Electron transfer between the quinones in the photosynthetic reaction center and its coupling to conformational changes

The electron transfer between the two quinones Q(A) and Q(B) in the bacterial photosynthetic reaction center (bRC) is coupled to a conformational rearrangement. Recently, the X-ray structures of the dark-adapted and light-exposed bRC from Rhodobacter sphaeroides were solved, and the conformational changes were characterized structurally. We computed the reaction free energy for the electron transfer from to Q(B) in the X-ray structures of the dark-adapted and light-exposed bRC from Rb. sphaeroides. The computation was done by applying an electrostatic model using the Poisson-Boltzmann equation and Monte Carlo sampling. We accounted for possible protonation changes of titratable groups upon electron transfer. According to our calculations, the reaction energy of the electron transfer from to Q(B) is +157 meV for the dark-adapted and -56 meV for the light-exposed X-ray structure; i.e., the electron transfer is energetically uphill for the dark-adapted structure and downhill for the light-exposed structure. A common interpretation of experimental results is that the electron transfer between and Q(B) is either gated or at least influenced by a conformational rearrangement: A conformation in which the electron transfer from to Q(B) is inactive, identified with the dark-adapted X-ray structure, changes into an electron-transfer active conformation, identified with the light-exposed X-ray structure. This interpretation agrees with our computational results if one assumes that the positive reaction energy for the dark-adapted X-ray structure effectively prevents the electron transfer. We found that the strongly coupled pair of titratable groups Glu-L212 and Asp-L213 binds about one proton in the dark-adapted X-ray structure, where the electron is mainly localized at Q(A), and about two protons in the light-exposed structure, where the electron is mainly localized at Q(B). This finding agrees with recent experimental and theoretical studies. We compare the present results for the bRC from Rb. sphaeroides to our recent studies on the bRC from Rhodopseudomonas viridis. We discuss possible mechanisms for the gated electron transfer from to Q(B) and relate them to theoretical and experimental results.