynthetic reaction centers from a green sulfur bacterium (GsbRC), the PscA/PscA proteins, and photosystem I (PSI), PsaA/PsaB proteins, share structural similarities. Here, we report the redox potential (E(m)) values of GsbRC by solving the linear Poisson–Boltzmann equation and considering the protonation states of all titratable sites in the entire GsbRC protein and identify the factors that shift the E(m) values with respect to PSI. The E(m) values for one-electron reduction of the accessory (A(–1)) and adjacent (A(0)) chlorophylls in GsbRC are 100–250 mV higher than those in PSI, whereas the E(m) values for the Fe(4)S(4) cluster (F(X)) are at the same level. The PsaA-Trp697/PsaB-Trp677 pair in PSI, which forms the A(1)-quinone binding site, is replaced with PscA-Arg638 in GsbRC. PsaB-Asp575 in PSI, which is responsible for the E(m) difference between A(1A) and A(1B) quinones in PSI, is absent in GsbRC. These discrepancies also contribute to the upshift in E(m)(A(–1)) and E(m)(A(0)) in GsbRC with respect to PSI. It seems likely that the upshifted E(m) for chlorophylls in GsbRC ultimately originates from the characteristics of the electrostatic environment that corresponds to the A(1) site of PSI.