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

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.