The position of QB in the photosynthetic reaction center depends on pH: a theoretical analysis of the proton uptake upon QB reduction

Abstract

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.

FIGURE 1

Illustration of the crystallographically determined equilibrium (Stowell et al., 1997). The left side represents the structure of dark-adapted RCs, in which Q_B is oxidized. Q_B is seen in two positions: proximal and distal. The structures with proximal and distal Q_B are called RC^prox and RC^dist, respectively. The right side represents the structure for light-exposed RCs, in which Q_B is reduced. No electron density is observed experimentally for RC^dist.

FIGURE 2

Illustration of the four models used to test the relationship between RC conformations and proton uptake upon reduction of Q_B. (a) Model 1 describes a pH-dependent conformational equilibrium between RC^prox and RC^dist for Q_B and Q_B⁻. (b) Model 2 uses the populations of RC^prox and RC^dist for Q_B and Q_B⁻ found with the first model at pH 8 and keeps the ratio of the different conformations constant over the whole pH range. (c) Model 3 uses only the RC^prox structure for both redox states of Q_B over the whole pH range. (d) Model 4 uses only the RC^dist structure for both redox states of Q_B over the whole pH range.

FIGURE 3

(a) Proton uptake upon reduction of Q_B, calculated using Model 1, which considers a conformational equilibrium between RC^prox and RC^dist for Q_B and Q_B⁻ is shown by the black line. The experimental proton uptake at the different pH values are shown by gray circles. (b) Equilibrium between RC^prox and RC^dist structures. The calculated ration RC^prox/(RC^prox + RC^dist) is shown for oxidized (dashed line) and reduced (solid line) Q_B.

FIGURE 4

Decomposition of the proton uptake obtained with Model 1. Experimental proton uptake is shown by gray circles, total proton uptake by a solid line, contribution of residues GLU-L212 and ASP-L213 by a dashed line, and the contribution of the remaining residues, which are listed in the main text, are shown by a dotted line.

FIGURE 5

Proton uptake of the RC upon reduction of Q_B. Comparison of calculated (solid line) and experimental data (circles). The calculations were performed with (a) Model 2. (b) Model 3. (c) Model 4. The proton uptake of Model 1 is shown as a dotted line for comparison.