Comparison of theory and experiment for electron transfers in proteins: where's the beef?

Friesner, Richard A.

doi:10.1016/s0969-2126(00)00035-6

Cited by 17 publications

(14 citation statements)

References 19 publications

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“…Some potential applications of conducting polymers as electromechanical materials have been evaluated conceptually (3), and macroscopic bilayer actuators have been demonstrated (4,5). Recently we produced micrometer-size bilayers (6), defining the geometrical elements photolithographically, and conducting polymer-polyimide cantilevers have been made (7).…”

Section: Controlled Folding Of Micrometer-size Structuresmentioning

confidence: 99%

Electron Tunneling in Proteins: Coupling Through a β Strand

Langen

Chang

Germanas

et al. 1995

Science

337

340

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Electron coupling through a beta strand has been investigated by measurement of the intramolecular electron-transfer (ET) rates in ruthenium-modified derivatives of the beta barrel blue copper protein Pseudomonas aeruginosa azurin. Surface histidines, introduced on the methionine-121 beta strand by mutagenesis, were modified with a Ru(2,2'-bipyridine)2(imidazole)2+ complex. The Cu+ to Ru3+ rate constants yielded a distance-decay constant of 1.1 per angstrom, a value close to the distance-decay constant of 1.0 per angstrom predicted for electron tunneling through an idealized beta strand. Activationless ET rate constants in combination with a tunneling-pathway analysis of the structures of azurin and cytochrome c confirm that there is a generally efficient network for coupling the internal (native) redox center to the surface of both proteins.

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Section: Controlled Folding Of Micrometer-size Structuresmentioning

confidence: 99%

Electron Tunneling in Proteins: Coupling Through a β Strand

Langen

Chang

Germanas

et al. 1995

Science

337

340

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“…First of all, as pointed out by , the separation between the cytochrome and the RC in docking models should be taken as an upper limit, because mutual amino acid side-chain rearrangements can lead to closer association of the redox partners. The limits of the approach of Moser et al (1992), as well as those of more sophisticated treatments considering specific electron tunneling pathways, have been thoroughly debated, emphasizing possible ambiguities in the identification of a proper boundary between the donor and acceptor molecules and the groups of the intervening medium (Farid et al, 1993;Friesner, 1994). Using the pathway model of Beratan and Onuchic, Aquino et al (1995) examined the electronic matrix element for two docked cyt c 2 structures, one based on the Tiede and Chang model , and the other based on maximizing electronic coupling, obtaining very similar electronic coupling decays.…”

Section: Estimation Of Electron Transfer Parameters For the Fastest Intracomplex Reaction: Assumptions And Implicationsmentioning

confidence: 99%

Effects of Temperature and ΔG° on Electron Transfer from Cytochrome c2 to the Photosynthetic Reaction Center of the Purple Bacterium Rhodobacter sphaeroides

Venturoli

Drepper

Williams

et al. 1998

Biophysical Journal

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The kinetics of electron transfer from cytochrome c2 to the primary donor (P) of the reaction center from the photosynthetic purple bacterium Rhodobacter sphaeroides have been investigated by time-resolved absorption spectroscopy. Rereduction of P+ induced by a laser pulse has been measured at temperatures from 300 K to 220 K in a series of specifically mutated reaction centers characterized by altered midpoint redox potentials of P+/P varying from 410 mV to 765 mV (as compared to 505 mV for wild type). Rate constants for first-order electron donation within preformed reaction center-cytochrome c2 complexes and for the bimolecular oxidation of free cytochrome c2 have been obtained by multiexponential deconvolution of the kinetics. At all temperatures the rate of the fastest intracomplex electron transfer increases by more than two orders of magnitude as the driving force -deltaGo is varied over a range of 350 meV. The temperature and deltaGo dependences of the rate constant fit the Marcus equation well. Global analysis yields a reorganization energy lambda = 0.96 +/- 0.07 eV and a set of electronic matrix elements, specific for each mutant, ranging from 1.2 10(-4) eV to 2.5 10(-4) eV. Analysis in terms of the Jortner equation indicates that the best fit is obtained in the classical limit and restricts the range of coupled vibrational modes to frequencies lower than approximately 200 cm(-1). An additional slower kinetic component of P+ reduction, attributed to electron transfer from cyt c2 docked in a nonoptimal configuration of the complex, displays a Marcus type dependence of the rate constant upon deltaGo, characterized by a similar value of lambda (0.8 +/- 0.1 eV) and by an average electronic matrix element smaller by more than one order of magnitude. In all of the mutants, as the temperature is decreased below 260 K, both intracomplex reactions are abruptly inhibited, their rate being negligible at 220 K. The free energy dependence of the second-order rate constant for oxidation of cyt c2 in solution suggests that the collisional reaction is partially diffusion controlled, reaching the diffusion limit at exothermicities between 150 and 250 meV over the temperature range investigated.

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“…[4][5][6][7][8] Electron exchange between reduced and oxidized cytochrome c 9,10 or between cytochrome c and a covalently attached metal 11 is among the systems most amenable to detailed study by computational and physical methods. 12,13 In early studies, Churg et al 4 converted the rather small difference between the crystal structures of the two redox states of tuna cytochrome c 14,15 to an estimate of the reorganization energy and found this energy to be quite small (6 kcal mol -1 ). This finding is consistent with the idea that proteins reduce activation barriers by minimizing the corresponding reorganization energies.…”

Section: Introductionmentioning

confidence: 99%

“…The emergence of microscopic simulation approaches has made it possible to evaluate the role of protein structure in setting the reorganization energy and the driving force of interprotein electron transfer. − Electron exchange between reduced and oxidized cytochrome c , or between cytochrome c and a covalently attached metal is among the systems most amenable to detailed study by computational and physical methods. , …”

Section: Introductionmentioning

confidence: 99%

The Reorganization Energy of Cytochrome c Revisited

et al. 1997

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The solution structures of the reduced and oxidized forms of the cytochrome c are used to reevaluate the reorganization energy for oxidation of cytochrome c. This is achieved by using the linear response approximation in concert with the NMR structures as pseudo energy constraints. Alternative estimates, obtained using a free energy perturbation approach employing umbrella sampling and a continuum dielectric approach, are also provided. The reorganization energy obtained is larger than that previously estimated using crystal structures of the protein. Nevertheless, the present estimate remains significantly smaller than the corresponding reorganization energy in water (9−15 kcal mol-1 as compared to ≈37 kcal mol-1 in water) and the protein contribution to the reorganization energy is only 8−10 kcal mol-1. This provides further support for the proposal that proteins assist in electron transfer reactions by reducing the relevant reorganization energies. The solution structures are also used to estimate the redox potential of cytochrome c. Several strategies are employed including a newly formulated scaled linear response approximation. The calculations agree reasonably well with the observed redox potential. Analysis of the group contributions to the reorganization energy and redox potential reveals a clear energetic linkage between these fundamental parameters of electron transfer and a redox-dependent surface feature likely to influence recognition of cytochrome c by its redox partners. Specifically, the rearrangement of Ile81 and other residues at the heme edge upon a change in oxidation state gives rise to a large contribution to both the redox potential and the reorganization energy. Finally this work is used to explore and illustrate the meaning of macroscopic dielectric models. It is shown that the “proper” dielectric constant depends strongly on the model used since it basically represents the implicit contributions of the given model rather than a fundamental physics. Thus we obtain different effective dielectric constants for different treatments of redox potential and reorganization energy.

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Comparison of theory and experiment for electron transfers in proteins: where's the beef?

Abstract: The correct way to describe the transfer of electrons across proteins has been the subject of some controversy. Recent advances in theoretical methods and an increase in the amount of experimental data may resolve this dispute.

Cited by 17 publications

References 19 publications

Electron Tunneling in Proteins: Coupling Through a β Strand

Electron Tunneling in Proteins: Coupling Through a β Strand

Effects of Temperature and ΔG° on Electron Transfer from Cytochrome c2 to the Photosynthetic Reaction Center of the Purple Bacterium Rhodobacter sphaeroides

The Reorganization Energy of Cytochrome c Revisited

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