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
(Em) 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 Em values with
respect to PSI. The Em values for one-electron
reduction of the accessory (A–1) and adjacent (A0) chlorophylls in GsbRC are 100–250 mV higher than
those in PSI, whereas the Em values for
the Fe4S4 cluster (FX) are at the
same level. The PsaA-Trp697/PsaB-Trp677 pair in PSI, which forms the
A1-quinone binding site, is replaced with PscA-Arg638 in
GsbRC. PsaB-Asp575 in PSI, which is responsible for the Em difference between A1A and A1B quinones in PSI, is absent in GsbRC. These discrepancies also contribute
to the upshift in Em(A–1) and Em(A0) in GsbRC with
respect to PSI. It seems likely that the upshifted Em for chlorophylls in GsbRC ultimately originates from
the characteristics of the electrostatic environment that corresponds
to the A1 site of PSI.