The mitochondrial cytochrome bc(1) complex catalyzes the transfer of electrons from ubiquinol to cyt c while generating a proton motive force for ATP synthesis via the "Q-cycle" mechanism. Under certain conditions electron flow through the Q-cycle is blocked at the level of a reactive intermediate in the quinol oxidase site of the enzyme, resulting in "bypass reactions," some of which lead to superoxide production. Using analogs of the respiratory substrates ubiquinol-3 and rhodoquinol-3, we show that the relative rates of Q-cycle bypass reactions in the Saccharomyces cerevisiae cyt bc(1) complex are highly dependent by a factor of up to 100-fold on the properties of the substrate quinol. Our results suggest that the rate of Q-cycle bypass reactions is dependent on the steady state concentration of reactive intermediates produced at the quinol oxidase site of the enzyme. We conclude that normal operation of the Q-cycle requires a fairly narrow window of redox potentials with respect to the quinol substrate to allow normal turnover of the complex while preventing potentially damaging bypass reactions.
Modification of quinone electrochemistry by the proteins in the biological electron transfer chains: examples from photosynthetic reaction centers
A semiquinone intermediate generated at the Qo site of the cytochrome bc1 complex: importance for the Q-cycle and superoxide production
EPR and ENDOR Investigation of Rhodosemiquinone in Bacterial Reaction Centers Formed by B-Branch Electron Transfer
Evidence that ubiquinone is a required intermediate for rhodoquinone biosynthesis in Rhodospirillum rubrum
The Q-cycle reviewed: How well does a monomeric mechanism of the bc(1) complex account for the function of a dimeric complex?
Discrimination between two possible reaction sequences that create potential risk of generation of deleterious radicals by cytochrome bc₁. Implications for the mechanism of superoxide production
Molecular mechanisms of superoxide production by complex III: a bacterial versus human mitochondrial comparative case study
Redox-linked protonation state changes in cytochrome bc1 identified by Poisson-Boltzmann electrostatics calculations
Computation of the redox and protonation properties of quinones: towards the prediction of redox cycling natural products
Similar transition states mediate the Q-cycle and superoxide production by the cytochrome bc1 complex
Polarographic studies on ubiquinone-10 and rhodoquinone bound with chromatophores from Rhodospirillum rubrum
Effect of substituents of the benzoquinone ring on electron-transfer activities of ubiquinone derivatives
Electron-transfer complexes of Ascaris suum muscle mitochondria. III. Composition and fumarate reductase activity of complex II
Purification of highly active cytochrome bc1 complexes from phylogenetically diverse species by a single chromatographic procedure
Thermodynamic and kinetic considerations of Q-cycle mechanisms and the oxidant-induced reduction of cytochromes b
Occurrence of ubiquinone and rhodoquinone in parasitic nematodes, Metastrangylus elongatus and Ascaris lumbricoides var. suis
Photooxidase system of Rhodospirillum rubrum III. The role of rhodoquinone and ubiquinone in the activity of preparations of chromatophores and photoreaction centers
Electrostatic calculations of amino acid titration and electron transfer, Q-AQB-->QAQ-B, in the reaction center
Cloning and characterization of COX14, whose product is required for assembly of yeast cytochrome oxidase
Cyclic electron transfer in Heliobacillus mobilis involving a menaquinol-oxidizing cytochrome bc complex and an RCI-type reaction center
Yeast and rat Coq3 and Escherichia coli UbiG polypeptides catalyze both O-methyltransferase steps in coenzyme Q biosynthesis
Ubiquinone at center N is responsible for triphasic reduction of cytochrome b in the cytochrome bc(1) complex
Free-living nematodes Caenorhabditis elegans possess in their mitochondria an additional rhodoquinone, an essential component of the eukaryotic fumarate reductase system
The energy landscape for ubihydroquinone oxidation at the Q(o) site of the bc(1) complex in Rhodobacter sphaeroides
Different sphingolipids show differential partitioning into sphingolipid/cholesterol-rich domains in lipid bilayers
Proton-coupled electron transfer at the Q(o) site: what type of mechanism can account for the high activation barrier?
Effects of mutations in mitochondrial cytochrome b in yeast and man. Deficiency, compensation and disease
Analysis of suppressor mutation reveals long distance interactions in the bc(1) complex of Saccharomyces cerevisiae
The electric field generated by photosynthetic reaction center induces rapid reversed electron transfer in the bc1 complex
Inhibitory analogs of ubiquinol act anti-cooperatively on the Yeast cytochrome bc1 complex. Evidence for an alternating, half-of-the-sites mechanism of ubiquinol oxidation
The bc(1) complex of the iron-grown acidophilic chemolithotrophic bacterium Acidithiobacillus ferrooxidans functions in the reverse but not in the forward direction. Is there a second bc(1) complex?
Interactions of quinone with the iron-sulfur protein of the bc(1) complex: is the mechanism spring-loaded?
A concerted, alternating sites mechanism of ubiquinol oxidation by the dimeric cytochrome bc(1) complex
Nonoxidizable ubiquinol derivatives that are suitable for the study of the ubiquinol oxidation site in the cytochrome bc1 complex
Complex II from phototrophic purple bacterium Rhodoferax fermentans displays rhodoquinol-fumarate reductase activity
The modified Q-cycle explains the apparent mismatch between the kinetics of reduction of cytochromes c1 and bH in the bc1 complex
COENZYME Q. LXII. STRUCTURE AND SYNTHESIS OF RHODOQUINONE, A NATURAL AMINOQUINONE OF THE COENZYME Q GROUP
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