Flash-induced H+ binding by bacterial photosynthetic reaction centers: Influences of the redox states of the acceptor quinones and primary donor

The flash-induced proton-binding behavior of reaction centers from Rhodobacter sphaeroides was examined, over a wide range of pH, as a function of the one-electron redox states of the acceptor quinones (QAQ−A and QBQ−B) and the primary donor (P+P). Below about pH 9, the P+Q− states (P+Q−A and P+Q−B), generated in the absence of exogenous electron donor to P+, fail to take up protons stoichiometrically, as established in the previous paper (Maroti, P. and Wraight, C.A. (1988) Biochim. Biophys. Acta 934, 314–328). When P+ is rereduced by a donor, to yield the state PQ−, proton binding is enhanced in this lower pH range. In the case of QB-reconstituted reaction centers, the net proton binding stoichiometry (H+P+) for PQ−B is about 0.85, between pH 7 and pH 9, approaching the stoichiometric value expected for a pH-dependent redox midpoint potential of QBQ−B. The shortfall from H+P+ = 1.0 can be accounted for by the involvement of four protonatable groups with different pK values depending on the redox state of QB. The pK shifts vary from 0.3 to 1.5 pH units, with the lower pK groups exhibiting the smaller pK shifts. The enhancement of proton binding, associated with the rereduction of P+, is interpreted as a response of the same groups to the redox state of P+P, with the lower pK groups exhibiting the largest pK shifts - up to 1.0 pH unit. A similar general behavior is seen for reaction centers lacking QB, or in the presence of terbutryn, a competitive inhibitor of QB-binding. Quantitatively, the rereduction of P+ does not restore such high levels of H+ binding for PQ−A or PQ−A+ terbutryn (H+P+ ⩽ 0.5 at all pH values), but the behavior can be similarly accounted for by four protonatable groups that are somewhat less responsive to the redox states of QA and P. The pK values for the three different acceptor configurations (QB, QA and QA+ terbutryn) are similar but not identical, and depend on the redox states of the primary donor and the acceptor quinones, and on the occupancy of the QB-binding site. The pK values are discussed in terms of possible structural determinants of the quinone binding sites. The data define protonation networks for the reaction center states PQ, P+Q− and PQ−, and allow one to deduce the properties of a fourth state: P+Q. The derived pK values predict the occurrence of H+ release from the state P+Q, at low pH, and this was confirmed by using ferricyanide to reoxidize Q− following a flash to generate P+Q. The proposed protonation scheme allows the calculation of the pH dependence of the one-electron transfer equilibrium between P+Q−AQB and P+QAQ−B. This agrees well with the measured value, derived from the kinetics of charge recombination. However, the pK changes, derived from the enhanced proton binding that accompanies rereduction of P+, give rise to a substantial discrepancy between the calculated and measured values for the PQ−AQB ↔ PQAQ−B equilibrium. The pH dependences of the redox midpoint potentials (Em) of QAQ−A, QBQ−B and P+P are also calculated. Good agreement between calculated and measured values is obtained for P+P, and between the calculated value for QBQ−B and that expected from studies on chromatophores. However, the calculated pH dependence of Em(QAQ−A) is at variance with that measured in isolated reaction centers or chromatophores. These discrepancies are discussed but not resolved.