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 associ ated with the different reduction states of the two quinone molecules const itute intrinsic probes of both the electrostatic interactions and the diffe rent kinetic events occurring within the protein in response to the light-g enerated introduction of a charge. The kinetics and stoichiometries of prot on uptake on formation of the primary semiquinone Q(A)(-) and the secondary acceptor Q(B)(-) after the first end 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 L 213Asp sites near Q(B); some of them carry additional mutations distant fro m the quinone sites (M231Arg --> Leu, M43Asn --> Asp, M5Asn --> Asp) that c ompensate 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 fan restore the normal response, and the act ivity of a tyrosine residue (M247Ala --> Tyr) in increasing and acceleratin g proton uptake. The data demonstrate a direct correlation between the kine tic events of proton uptake that are observed with the formation of either Q(A)(-) or Q(B)(-), suggesting that the same residues respond to the genera tion of either semiquinone species. Therefore, the efficiency of transferri ng the first proton to Q(B) is evident from examination of the pattern of H +/Q(A)(-) proton uptake. This delocalized response of the protein complex t o the introduction of a charge is coordinated by an interactive network tha t links the Q(-) species, polarizable residues, and numerous water molecule s that are located in this region of the reaction center structure. This co uld be a general property of transmembrane redox proteins that couple elect ron transfer to proton uptake/release reactions.