Proton translocation by the cytochrome bc(1) complexes of phototrophic bacteria: introducing the activated Q-cycle
|Mulkidjanian, Armen Y.
|B-C2 OXIDOREDUCTASE; Biochemistry & Molecular Biology; Biophysics; Chemistry; Chemistry, Physical; CRYSTAL-STRUCTURE; IRON-SULFUR PROTEIN; OXIDOREDUCTASE COMPLEX; PHOTOSYNTHETIC ELECTRON-TRANSFER; RESPIRATORY-CHAIN; RHODOBACTER-CAPSULATUS; RHODOPSEUDOMONAS-SPHAEROIDES; SINGLE TURNOVER FLASH; UBIQUINOL-OXIDATION
|PHOTOCHEMICAL & PHOTOBIOLOGICAL SCIENCES
The cytochrome bc(1) complexes are proton-translocating, dimeric membrane ubiquinol:cytochrome c oxidoreductases that serve as ``hubs'' in the vast majority of electron transfer chains. After each ubiquinol molecule is oxidized in the catalytic center P at the positively charged membrane side, the two liberated electrons head out, according to the Mitchell's Q-cycle mechanism, to different acceptors. One is taken by the [2Fe-2S] iron-sulfur Rieske protein to be passed further to cytochrome c(1). The other electron goes across the membrane, via the low- and high-potential hemes of cytochrome b, to another ubiquinone-binding site N at the opposite membrane side. It has been assumed that two ubiquinol molecules have to be oxidized by center P to yield first a semiquinone in center N and then to reduce this semiquinone to ubiquinol. This review is focused on the operation of cytochrome bc(1) complexes in phototrophic purple bacteria. Their membranes provide a unique system where the generation of membrane voltage by light-driven, energy-converting enzymes can be traced via spectral shifts of native carotenoids and correlated with the electron and proton transfer reactions. An ``activated Q-cycle'' is proposed as a novel mechanism that is consistent with the available experimental data on the electron/proton coupling. Under physiological conditions, the dimeric cytochrome bc(1) complex is suggested to be continually primed by prompt oxidation of membrane ubiquinol via center N yielding a bound semiquinone in this center and a reduced, high-potential heme b in the other monomer of the enzyme. Then the oxidation of each ubiquinol molecule in center P is followed by ubiquinol formation in center N, proton translocation and generation of membrane voltage.
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