This Article Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Taly, A. Articles by Ullmann, G. M. Search for Related Content PubMed PubMed Citation Articles by Taly, A. Articles by Ullmann, G. M.

The Position of Q_B in the Photosynthetic Reaction Center Depends on pH: A Theoretical Analysis of the Proton Uptake upon Q_B Reduction

Antoine Taly^*,, Pierre Sebban, Jeremy C. Smith^* and G. Matthias Ullmann^*

^* Biocomputing Group, IWR, INF 368, Universität Heidelberg, D-69120 Heidelberg, Germany; and Centre de Génétique Moléculaire, Avenue de la Terrasse, F-91198 Gif-sur-Yvette, France

Correspondence: Address reprint requests to G. Matthias Ullmann, E-mail: matthias.ullmann{at}iwr.uni-heidelberg.de.

Electrostatics-based calculations have been performed to examine the proton uptake upon reduction of the terminal electron acceptor Q_B in the photosynthetic reaction center of Rhodobacter sphaeroides as a function of pH and the associated conformational equilibrium. Two crystal structures of the reaction center were considered: one structure was determined in the dark and the other under illumination. In the two structures, the Q_B was found in two different positions, proximal or distal to the nonheme iron. Because Q_B was found mainly in the distal position in the dark and only in the proximal position under illumination, the two positions have been attributed mostly to the oxidized and the reduced forms of Q_B, respectively. We calculated the proton uptake upon Q_B reduction by four different models. In the first model, Q_B is allowed to equilibrate between the two positions with either oxidation state. This equilibrium was allowed to vary with pH. In the other three models the distribution of Q_B between the proximal position and the distal position was pH-independent, with Q_B occupying only the distal position or only the proximal position or populating the two positions with a fixed ratio. Only the first model, which includes the pH-dependent conformational equilibrium, reproduces both the experimentally measured pH dependence of the proton uptake and the crystallographically observed conformational equilibrium at pH 8. From this model, we find that Q_B occupies only the distal position below pH 6.5 and only the proximal position above pH 9.0 in both oxidation states. Between these pH values both positions are partially occupied. The reduced Q_B has a higher occupancy in the proximal position than the oxidized Q_B. In summary, the present results indicate that the conformational equilibrium of Q_B depends not only on the redox state of Q_B, but also on the pH value of the solution.

This article has been cited by other articles:

				R. H. G. Baxter, N. Ponomarenko, V. Srajer, R. Pahl, K. Moffat, and J. R. Norris Time-resolved crystallographic studies of light-induced structural changes in the photosynthetic reaction center PNAS, April 20, 2004; 101(16): 5982 - 5987. [Abstract] [Full Text] [PDF]