Exploring the energy profile of the Q(A)(-) to Q(B) electron transfer reaction in bacterial photosynthetic reaction centers: pH dependence of the conformational gating step

Both large- and small-scale conformational changes are needed as proteins carry out reactions. However, little is known about the identity, energy of, and barriers between functional substates on protein reaction coordinates. In isolated bacterial photosynthetic reaction centers, the electron transfer from the reduced primary quinone, Q(A)(-), to the secondary quinone, Q(B), is rate limited by conformational changes at low pH and by proton binding at high pH. The kinetics and thermodynamics of this reaction were determined between 200 and 300 K from pH 6 to pH 10.5. A model with two substates of the reactant, P(+)Q(A)(-)Q(B), one protonated (state A) and one unprotonated (alpha), and one state of the product, P(+)Q(A)Q(B)(-) (B), was able to simulate the dependence of the rate on temperature and pH fairly well. The equilibrium between the three states were measured in situ at each temperature. Proton binding (alpha to A transition) has a favorable DeltaH and unfavorable DeltaS as does the conformational changes required for electron transfer at low pH (A to B). The pK for the A to alpha transition is 9.7 at room temperature, consistent with previous measurements, and equivalent to 13.5 at 200 K. The activation barriers were determined for each transition. Both the alpha to A and the A to B transitions are limited primarily by the activation enthalpy with modest DeltaS.