Quantum mechanical tunnelling in biological systems

DeVault, Don

doi:10.1017/s003358350000175x

Cited by 327 publications

(194 citation statements)

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“…We discuss constraints on mechanism from the structure and from electron transfer theory, and we suggest that the restricted movement of substrates (the extrinsic domain of the ISP and the semiquinone intermediate) allows electron transfer over distances that would otherwise by forbidden by the energy-gap law of Marcus (reviewed in Ref. 29).…”

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confidence: 99%

The Energy Landscape for Ubihydroquinone Oxidation at the Qo Site of the bc 1 Complex inRhodobacter sphaeroides

Hong¹,

Ugulava²,

Guergova-Kuras³

et al. 1999

Journal of Biological Chemistry

174

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Activation energies for partial reactions involved in oxidation of quinol by the bc 1 complex were independent of pH in the range 5.5-8.9. Formation of enzyme-substrate complex required two substrates, ubihydroquinone binding from the lipid phase and the extrinsic domain of the iron-sulfur protein. The activation energy for ubihydroquinone oxidation was independent of the concentration of either substrate, showing that the activated step was in a reaction after formation of the enzyme-substrate complex. At all pH values, the partial reaction with the limiting rate and the highest activation energy was oxidation of bound ubihydroquinone. The pH dependence of the rate of ubihydroquinone oxidation reflected the pK on the oxidized iron-sulfur protein and requirement for the deprotonated form in formation of the enzyme-substrate complex. We discuss different mechanisms to explain the properties of the bifurcated reaction, and we preclude models in which the high activation barrier is in the second electron transfer or is caused by deprotonation of QH 2 . Separation to products after the first electron transfer and movement of semiquinone formed in the Q o site would allow rapid electron transfer to heme b L . This would also insulate the semiquinone from oxidation by the iron-sulfur protein, explaining the efficiency of bifurcation.The ubihydroquinone:cytochrome c oxidoreductase (EC 1.10.2.2) (bc 1 complex) 1 family of proteins is the central component of all major energy-conserving electron transfer chains (1-5). Crystallographic structures for mitochondrial complexes from three vertebrate species in six different crystals forms, with and without inhibitors, have recently been solved (6 -8).Although the mitochondrial structures have 10 or 11 subunits, the mechanism is determined by a catalytic core consisting of three subunits, cytochrome (cyt) b, cyt c 1 , and the Rieske ironsulfur protein (ISP), which are highly conserved between mitochondria and the ␣-proteobacteria from which they evolved and are the only subunits in some species (9, 10). These enzymes operate through a modified Q-cycle (4, 11, 12), involving catalytic sites for oxidation of ubihydroquinone (quinol or QH 2 ), reduction of ubiquinone (quinone or Q), and reduction of cyt c (in mitochondria) or c 2 (in photosynthetic bacteria). The oxidation of quinol occurs at the Q o site through the bifurcated reaction (13), so called because it involves two 1-electron transfers to separate acceptor chains, one to the high potential chain (the Fe 2 S 2 cluster of the ISP, cyt c 1 , cyt c, or c 2 ), and the second to the low potential chain (heme b L , heme b H , and the quinone or semiquinone acceptor at the Q i site).The reaction at the Q o site is remarkable because of the high efficiency of the bifurcation, which ensures that the second electron is delivered to the low potential chain despite the strong thermodynamic gradient favoring delivery to the high potential chain. The reaction can be initiated either by addition of quinol to the oxidized complex or by...

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The Energy Landscape for Ubihydroquinone Oxidation at the Qo Site of the bc 1 Complex inRhodobacter sphaeroides

Hong¹,

Ugulava²,

Guergova-Kuras³

et al. 1999

Journal of Biological Chemistry

174

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“…Furthermore, it is possible in this complex to employ metal substitution to systematically vary parameters such as the reaction exothermicity, which play an important role in controlling biological electron transfer rates. For example, theory predicts (7)(8)(9)(10)(11) that the electron transfer rate constant kt = k~v01vn, where k0 1014 s-1 and the electronic (vel) and nuclear (vn) frequency factors are given by exp(-ocR) and exp-(AGo+M)2/4XkT, respectively. Electron transfer within the 1:1 (cyt c:cyt c peroxidase-enzyme-substrate) complex [cyt c peroxidase-enzyme-substrate is formed on reaction of H202 with cyt c peroxidase(III)] is believed to be very rapid, based on the large turnover number observed for cyt c peroxidase (2).…”

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confidence: 99%

“…Samples for pulse radiolysis were prepared by mixing equal concentrations (2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20) AM) of cyt c peroxidase(III) and cyt c(III) in phosphate buffer (1 mM, pH 7.0). The solutions were degassed under Ar and transferred by syringe to anaerobic quartz cells for pulse radiolysis.…”

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confidence: 99%

Direct measurements of intramolecular electron transfer rates between cytochrome c and cytochrome c peroxidase: effects of exothermicity and primary sequence on rate.

Cheung¹,

Taylor²,

Kornblatt³

et al. 1986

Proc. Natl. Acad. Sci. U.S.A.

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Rapid mixing of ferrocytochrome c peroxi- sition (15). Corroboration of this rate by a more direct measurement is desirable in the context of understanding biological electron transfer rates.We, therefore, wish to report the direct measurement of electron transfer from cyt c peroxidase(II) to cyt c(III) within the cyt c(III)-cyt c peroxidase(II) complex. In addition, we report data for electron transfer in the derivative system, cyt c peroxidase-porphyrin (Por) cyt c7 (AP0 1 V), where Por-cyt c7 is the anion radical of Por-cyt c. Por-cyt c is iron-free cyt c. These systems together provide unique data on the dependence of protein electron transfer rates on AG.Finally, Ho et al. (16) reported an interesting study on the rate of oxidative quenching of the Zn-cyt c peroxidase triplet by cyt c(III). They found that 3kq depends markedly on the primary structure of the cytochrome with 3kq (yeast C) > 10 3kq (horse C). With the methods in hand to study a simple Fe(II)/Fe(III) redox reaction in the cyt c/cyt c peroxidase system, we report a comparative study of how the cyt c peroxidase(II) + cyt c(III) reaction depends on the primary structure of cyt c. (18). The product was purified by G-200 Abbreviations: cyt c, cytochrome c; Por, porphyrin. 1To whom reprint requests should be addressed. MATERIALS AND METHODS 1330The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

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“…However, the electron transfer rates, cz~ and fit (i= 1--4) may be quite voltage-sensitive. From the Marcus' theory [17,18],…”

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“…The electric field may alter the energy gap, Ev, -Ewe, thereby changing the transfer rate [17,18]. As calculated in [6], the energy gap is not very sensitive to the depolarizing field itself, but may be altered significantly through the change of the electric field from nearby charged residues (the voltage sensors) in response to the depolarizing field.…”

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confidence: 99%

On the activation—inactivation coupling in Shaker potassium channels

Lee

1992

FEBS Letters

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The "ball-and-chain' model suggests the e~istence of a tlegativc site which may attract the positively charged inactivation bali to occlude the pore when the channel is in the open state. For Shah'er K + channels, we propose that the state-dependent negative site be tryptophan435, which becomes negatively charged after receiving an electron from tyrosine.445. The kinetic scheme for the channel's activation-inactivation coupling as derived from the YW.g:tted model resembles a successful 'scheme 8' proposed by Zagotta and Aldrich. Oar model suggests that the final rapid voltageindependent transition to the open state is due to the deprotonation of tyrosine-445.

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Quantum mechanical tunnelling in biological systems

Cited by 327 publications

References 386 publications

The Energy Landscape for Ubihydroquinone Oxidation at the Qo Site of the bc 1 Complex inRhodobacter sphaeroides

The Energy Landscape for Ubihydroquinone Oxidation at the Qo Site of the bc 1 Complex inRhodobacter sphaeroides

Direct measurements of intramolecular electron transfer rates between cytochrome c and cytochrome c peroxidase: effects of exothermicity and primary sequence on rate.

On the activation—inactivation coupling in Shaker potassium channels

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