Demethyl Ubiquinone Inhibits Catalytic Qb Activity In Reaction Centers From Rhodobacter Sphaeroides

Catalytic light induced electron transfer in the photosynthetic reaction center (RC) involves reduction of the loosely bound secondary quinone QB. In this study we investigated the activity of demethyl ubiquionone (De-Q), the immediate precursor for the synthesis of ubiquinone (UQ), a critical component of the electron transport pathways in both prokaryotes and eukaryotes (1). Upon addition of 2 microM De-Q (KD ∼ 0.2 microM), catalytic QB activity was inhibited > 10-fold at pH 8 (Figure). These results show that De-Q binds more tightly than UQ thereby inhibiting its reduction. In addition, optical shifts of bacteriocholorophyll were observed consistent with an anionic De-Q; the solution pKa of De-Q was measured to be ∼ 6 (not shown). We propose that De-Q near neutral pH is anionic which facilitates binding acting as a non-reducible analog of ubisemiquinone in the RC. (1) Poon et al. (1999) J Biol Chem. 274, 21665-21672. ∗Supported by NIH (GM 41637).