Long-range electron transfer within metal-substituted protein complexes

Hoffman, Brian M.; Natan, Michael J.; Nocek, Judith M.; Wallin, Sten A.

doi:10.1007/3-540-53260-9_3

Cited by 43 publications

(39 citation statements)

References 22 publications

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“…Many biological electron-transfer (ET) reactions occur between protein-bound cofactors that are separated by 10-15A, or even longer distances [1][2][3][4][5][6][7]. According to theory, the rate constant, kET, for ET between a distant donor (D) and an acceptor (A) can be expressed as the product of the square of an electronic-coupling matrix element (HDA) and a nuclear (or Franck-Condon) factor (FC) [2,3]:…”

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confidence: 99%

Electron transfer in cytochrome c depends upon the structure of the intervening medium

et al. 1994

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Section: Introductionmentioning

confidence: 99%

Electron transfer in cytochrome c depends upon the structure of the intervening medium

et al. 1994

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“…4 The pair has been particularly amenable to study because the heme-iron of either partner can be substituted by Zn (or Mg) to form a complex that exhibits an ET photocycle. 8 When the Fe of CcP is substituted to form ZnPCcP and is in complex with the iron form of the partner protein, Fe 3+ Cc, the complex can undergo a laser-initiated ET photocycle comprised of ‘forward’ charge-separation (CS) ET ( 3 ZnPCcP;Fe 3+ Cc→ZnP + CcP;Fe 2+ Cc), rate constant k f , to produce the charge-separated intermediate protein pair, ZnP + CcP and Fe 2+ Cc—denoted I representing all states involving ZnP + CcP—followed by ‘back’ charge-recombination (CR) ET to regenerate the initial state (ZnPCcP;Fe 3+ Cc←ZnP + CcP;Fe 2+ Cc) with rate constant k b , 13 a process analogous to physiological ET wherein Fe 2+ Cc reduces CcP Compound ES.…”

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Control of Cyclic Photoinitiated Electron Transfer between Cytochrome c Peroxidase (W191F) and Cytochrome c by Formation of Dynamic Binary and Ternary Complexes

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Hoffman

2015

Biochemistry

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Extensive studies of the physiological protein—protein electron-transfer (ET) complex between yeast cytochrome c peroxidase (CcP) and cytochrome c (Cc) have left unresolved questions about how formation/dissociation of binary and ternary complexes influence ET. We probe this issue through study of the photocycle of ET between Zn-ProtoporphyrinIX-substituted CcP(W191F) (ZnPCcP) and Cc. Photoexcitation of ZnPCcP in complex with Fe3+Cc, initiates the photocycle: charge-separation ET [3ZnPCcP, Fe3+Cc]→[ZnP+CcP, Fe2+Cc] followed by charge recombination, [ZnP+CcP, Fe2+Cc] → [ZnPCcP, Fe3+Cc]. The W191F mutation eliminates fast hole hopping through W191, enhancing accumulation of charge-separated intermediate and extending the timescale for binding/dissociation of the charge-separated complex. Both triplet quenching and the charge-separated intermediate were monitored during titrations of ZnPCcP with Fe3+Cc, Fe2+Cc, and redox-inert CuCc. The results require a photocycle that includes dissociation/recombination of the charge-separated binary complex and a charge-separated ternary complex, [ZnP+CcP, Fe2+Cc, Fe3+Cc]. The expanded kinetic scheme formalizes earlier proposals of “substrate-assisted product dissociation” within the photocycle. The measurements yield the thermodynamic affinity constants for binding the first and second Cc: KI = 10−7 M−1, KII = 10−4 M−1. However, two-site analysis of the thermodynamics of formation of the ternary reveals that Cc binds at the weaker-binding site with much greater affinity than previously recognized, and places upper bounds on the contributions of repulsion between the two Cc of the ternary complex. In conjunction with recent NMR studies, the analysis further suggests a dynamic view of the ternary complex, wherein neither Cc necessarily faithfully adopts the crystal-structure configuration because of Cc-Cc repulsion.

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“…Substitution of the heme (FeP) of one partner of an ET complex by a closed-shell metalloporphyrin (ZnP in the present case) offers a means of studying binding and intracomplex ET (12)(13)(14)(15). The metalloporphyrin triplet state, 3 ZnP, produced by laser-flash excitation is a strong reductant, and in a complex with an Fe 3+ P quencher, its lifetime is decreased by long-range, intracomplex 3 ZnP f Fe 3+ P ET (eq 2).…”

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Determination of the Hemoglobin Surface Domains That React with Cytochrome b₅

et al. 2001

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We have compared the photoinitiated electron-transfer (ET) reaction between cytochrome b(5) (b(5)) and zinc mesoporphyrin-substituted hemoglobin [(ZnM)Hb] and Hb variants in order to determine whether b(5) binds to the subunit surface of either or both Hb chains, or to sites which span the dimer--dimer interface. Because the dimer--dimer interface would be disrupted for monomers or alpha beta dimers, we studied the reaction of b(5) with alpha ZnM chains and (ZnM)Hb beta W37E, which exists as alpha beta dimers in solution. Triplet quenching titrations of the ZnHb proteins with Fe(3+)b(5) show that the binding affinity and ET rate constants for the alpha-chains are the same when they are incorporated into a Hb tetramer or dimer, or exist as monomers. Likewise, the parameters for beta-chains in tetramers and dimers differ minimally. In parallel, we have modified the surface of the Hb chains by neutralizing the heme propionates through the preparation of zinc deuterioporphyrin dimethyl ester hemoglobin, (ZnD-DME)Hb. The charge neutralization increases the ET rate constants 100-fold for the alpha-chains and 40-fold for the beta-chains (but has has little effect on the affinity of either chain type for b(5), similar to earlier results for myoglobin). Together, these results indicate that b(5) binds to sites at the subunit surface of each chain rather than to sites which span the dimer-dimer interface. The charge-neutralization results further suggest that b(5) binds over a broad area of the subunit face, but reacts only in a minority population of binding geometries.

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Long-range electron transfer within metal-substituted protein complexes

Cited by 43 publications

References 22 publications

Electron transfer in cytochrome c depends upon the structure of the intervening medium

Electron transfer in cytochrome c depends upon the structure of the intervening medium

Control of Cyclic Photoinitiated Electron Transfer between Cytochrome c Peroxidase (W191F) and Cytochrome c by Formation of Dynamic Binary and Ternary Complexes

Determination of the Hemoglobin Surface Domains That React with Cytochrome b₅

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Long-range electron transfer within metal-substituted protein complexes

Cited by 43 publications

References 22 publications

Electron transfer in cytochrome c depends upon the structure of the intervening medium

Electron transfer in cytochrome c depends upon the structure of the intervening medium

Control of Cyclic Photoinitiated Electron Transfer between Cytochrome c Peroxidase (W191F) and Cytochrome c by Formation of Dynamic Binary and Ternary Complexes

Determination of the Hemoglobin Surface Domains That React with Cytochrome b5

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Determination of the Hemoglobin Surface Domains That React with Cytochrome b₅