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 QA ubiquinone. Reaction centers that contain this AM260W mutation are proposed to photoaccumulatethe 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 theP+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 demonstratethat photooxidation of the reaction center's primary donor in QA-deficient reaction centers results information of a semiquinone at the QB site by B-branch electron transfer. Reduction of QB by the B-branchpathway still occurs at 100 K, with a yield of approximately 10% relative to that at room temperature, incontrast to the QA- to QB reaction in the wild-type reaction center, which is not active at cryogenictemperatures. These FTIR results suggest that the conformational changes that "gate" the QA- to QBreaction do not necessarily have the same influence on QB reduction when the electron donor is the HBanion, at least in a minority of reaction centers.