Pathway of proton transfer in bacterial reaction centers: replacement of glutamic acid 212 in the L subunit by glutamine inhibits quinone (secondary acceptor) turnover.

The mechanism of proton transfer in the reaction centers (RCs) from Rhodobacter sphaeroides was investigated by site-directed mutagenesis. Replacement of Glu-212 of the L subunit, a protonatable residue located near the secondary acceptor (QB) binding site, by glutamine reduced the in vitro electron turnover from cytochrome c to 2,3-dimethoxy-5-methylbenzoquinone (UQ0) by a factor of 25. The electron transfer rate to QB remained essentially unimpaired. Consequently, it is postulated that the reduced turnover in the mutant is due to a reduced rate of proton transfer to QB2-. The lack of pH dependence of the forward electron transfer rate DQA-QB----DQAQB- and the back reaction rate D+QAQB- ----DQAQB (where D = primary donor and QA = primary acceptor) in the mutant RC indicate that the observed pH dependence in the native RC is due to Glu-212, which has an anomalously high pKa value of 9.5 +/- 0.3. These results support the involvement of Glu-212 as a proton donor to reduced QB.