Energetics of proton transfer pathways in reaction centers from Rhodobacter sphaeroides. The Glu-H173 activated mutants.
Electron transfer between the primary and secondary quinones (Q(A), Q(B)) in the bacterial photosynthetic reaction center (bRC) is coupled with proton uptake at Q(B). The protons are conducted from the cytoplasmic side, probably with the participation of two water channels. Mutations of titratable residues like Asp-L213 to Asn (inhibited mutant) or the double mutant Glu-L212 to Ala/Asp-L213 to Ala inhibit these electron transfer-coupled proton uptake events. The inhibition of the proton transfer (PT) process in the single mutant can be restored by a second mutation of Arg-M233 to Cys or Arg-H177 to His (revertant mutant). These revertant mutants shed light on the location of the main proton transfer pathway of wild type bRC. In contrast to the wild type and inhibited mutant bRC, the revertant mutant bRC showed notable proton uptake at Glu-H173 upon formation of the Q(B)- state. In all of these mutants, the pK(a) of Asp-M17 decreased by 1.4-2.4 units with respect to the wild type bRC, whereas a significant pK(a) upshift of up to 5.8 units was observed at Glu-H122, Asp-H170, Glu-H173, and Glu-H230 in the revertant mutants. These residues belonging to the main PT pathway are arranged along water channel P1 localized mainly in subunit H. bRC possesses subunit H, which has no counterpart in photosystem II. Thus, bRC may possess alternative PT pathways involving water channels in subunit H, which becomes active in case the main PT pathway is blocked.
Ishikita
Hiroshi
H
Institute of Chemistry, Free University of Berlin, Takustrasse 6, Berlin D-14195, Germany.
Knapp
Ernst-Walter
EW
D000409:Alanine / Q000737:chemistry; D003545:Cysteine / Q000737:chemistry; D003593:Cytoplasm / Q000378:metabolism; D004579:Electron Transport; D004583:Electrons; D018698:Glutamic Acid / Q000737:chemistry; D006639:Histidine / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008956:Models, Chemical; D008958:Models, Molecular; D009154:Mutation; D009584:Nitrogen / Q000737:chemistry; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry; D011522:Protons; D012242:Rhodobacter sphaeroides / Q000201:enzymology* / Q000235:genetics; D012984:Software
D011522:Protons; D018698:Glutamic Acid; D006639:Histidine; D003545:Cysteine; D009584:Nitrogen; D000409:Alanine; D010100:Oxygen