Proton Uptake by Bacterial Reaction Centers: The Protein Complex Responds in a Similar Manner to the Reduction of Either Quinone Acceptor

In bacterial photosynthetic reaction centers, the protonation events associated with the different reduction states of the two quinone molecules constitute intrinsic probes of both the electrostatic interactions and the different kinetic events occurring within the protein in response to the light-generated introduction of a charge. The kinetics and stoichiometries of proton uptake on formation of the primary semiquinone QA -and the secondary acceptor QB -after the first and second flashes have been measured, at pH 7.5, in reaction centers from genetically modified strains and from the wild type. The modified strains are mutated at the L212Glu and/or at the L213Asp sites near QB; some of them carry additional mutations distant from the quinone sites (M231Arg→ Leu, M43Asn→ Asp, M5Asn→ Asp) that compensate for the loss of L213Asp. Our data show that the mutations perturb the response of the protein system to the formation of a semiquinone, how distant compensatory mutations can restore the normal response, and the activity of a tyrosine residue (M247Ala→ Tyr) in increasing and accelerating proton uptake. The data demonstrate a direct correlation between the kinetic events of proton uptake that are observed with the formation of either QA -or QB -,, suggesting that the same residues respond to the generation of either semiquinone species. Therefore, the efficiency of transferring the first proton to QBis evident from examination of the pattern of H+/QA -proton uptake. This delocalized response of the protein complex to the introduction of a charge is coordinated by an interactive network that links the Q-species, polarizable residues, and numerous water molecules that are located in this region of the reaction center structure. This could be a general property of transmembrane redox proteins that couple electron transfer to proton uptake/release reactions.