Proton Uptake of Rhodobacter capsulatus Reaction Center Mutants Modified in the Primary Quinone Environment

Flash-induced absorbance spectroscopy was used to analyze the proton uptake and electron transfer properties of photosynthetic reaction centers (RC) of Rhodobacter capsulatus that have been genetically modified near the primary quinone electron acceptor (QA). M246Ala and M247Ala, which are symmetry-related to the positions of two acidic groups, L212Glu and L213Asp, in the secondary quinone electron acceptor (QB) protein environment, have been mutated to Glu and Asp, respectively. The pH dependence of the stoichiometry of proton uptake upon formation of the P+QA− (H+/P+QA−) and PQA− (H+/QA−) (P is the primary electron donor, a noncovalently linked bacteriochlorophyll dimer) states have been measured in the M246Ala → Glu and the M247Ala → Asp mutant RC, in the M246Ala–M247Ala → Glu–Asp double mutant and in the wild type (WT). Our results show that the introduction of an acidic group (Glu or Asp) in the QA protein region induces notable additional proton uptake over a large pH region (∼6–9), which reflects a delocalized response of the protein to the formation of QA−. This may indicate the existence of a widely spread proton reservoir in the cytoplasmic region of the protein. Interestingly, the pH titration curves of the proton release caused by the formation of P+ (H+/P+: difference between H+/P+QA− and H+/PQA− curves) are nearly superimposable in the WT and the M246Ala → Glu mutant RC, but substantial additional proton release is detected between pH 7 and 9 in the M247Ala → Asp mutant RC. This effect can be accounted for by an increased proton release by the P+ environment in the M247Ala → Asp mutant. The M247Ala → Asp mutation reveals the existence of an energetic and conformational coupling between donor and acceptor sides of the RC at a distance of nearly 30Å.