title: Formation of a semiquinone at the QB site by A- or B-branch electron transfer in the reaction center from Rhodobacter sphaeroides.
abstract: In Rhodobacter sphaeroides reaction centers containing the mutation Ala M260 to Trp (AM260W), transmembrane electron transfer along the A-branch of cofactors is prevented by the loss of the QA ubiquinone. Reaction centers that contain this AM260W mutation are proposed to photoaccumulate the P(+)QB- radical pair following transmembrane electron transfer along the B-branch of cofactors (Wakeham, M. C., Goodwin, M. G., McKibbin, C., and Jones, M. R. (2003) Photoaccumulation of the P(+)QB- radical pair state in purple bacterial reaction centers that lack the QA ubiquinone. FEBS Lett. 540, 234-240). The yield of the P(+)QB- state appears to depend upon which additional mutations are present. In the present paper, Fourier transform infrared (FTIR) difference spectroscopy was used to demonstrate that photooxidation of the reaction center's primary donor in QA-deficient reaction centers results in formation of a semiquinone at the QB site by B-branch electron transfer. Reduction of QB by the B-branch pathway still occurs at 100 K, with a yield of approximately 10% relative to that at room temperature, in contrast to the QA- to QB reaction in the wild-type reaction center, which is not active at cryogenic temperatures. These FTIR results suggest that the conformational changes that "gate" the QA- to QB reaction do not necessarily have the same influence on QB reduction when the electron donor is the HB anion, at least in a minority of reaction centers.
authors.affiliation: Department of Biochemistry, School of Medical Sciences, University of Bristol, University Walk, Bristol, BS8 1TD, United Kingdom.
authors.forename: Marion C
authors.initials: MC
authors.lastname: Wakeham
authors.forename: Jacques
authors.initials: J
authors.lastname: Breton
authors.forename: Eliane
authors.initials: E
authors.lastname: Nabedryk
authors.forename: Michael R
authors.initials: MR
authors.lastname: Jones
mesh_terms: D000409:Alanine / Q000235:genetics; D000900:Anti-Bacterial Agents / Q000737:chemistry; D016227:Benzoquinones / Q000037:antagonists & inhibitors / Q000737:chemistry* / Q000378:metabolism; D001665:Binding Sites / Q000235:genetics; D004579:Electron Transport / Q000235:genetics; D005615:Freezing; D008027:Light; D016297:Mutagenesis, Site-Directed; D010084:Oxidation-Reduction; D045322:Photosynthetic Reaction Center Complex Proteins / Q000037:antagonists & inhibitors / Q000737:chemistry* / Q000235:genetics; D011090:Polyenes / Q000737:chemistry; D012242:Rhodobacter sphaeroides / Q000737:chemistry* / Q000235:genetics; D017550:Spectroscopy, Fourier Transform Infrared; D014364:Tryptophan / Q000235:genetics; D014451:Ubiquinone / Q000235:genetics / Q000378:metabolism
chemical_list: D000900:Anti-Bacterial Agents; D016227:Benzoquinones; D045322:Photosynthetic Reaction Center Complex Proteins; D011090:Polyenes; D014451:Ubiquinone; C025232:semiquinone radicals; D014364:Tryptophan; C041573:stigmatellin; D000409:Alanine