Posters P4Ab Type II reaction centres: Acceptor side. Abstracts (272-288)
Characteristics of ubiquinone reduction at the QB site following B-branch electron transfer in the Rhodobacter sphaeroides reaction centre. Marion Wakeham*,1, Jacques Breton*,2, Paul Fyfe*,1, Kathryn Norris1, Eliane Nabedryk2, Michael Jones1, 1 Department of Biochemistry, Bristol, United Kingdom2 Service de Bioénergétique, France
ABSTRACT- In Rhodobacter sphaeroides reaction centres (RCs) containing the mutation Ala M260 to Trp (AM260W), transmembrane electron transfer along the full length of the A-branch of cofactors is prevented by the loss of the QA ubiquinone. Reaction centres that contain this AM260W mutation are proposed to photoaccumulate the P+QB- radical pair following transmembrane electron transfer along the B-branch of cofactors. The yield of this state appears to depend upon which additional mutations are present. Light induced FTIR difference spectroscopy has been used to probe the neutral QB and the semiquinone QB- states in reaction centres containing mutations in addition to the AM260W mutation. The resulting P+Q-/PQ difference spectra were compared with spectra obtained with reaction centres where the normal A-branch is functional. Using continuous illumination, reduction of QB via the B-branch pathway is seen to occur in the entire reaction centre population at room temperature, and in approximately 10 % of reaction centres at cryogenic temperature. Isotope edited IR fingerprint spectra were obtained for reaction centres reconstituted with site-specific 13C-labelled ubiquinone. It is concluded that QB occupies the same binding position, proximal to the non-haem iron, prior to reduction by either A- or B-branch electron transfer.
KEY WORDS: reaction centre, Rhodobacter sphaeroides, ubiquinone, B-branch