PARENT SESSION
Posters P4Ab Type II reaction centres: Acceptor side. Abstracts (272-288)

Characteristics of ubiquinone reduction at the Q_B 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 Q_A ubiquinone. Reaction centres that contain this AM260W mutation are proposed to photoaccumulate the P⁺Q_B^- 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 Q_B and the semiquinone Q_B^- 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 Q_B 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 Q_B 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