ENERGETICS OF ELECTRON-TRANSFER AND PROTONATION REACTIONS OF THE QUINONES IN THE PHOTOSYNTHETIC REACTION-CENTER OF RHODOPSEUDOMONAS-VIRIDIS

The electron-transfer reactions involving the quinones in the bacteria l photosynthetic reaction center (bRC) are coupled to a proton uptake by the bRC. In this study, we calculated the energies of the different states of the bRC occurring during these electron-transfer and proton ation reactions by an electrostatic model. We considered the possibili ty that titratable groups of the bRC can change their protonation duri ng these reactions, The protonation probabilities of titratable groups were obtained by a Monte Carlo calculation. In contrast to earlier st udies by other groups, we used atomic partial charges derived from qua ntum-chemical calculations, Our calculated reaction energies are in ag reement with experiments. We found that the proton uptake by the bRC i s coupled more strongly to changes of the redox state of the quinones than to changes of their protonation state. Thus, the proton uptake by the bRC occurs predominantly before the protonation of QB. According to our computations, the reduction of Q(B)(.-) to the doubly negative state Q(B)(2-) is energetically even more unfavorable in the bRC than in solution. Therefore, we suggest that the second electron transfer f rom Q(A), to QB occurs after QB has received its first proton. We foun d that the Q(A)(.-)Q(B)(.-) state is more populated at pH 7.5 than the Q(A)(.-)Q(B)(.)H state. The low population of the Q(A)(.)-Q(B)(.)H st ate may be the reason why the singly protonated QB could not be detect ed spectroscopically. Our calculations imply that the first protonatio n of Q(B)(.-) is a prerequisite for the second electron transfer betwe en QA and Qs Therefore, a pH dependence of the equilibrium between the states Q(A)(.-)Q(B)(.-) and Q(A)(.-)Q(B)(.)H can also explain the exp erimentally observed pH dependence of the rate for the second electron -transfer step. On the basis of our calculated reaction energies, we p ropose the following sequence for the electron-transfer and protonatio n reactions: (1) first electron transfer from Q(A) to Q(B). (2) first protonation of Q(B) (at the distal oxygen close to Ser L223), (3) seco nd electron transfer from Q(A) to Q(B), and (4) second protonation of QB (at the proximal oxygen close to His L190).