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 bacteri al photosynthetic reaction center (bRC) is coupled to a conformational rear rangement. Recently, the X-ray structures of the dark-adapted and light-exp osed bRC from Rhodobacter sphaeroides were solved, and the conformational c hanges were characterized structurally. We computed the reaction free energ y for the electron transfer from Q(A)(.-) to Q(B) in the X-ray structures o f the dark-adapted and light-exposed bRC from Rb, sphaeroides. The computat ion was done by applying an electrostatic model using the Poisson-Boltzmann equation and Monte Carlo sampling. We accounted for possible protonation c hanges of titratable groups upon electron transfer. According to our calcul ations, the reaction energy of the electron transfer from Q(A)(.-) to Q(B) is +157 meV for the dark-adapted and -56 meV for the light-exposed X-ray st ructure; i.e., the electron transfer is energetically uphill for the dark-a dapted structure and downhill for the Light-exposed structure. A common int erpretation of experimental results is that the electron transfer between Q (A)(.-) and Q(B) is either gated or at least influenced by a conformational rearrangement: A conformation in which the electron transfer from Q(A)(.-) to Q(B) is inactive, identified with the dark-adapted X-ray structure, cha nges into an electron-transfer active conformation, identified with the lig ht-exposed X-ray structure. This interpretation agrees with our computation al results if one assumes that the positive reaction energy for the dark-ad apted X-ray structure effectively prevents the electron transfer. We found that the strongly coupled pair of titratable groups Glu-L212 and Asp-L213 b inds about one proton in the dark-adapted X-ray structure, where the electr on 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 agre es 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 Q(A)(.-) to Q(B) and relate them to theoretical and experimental results.