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Discrimination between two possible reaction sequences that create potential risk of generation of deleterious radicals by cytochrome bc?. Implications for the mechanism of superoxide production.

ABSTRACT: In addition to its bioenergetic function of building up proton motive force, cytochrome bc? can be a source of superoxide. One-electron reduction of oxygen is believed to occur from semiquinone (SQ(o)) formed at the quinone oxidation/reduction Q(o) site (Q(o)) as a result of single-electron oxidation of quinol by the iron-sulfur cluster (FeS) (semiforward mechanism) or single-electron reduction of quinone by heme b(L) (semireverse mechanism). It is hotly debated which mechanism plays a major role in the overall production of superoxide as experimental data supporting either reaction exist. To evaluate a contribution of each of the mechanisms we first measured superoxide production under a broad range of conditions using the mutants of cytochrome bc? that severely impeded the oxidation of FeS by cytochrome c?, changed density of FeS around Q(o) by interfering with its movement, or combined these two effects together. We then compared the amount of generated superoxide with mathematical models describing either semiforward or semireverse mechanism framed within a scheme assuming competition between the internal reactions at Q(o) and the leakage of electrons on oxygen. We found that only the model of semireverse mechanism correctly reproduced the experimentally measured decrease in ROS for the FeS motion mutants and increase in ROS for the mutants with oxidation of FeS impaired. This strongly suggests that this mechanism dominates in setting steady-state levels of SQ(o) that present a risk of generation of superoxide by cytochrome bc?. Isolation of this reaction sequence from multiplicity of possible reactions at Q(o) helps to better understand conditions under which complex III might contribute to ROS generation in vivo.

SUBMITTER: Sarewicz M

PROVIDER: S-EPMC3057645 | biostudies-literature | 2010 Nov

REPOSITORIES: biostudies-literature

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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.

Sarewicz Marcin M Borek Arkadiusz A Cieluch Ewelina E Swierczek Monika M Osyczka Artur A

Biochimica et biophysica acta 20100715 11

In addition to its bioenergetic function of building up proton motive force, cytochrome bc₁ can be a source of superoxide. One-electron reduction of oxygen is believed to occur from semiquinone (SQ(o)) formed at the quinone oxidation/reduction Q(o) site (Q(o)) as a result of single-electron oxidation of quinol by the iron-sulfur cluster (FeS) (semiforward mechanism) or single-electron reduction of quinone by heme b(L) (semireverse mechanism). It is hotly debated which mechanism plays a major rol ...[more]

PMID: 20637719

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Project description:Aspects of the crystal structures of the hetero-oligomeric cytochrome bc(1) and b(6)f ("bc") complexes relevant to their electron/proton transfer function and the associated redox reactions of the lipophilic quinones are discussed. Differences between the b(6)f and bc(1) complexes are emphasized. The cytochrome bc(1) and b(6)f dimeric complexes diverge in structure from a core of subunits that coordinate redox groups consisting of two bis-histidine coordinated hemes, a heme b(n) and b(p) on the electrochemically negative (n) and positive (p) sides of the complex, the high potential [2Fe-2S] cluster and c-type heme at the p-side aqueous interface and aqueous phase, respectively, and quinone/quinol binding sites on the n- and p-sides of the complex. The bc(1) and b(6)f complexes diverge in subunit composition and structure away from this core. b(6)f Also contains additional prosthetic groups including a c-type heme c(n) on the n-side, and a chlorophyll a and β-carotene. Common structure aspects; functions of the symmetric dimer. (I) Quinone exchange with the bilayer. An inter-monomer protein-free cavity of approximately 30Å along the membrane normal×25Å (central inter-monomer distance)×15Å (depth in the center), is common to both bc(1) and b(6)f complexes, providing a niche in which the lipophilic quinone/quinol (Q/QH(2)) can be exchanged with the membrane bilayer. (II) Electron transfer. The dimeric structure and the proximity of the two hemes b(p) on the electrochemically positive side of the complex in the two monomer units allow the possibility of two alternate routes of electron transfer across the complex from heme b(p) to b(n): intra-monomer and inter-monomer involving electron cross-over between the two hemes b(p). A structure-based summary of inter-heme distances in seven bc complexes, representing mitochondrial, chromatophore, cyanobacterial, and algal sources, indicates that, based on the distance parameter, the intra-monomer pathway would be favored kinetically. (III) Separation of quinone binding sites. A consequence of the dimer structure and the position of the Q/QH(2) binding sites is that the p-side QH(2) oxidation and n-side Q reduction sites are each well separated. Therefore, in the event of an overlap in residence time by QH(2) or Q molecules at the two oxidation or reduction sites, their spatial separation would result in minimal steric interference between extended Q or QH(2) isoprenoid chains. (IV) Trans-membrane QH(2)/Q transfer. (i) n/p-side QH(2)/Q transfer may be hindered by lipid acyl chains; (ii) the shorter less hindered inter-monomer pathway across the complex would not pass through the center of the cavity, as inferred from the n-side antimycin site on one monomer and the p-side stigmatellin site on the other residing on the same surface of the complex. (V) Narrow p-side portal for QH(2)/Q passage. The [2Fe-2S] cluster that serves as oxidant, and whose histidine ligand serves as a H(+) acceptor in the oxidation of QH(2), is connected to the inter-monomer cavity by a narrow extended portal, which is also occupied in the b(6)f complex by the 20 carbon phytyl chain of the bound chlorophyll.

Dataset Information

Discrimination between two possible reaction sequences that create potential risk of generation of deleterious radicals by cytochrome bc?. Implications for the mechanism of superoxide production.

Publications

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.

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