Why the electron chooses one of two symmetry-related paths in the type II bacterial photosynthetic reaction centers

A series of electron and proton transfer reactions are carried out in photosynthetic reaction centers that convert the energy of light into high energy reduced products and add to the proton gradient. All photosynthetic reaction centers (photosystems I, II, and different bacterial reaction centers (bRCs)) have their charge separating cofactors arranged with two, <i>c</i>₂-symmetric paths from a primary donor to a terminal acceptor. In Type II reaction centers (PSII and bRCs), electrons utilize only one active branch. In bRCs the electron transfer occurs in the A branch from the excited state of the bacteriochlorophyll (BChl) dimer P₈₆₀ through BChl_A, bacteriopheophytin_A (BPh_A), to ubiquinone Q_A, and then to the B-branch Q_B. B-branch BChl_B and BPh_B do not participate. A similar path is found in PSII. These proteins challenge our understanding of how proteins can turn on or turn off the activity of bound ligands. The shift of the electrochemical midpoint potential (<i>E</i>_m) of each cofactor by the protein environment is calculated using continuum electrostatics within the Multi-Conformation Continuum Electrostatic (MCCE) program. In bRCs from <i>Rhodobacter sphaeroides</i>, the electrostatic potentials favor reduction of the active branch cofactors. The residues that contribute to a more positive potential on the A branch are identified. These are often distant from the cofactors. The large difference of the potential at the competing BChls may direct the electron transfer to the A branch. Residues TyrM210 and PheL181 are at symmetrical positions on the A and B branches, near P₈₆₀, between BChl_A and BChl_B. Mutated bRCs, that swap the Tyr and Phe (YF mutant), have been shown to support some electron transfer to the B-branch cofactors. The calculated <i>E</i>_ms with the YF swap show more positive B-branch potentials, which help explain why there is now electron transfer along the B branch.