Formation of a Semiquinone at the QB Site by A- or B-Branch Electron Transfer in the Reaction Center from Rhodobacter sphaeroides

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• 作者：Marion C. Wakeham ; Jacques Breton ; Eliane Nabedryk ; and Michael R. Jones
• 刊名：Biochemistry
• 出版年：2004
• 出版时间：April 27, 2004
• 年：2004
• 卷：43
• 期：16
• 页码：4755 - 4763
• 全文大小：139K
• 年卷期：v.43,no.16(April 27, 2004)
• ISSN：1520-4995

文摘

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 ofthe Q_A ubiquinone. Reaction centers that contain this AM260W mutation are proposed to photoaccumulatethe P⁺Q_B^- 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 theP⁺Q_B^- radical pair state in purple bacterial reaction centers that lack the Q_A ubiquinone. FEBS Lett. 540,234-240). The yield of the P⁺Q_B^- state appears to depend upon which additional mutations are present.In the present paper, Fourier transform infrared (FTIR) difference spectroscopy was used to demonstratethat photooxidation of the reaction center's primary donor in Q_A-deficient reaction centers results information of a semiquinone at the Q_B site by B-branch electron transfer. Reduction of Q_B by the B-branchpathway still occurs at 100 K, with a yield of approximately 10% relative to that at room temperature, incontrast to the Q_A^- to Q_B reaction in the wild-type reaction center, which is not active at cryogenictemperatures. These FTIR results suggest that the conformational changes that "gate" the Q_A^- to Q_Breaction do not necessarily have the same influence on Q_B reduction when the electron donor is the H_Banion, at least in a minority of reaction centers.