Recent advances in cytochrome <i>bc</i><sub>1</sub>: Inter monomer electronic communication?

Khalfaoui-Hassani, Bahia; Lanciano, Pascal; Lee, Dong Woo; Darrouzet, Élisabeth; Daldal, Fevzi

doi:10.1016/j.febslet.2011.08.032

Cited by 21 publications

(27 citation statements)

References 40 publications

(69 reference statements)

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“…We demonstrated that strains in which the heterodimeric constructs enforced an inter-monomer path could not compete with monomeric function. Although this problem had not been discussed in the earlier papers, in subsequent papers [91, 92] two groups acknowledged that recombination occurred with high frequency in their systems, and addressed the question in greater detail. In a more recent paper [93], the rates reported in isolated heterodimeric complexes constructed so as to enforce the inter-monomer path were <10% those measured in similar constructs with wildtype sequence, - much too slow to be commensurate with the original claims, or with a half-of-sites mechanism.…”

Section: Discussionmentioning

confidence: 99%

Role of the -PEWY-glutamate in catalysis at the Qo-site of the Cyt bc1 complex

Victoria

Burton

Crofts

2013

Biochimica et Biophysica Acta (BBA) - Bioenergetics

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We re-examine the pH dependence of partial processes of QH2 turnover in Glu-295 mutants in Rhodobacter sphaeroides to clarify the mechanistic role. In more crippled mutants, the bell-shaped pH profile of wildtype was replaced by dependence on a single pK at ~8.5 favoring electron transfer. Loss of the pK at 6.5 reflects a change in the rate-limiting step from the first to the second electron transfer. Over the range of pH 6–8, no major pH dependence of formation of the initial reaction complex was seen, and the rates of bypass reactions were similar to wildtype. Occupancy of the Qo-site by semiquinone (SQ) was similar in wildtype and the Glu→Trp mutant. Since heme bL is initially oxidized in the latter, the bifurcated reaction can still occur, allowing estimation of an empirical rate constant <103 s−1 for reduction of heme bL by SQ from the domain distal from heme bL, a value 1000-fold smaller than that expected from distance. If the pK ~8.5 in mutant strains is due to deprotonation of the neutral semiquinone, with Q.- as electron donor to heme bL, then in wildtype this low value would preclude mechanisms for normal flux in which semiquinone is constrained to this domain. A kinetic model in which Glu-295 catalyzes H+ transfer from QH., and delivery of the H+ to exit channel(s) by rotational displacement, and facilitates rapid electron transfer from SQ to heme bL by allowing Q.- to move closer to the heme, accounts well for the observations.

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Section: Discussionmentioning

confidence: 99%

Role of the -PEWY-glutamate in catalysis at the Qo-site of the Cyt bc1 complex

Victoria

Burton

Crofts

2013

Biochimica et Biophysica Acta (BBA) - Bioenergetics

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“…To a greater or lesser extent, these alternative mechanisms would require re-evaluation of the data that had led to identification in the 1980s of Garland’s modified Q-cycle [2, 5, 25–29] as the most economical hypothesis. Rather than repeat the detailed arguments in the previous review, we will focus on developments since then that have been claimed to demonstrate that intermonomer electron transfer occurs at rates compatible with an important role in normal flux [14, 16, 19, 30–32]. Does the proposed intermonomer electron transfer represent a challenge demanding a paradigmatic change in our thinking, or a diversion?…”

Section: Monomeric or Dimeric Functionmentioning

confidence: 99%

The mechanism of ubihydroquinone oxidation at the Qo-site of the cytochrome bc1 complex

Crofts

Hong

Wilson

et al. 2013

Biochimica et Biophysica Acta (BBA) - Bioenergetics

117

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1. Recent results suggest that the major flux is carried by a monomeric function, not by intermonomer electron flow. 2. The bifurcated reaction at the Qo-site involves sequential partial processes, - a rate limiting first electron transfer generating a semiquinone (SQ) intermediate, and a rapid second electron transfer in which the SQ is oxidized by the low potential chain. 3. The rate constant for the first step in a strongly endergonic, proton-first-then-electron mechanism, is given by a Marcus-Brønsted treatment in which a rapid electron transfer is convoluted with a weak occupancy of the proton configuration needed for electron transfer. 4. A rapid second electron transfer pulls the overall reaction over. Mutation of Glu-295 of cyt b shows it to be a key player. 5. In more crippled mutants, electron transfer is severely inhibited and the bell-shaped pH dependence of wildtype is replaced by a dependence on a single pK at ~8.5 favoring electron transfer. Loss of a pK ~6.5 is explained by a change in the rate limiting step from the first to the second electron transfer; the pK ~8.5 may reflect dissociation of QH·. 6. A rate constant (<103 s−1) for oxidation of SQ in the distal domain by heme bL has been determined, which precludes mechanisms for normal flux in which SQ is constrained there. 7. Glu-295 catalyzes proton exit through H+ transfer from QH·, and rotational displacement to delivers the H+ to exit channel(s). This opens a volume into which Q·− can move closer to the heme to speed electron transfer. 8. A kinetic model accounts well for the observations, but leaves open the question of gating mechanisms. For the first step we suggest a molecular “escapement”; for the second a molecular ballet choreographed through coulombic interactions.

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“…19 We also observed that strains carrying the two-plasmid system formed rare (∼10 −4 ) large Ps + colonies among a population of predominantly small Ps +/− colonies. 19,20 …”

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confidence: 99%

Intermonomer Electron Transfer between the b Hemes of Heterodimeric Cytochrome bc₁

et al. 2013

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The ubihydroquinone:cytochrome c oxidoreductase, or cytochrome bc1, is a central component of respiratory and photosynthetic energy transduction pathways in many organisms. It contributes to the generation of membrane potential and proton gradient used for cellular energy (ATP) production. The three-dimensional structures of cytochrome bc1 show a homodimeric organization of its three catalytic subunits. The unusual architecture revived the issue of whether the monomers operate independently or function cooperatively during the catalytic cycle of the enzyme. In recent years, different genetic approaches allowed the successful production of heterodimeric cytochrome bc1 variants and evidenced the occurrence of intermonomer electron transfer between the monomers of this enzyme. Here we used a version of the “two-plasmid” genetic system, also described in the preceding paper (DOI: 10.1021/bi400560p), to study a new heterodimeric mutant variant of cytochrome bc1. The strain producing this heterodimeric variant sustained photosynthetic growth of Rhodobacter capsulatus and yielded an active heterodimer. Interestingly, kinetic data showed equilibration of electrons among the four b heme cofactors of the heterodimer, via “reverse” intermonomer electron transfer between the bL hemes. Both inactive homodimeric and active heterodimeric cytochrome bc1 variants were purified to homogeneity from the same cells, and purified samples were subjected to mass spectrometry analyses. The data unequivocally supported the idea that the cytochrome b subunits carried the expected mutations and their associated epitope tags. Implications of these findings on our interpretation of light-activated transient cytochrome b and c redox kinetics and the mechanism of function of a dimeric cytochrome bc1 are discussed with respect to the previously proposed heterodimeric Q cycle model.

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Recent advances in cytochrome bc₁: Inter monomer electronic communication?

Cited by 21 publications

References 40 publications

Role of the -PEWY-glutamate in catalysis at the Qo-site of the Cyt bc1 complex

Role of the -PEWY-glutamate in catalysis at the Qo-site of the Cyt bc1 complex

The mechanism of ubihydroquinone oxidation at the Qo-site of the cytochrome bc1 complex

Intermonomer Electron Transfer between the b Hemes of Heterodimeric Cytochrome bc₁

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Recent advances in cytochrome bc1: Inter monomer electronic communication?

Cited by 21 publications

References 40 publications

Role of the -PEWY-glutamate in catalysis at the Qo-site of the Cyt bc1 complex

Role of the -PEWY-glutamate in catalysis at the Qo-site of the Cyt bc1 complex

The mechanism of ubihydroquinone oxidation at the Qo-site of the cytochrome bc1 complex

Intermonomer Electron Transfer between the b Hemes of Heterodimeric Cytochrome bc1

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Recent advances in cytochrome bc₁: Inter monomer electronic communication?

Intermonomer Electron Transfer between the b Hemes of Heterodimeric Cytochrome bc₁