Electron transfer in the cytochrome c/cytochrome b2 complex: evidence for conformational gating

McLendon, George; Pardue, Kelli; Bak, Phillip

doi:10.1021/ja00258a054

Cited by 67 publications

(36 citation statements)

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“…However, this rate reached a saturation behaviour of 380 + 30 s'I at low ionic strength [7]. Such an intramolecular transfer was recently confirmed to take place in reaction between S.c. flavocytochrome b2 and horse heart cytochrome c derivatives, with a first-order rate constant 700 + 100 s-1 at 25°C [38]. The present paper reports a parallel study with ferricyanide as acceptor, which affords valuable information on the reactivity of the two possible one-electron donors, haem b2 and flavin semiquinone.…”

Section: Electron-transfer Reactions Between Flavocytochrome B2 and Ementioning

confidence: 81%

Electron-transfer steps involved in the reactivity of Hansenula anomala flavocytochrome b2 as deduced from deuterium isotope effects and simulation studies

Capeillère‐Blandin

1991

Biochemical Journal

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The L-lactate-flavocytochrome b2-ferricyanide electron-transfer system from the yeast Hansenula anomala was investigated by rapid-reaction techniques. The kinetics of reduction of oxidized flavocytochrome b2 by L-lactate and L-[2H]lactate were biphasic both for flavin and haem prosthetic groups and at all concentrations tested. The first-order rate constants of the rapid and slow phases depended upon substrate concentrations, a saturation behaviour being exhibited. Substitution of the C alpha-H atom by 2H was found to cause appreciable changes in the rate constants for the initial reduction of flavin and haem (phase I), which were respectively about 3-fold and 2-fold less than with L-lactate. In contrast, no significant isotope effect was noted on the apparent reduction rate constants of the slow phase, phase II. Under steady-state conditions an isotope effect of 2.0 was found on the overall electron transfer from L-lactate to ferricyanide. These transient reduction results were discussed in terms of a kinetic model implying intra- and inter-protomer electron exchanges between flavin and haem b2, all of which have been experimentally described. Computer simulations indicate that the reaction scheme provides a reasonable explanation of the fast-reduction phase, phase I (in absence of acceptor). The pseudo-first-order rate constant for oxidation of reduced haem b2 in flavocytochrome b2 increased with increasing ferricyanide concentration in a hyperbolic fashion. The limiting value at infinite ferricyanide concentration, which was attributed to the intramolecular electron-transfer rate from ferroflavocytochrome b2 to the iron of ferricyanide within a complex, was 920 +/- 50 s-1 at pH 7.0 and 5 degrees C. Stopped-flow and rapid-freezing measurements showed haem b2 and flavin to be 90 and 44% oxidized respectively under steady-state conditions in presence of ferricyanide. Simulation studies were carried out to check the participation of the proposed reduction sequence in the overall catalytic reaction together with the role of reduced haem b2 (Hr) and flavin semiquinone (Fsq) as electron donors to ferricyanide. When the rate of the intramolecular electron-transfer exchange between Fsq and ferricyanide was adjusted to 200 s-1, simulated data accounted for molar activities defined under various conditions of L-lactate, [2H]lactate and ferricyanide concentrations. Simulation studies were extended to data obtained using cytochrome c as acceptor and reaction catalysed by Saccharomyces cerevisiae flavocytochrome b2. The differences in reactivity observed for Hr and Fsq with ferricyanide and cytochrome c were discussed in terms of redox potentials, electrostatic interactions, distances and accessibility of the participating groups.

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Section: Electron-transfer Reactions Between Flavocytochrome B2 and Ementioning

confidence: 81%

Electron-transfer steps involved in the reactivity of Hansenula anomala flavocytochrome b2 as deduced from deuterium isotope effects and simulation studies

Capeillère‐Blandin

1991

Biochemical Journal

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“…A pair of metalloproteins can form multiple complexes in solution and an orientation optimal for recognition and binding need not be optimal for ET. Known examples include the complexes between cytochrome c and plastocyanin [49] and between cytochrome c and cytochrome b # in yeast lactate dehydrogenase [50] in which, however, the configurational changes accompanying ET are ratelimiting. In such processes (called electron gating, conformational gating, or diffusive electron transfer) the well known dependence of ET rate on driving force [51] would not hold.…”

Section: Discussionmentioning

confidence: 99%

Probing the high-affinity site of beef heart cytochrome c oxidase by cross-linking

Malatesta

Antonini

Nicoletti

et al. 1996

Biochemical Journal

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A covalent complex between cytochrome c oxidase and Saccharomyces cerevisiae iso-1-cytochrome c (called caa3) has been prepared at low ionic strength. Subunit III Cys-115 of beef heart cytochrome c oxidase cross-links by disulphide bond formation to thionitrobenzoate-modified yeast cytochrome c, a derivative shown to bind into the high-affinity site for substrate [Fuller, Darley-Usmar and Capaldi (1981) Biochemistry 20, 7046-7053]. Stopped-flow experiments show that (1) covalently bound yeast cytochrome c cannot donate electrons to cytochrome oxidase, whereas oxidation of exogenously added cytochrome c and electron transfer to cytochrome a are only slightly affected; (2) the steady-state reduction levels of cytochrome c and cytochrome a in the covalent complex caa3 are higher than those found in the native aa3 enzyme. However, (3) K(m) and Vmax values obtained from the non-linear Eadie-Hofstee plots are very similar in both caa3 and aa3. The results imply that cytochrome c bound to the high-affinity site is not in a configuration optimal for electron transfer.

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“…77 This correspondence suggests the possibility that the transport dynamics are influenced or controlled by geometric change(s) that occur within the PtÀacetylide chain that modulate electronic couplings between adjacent segments (e.g., conformational gating). 78 It is also of interest to compare the observed polaron and exciton transport rates in the PtÀacetylides with related work on transport in π-conjugated polymers. In order to make these comparisons, we use the triplet hopping time (27 ps) and the major decay component for the negative polaron (59 ps) to estimate the diffusion coefficient (D) for the triplet exciton and mobility (μ) for the polaron.…”

Section: Journal Of the American Chemical Societymentioning

confidence: 99%

Negative Polaron and Triplet Exciton Diffusion in Organometallic “Molecular Wires”

et al. 2011

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The dynamics of negative polaron and triplet exciton transport within a series of monodisperse platinum (Pt) acetylide oligomers is reported. The oligomers consist of Pt-acetylide repeats, [PtL(2)-C≡C-Ph-C≡C-](n) (where L = PBu(3) and Ph = 1,4-phenylene, n = 2, 3, 6, and 10), capped with naphthalene diimide (NDI) end groups. The Pt-acetylide segments are electro- and photoactive, and they serve as conduits for transport of electrons (negative polaron) and triplet excitons. The NDI end groups are relatively strong acceptors, serving as traps for the carriers. Negative polaron transport is studied by using pulse radiolysis/transient absorption at the Brookhaven National Laboratory Laser-Electron Accelerator Facility (LEAF). Electrons are rapidly attached to the oligomers, with some fraction initially residing upon the Pt-acetylide chains. The dynamics of transport are resolved by monitoring the spectral changes associated with transfer of electrons from the chain to the NDI end group. Triplet exciton transport is studied by femtosecond-picosecond transient absorption spectroscopy. Near-UV excitation leads to rapid production of triplet excitons localized on the Pt-acetylide chains. The excitons transport to the chain ends, where they are annihilated by charge separation with the NDI end group. The dynamics of triplet transport are resolved by transient absorption spectroscopy, taking advantage of the changes in spectra associated with decay of the triplet exciton and rise of the charge-separated state. The results indicate that negative polarons and excitons are transported rapidly, on average moving distances of ~3 nm in less than 200 ps. Analysis of the dynamics suggests diffusive transport by a site-to-site hopping mechanism with hopping times of ~27 ps for triplets and <10 ps for electrons.

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Electron transfer in the cytochrome c/cytochrome b2 complex: evidence for conformational gating

Cited by 67 publications

References 0 publications

Electron-transfer steps involved in the reactivity of Hansenula anomala flavocytochrome b2 as deduced from deuterium isotope effects and simulation studies

Electron-transfer steps involved in the reactivity of Hansenula anomala flavocytochrome b2 as deduced from deuterium isotope effects and simulation studies

Probing the high-affinity site of beef heart cytochrome c oxidase by cross-linking

Negative Polaron and Triplet Exciton Diffusion in Organometallic “Molecular Wires”

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