title: Pathway of proton transfer in bacterial reaction centers: replacement of glutamic acid 212 in the L subunit by glutamine inhibits quinone (secondary acceptor) turnover.
abstract: 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.
authors.affiliation: Department of Physics, University of California, San Diego, La Jolla 92093.
authors.forename: M L
authors.initials: ML
authors.lastname: Paddock
authors.forename: S H
authors.initials: SH
authors.lastname: Rongey
authors.forename: G
authors.initials: G
authors.lastname: Feher
authors.forename: M Y
authors.initials: MY
authors.lastname: Okamura
mesh_terms: D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism*; D002872:Chromosome Deletion; D003574:Cytochrome c Group / Q000378:metabolism; D004579:Electron Transport; D005796:Genes; D005798:Genes, Bacterial; D005971:Glutamates; D018698:Glutamic Acid; D005973:Glutamine; D007700:Kinetics; D008027:Light; D008958:Models, Molecular; D009154:Mutation; D045322:Photosynthetic Reaction Center Complex Proteins; D011487:Protein Conformation; D011809:Quinones / Q000378:metabolism*; D015183:Restriction Mapping; D012242:Rhodobacter sphaeroides / Q000235:genetics / Q000378:metabolism*
chemical_list: D001426:Bacterial Proteins; D003574:Cytochrome c Group; D005971:Glutamates; D045322:Photosynthetic Reaction Center Complex Proteins; D011809:Quinones; D005973:Glutamine; D018698:Glutamic Acid