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  • 1.
    Ejdebäck, Mikael
    et al.
    University of Skövde, Department of Natural Sciences. Biochemistry and Biophysics, Department of Chemistry, Göteborg University, Sweden.
    Bergkvist, Anders
    Biochemistry and Biophysics, Department of Chemistry, Göteborg University, Sweden.
    Karlsson, B. Göran
    Deptartment of Molecular Biotechnology, Chalmers University of Technology, Göteborg, Sweden.
    Ubbink, Marcellus
    Leiden Institute of Chemistry, Leiden University, Gorlaeus Laboratories, Netherlands.
    Side-chain interactions in the plastocyanin-cytochrome f complex2000In: Biochemistry, ISSN 0006-2960, E-ISSN 1520-4995, Vol. 39, no 17, p. 5022-5027Article in journal (Refereed)
    Abstract [en]

    Cytochrome f and plastocyanin are redox partners in the photosynthetic electron-transfer chain. Electron transfer from cytochrome f to plastocyanin occurs in a specific short-lived complex. To obtain detailed information about the binding interface in this transient complex, the effects of binding on the backbone and side-chain protons of plastocyanin have been analyzed by mapping NMR chemical-shift changes. Cytochrome f was added to plastocyanin up to 0.3 M equiv, and the plastocyanin proton chemical shifts were measured. Out of approximately 500 proton resonances, 86% could be observed with this method. Nineteen percent demonstrate significant chemical-shift changes and these protons are located in the hydrophobic patch (including the copper ligands) and the acidic patches of plastocyanin, demonstrating that both areas are part of the interface in the complex. This is consistent with the recently determined structure of the complex [Ubbink, M., Ejdebäck, M., Karlsson, B. G., and Bendall, D. S. (1998) Structure 6, 323-335]. The largest chemical-shift changes are found around His87 in the hydrophobic patch, which indicates tight contacts and possibly water exclusion from this part of the protein interface. These results support the idea that electron transfer occurs via His87 to the copper in plastocyanin and suggest that the hydrophobic patch determines the specificity of the binding. The chemical-shift changes in the acidic patches are significant but small, suggesting that the acidic groups are involved in electrostatic interactions but remain solvent exposed. The existence of small differences between the present data and those used for the structure may imply that the redox state of the metals in both proteins slightly affects the structure of the complex. The chemical-shift mapping is performed on unlabeled proteins, making it an efficient way to analyze effects of mutations on the structure of the complex.

  • 2.
    Ivković-Jensen, Maja M.
    et al.
    Department of Microbiology, University of Iowa, Iowa City, United States / Department of Chemistry, Iowa State University, Ames, United States.
    Ullmann, G. Matthias
    Institut für Kristallographie, Freie Universität Berlin, Germany.
    Crnogorac, Milan M.
    Department of Microbiology, University of Iowa, Iowa City, United States.
    Ejdebäck, Mikael
    Department of Biochemistry and Biophysics, Lundberg Institute, Göteborg University, Sweden.
    Young, Simon
    Department of Biochemistry and Biophysics, Lundberg Institute, Göteborg University, Sweden.
    Hansson, Örjan
    Department of Biochemistry and Biophysics, Lundberg Institute, Göteborg University, Sweden.
    Kostić, Nenad M.
    Department of Microbiology, University of Iowa, Iowa City, United States.
    Comparing the rates and the activation parameters for the forward reaction between the triplet state of zinc cytochrome c and cupriplastocyanin and the back reaction between the zinc cytochrome c cation radical and cuproplastocyanin1999In: Biochemistry, ISSN 0006-2960, E-ISSN 1520-4995, Vol. 38, no 5, p. 1589-1597Article in journal (Refereed)
    Abstract [en]

    This is a comparative study of the photoinduced (so-called forward) electron-transfer reaction 3Zncyt/pc(II) --> Zncyt+/pc(I), between the triplet state of zinc cytochrome c (3Zncyt) and cupriplastocyanin [pc(II)], and the thermal (so-called back) electron-transfer reaction Zncyt+/pc(I) --> Zncyt/pc(II), between the cation (radical) of zinc cytochrome c (Zncyt+) and cuproplastocyanin [pc(I)], which follows it. Both reactions occur between associated (docked) reactants, and the respective unimolecular rate constants are kF and kB. Our previous studies showed that the forward reaction is gated by a rearrangement of the diprotein complex. Now we examine the back reaction and complare the two. We study the effects of temperature (in the range 273.3-302.9 K) and viscosity (in the range 1.00-17.4 cP) on the rate constants and determine enthalpies (DeltaH), entropies (DeltaS), and free energies (DeltaG) of activation. We compare wild-type spinach plastocyanin, the single mutants Tyr83Leu and Glu59Lys, and the double mutant Glu59Lys/Glu60Gln. The rate constant kB for wild-type spinach plastocyanin and its mutants markedly depends on viscosity, an indication that the back reaction is also gated. The activation parameters DeltaH and DeltaS show that the forward and back reactions have similar mechanisms, involving a rearrangement of the diprotein complex from the initial binding configuration to the reactive configuration. The rearrangements of the complexes 3Zncyt/pc(II) and Zncyt+/pc(I) that gate their respective reactions are similar but not identical. Since the back reaction of all plastocyanin variants is faster than the forward reaction, the difference in free energy between the docking and the reactive configuration is smaller for the back reaction than for the forward reaction. This difference is explained by the change in the electrostatic potential on the plastocyanin surface as Cu(II) is reduced to Cu(I). It is the smaller DeltaH that makes DeltaG smaller for the back reaction than for the forward reaction.

  • 3.
    Ivković-Jensen, Maja M.
    et al.
    Department of Chemistry, Iowa State University, Ames, United States.
    Ullmann, G. Matthias
    Institut für Kristallographie, Freie Universität Berlin, Germany.
    Young, Simon
    Department of Biochemistry and Biophysics, Göteborg University, Chalmers University of Technology, Göteborg, Sweden.
    Hansson, Örjan
    Department of Biochemistry and Biophysics, Göteborg University, Chalmers University of Technology, Göteborg, Sweden.
    Crnogorac, Milan M.
    Department of Chemistry, Iowa State University, Ames, United States.
    Ejdebäck, Mikael
    Department of Biochemistry and Biophysics, Göteborg University, Chalmers University of Technology, Göteborg, Sweden.
    Kostić, Nenad M.
    Department of Chemistry, Iowa State University, Ames, United States.
    Effects of single and double mutations in plastocyanin on the rate constant and activation parameters for the rearrangement gating the electron-transfer reaction between the triplet state of zinc cytochrome c and cupriplastocyanin1998In: Biochemistry, ISSN 0006-2960, E-ISSN 1520-4995, Vol. 37, no 26, p. 9557-9569Article in journal (Refereed)
    Abstract [en]

    The unimolecular rate constant for the photoinduced electron-transfer reaction 3Zncyt/pc(II) --> Zncyt+/pc(I) within the electrostatic complex of zinc cytochrome c and spinach cupriplastocyanin is kF. We report the effects on kF of the following factors, all at pH 7.0: 12 single mutations on the plastocyanin surface (Leu12Asn, Leu12Glu, Leu12Lys, Asp42Asn, Asp42Lys, Glu43Asn, Glu59Gln, Glu59Lys, Glu60Gln, Glu60Lys, Gln88Glu, and Gln88Lys), the double mutation Glu59Lys/Glu60Gln, temperature (in the range 273.3-302.9 K), and solution viscosity (in the range 1. 00-116.0 cP) at 283.2 and 293.2 K. We also report the effects of the plastocyanin mutations on the association constant (Ka) and the corresponding free energy of association (DeltaGa) with zinc cytochrome c at 298.2 K. Dependence of kF on temperature yielded the activation parameters DeltaH, DeltaS, and DeltaG. Dependence of kF on solution viscosity yielded the protein friction and confirmed the DeltaG values determined from the temperature dependence. The aforementioned intracomplex reaction is not a simple electron-transfer reaction because donor-acceptor electronic coupling (HAB) and reorganizational energy (lambda), obtained by fitting of the temperature dependence of kF to the Marcus equation, deviate from the expectations based on precedents and because kF greatly depends on viscosity. This last dependence and the fact that certain mutations affect Ka but not kF are two lines of evidence against the mechanism in which the electron-transfer step is coupled with the faster, but thermodynamically unfavorable, rearrangement step. The electron-transfer reaction is gated by the slower, and thus rate determining, structural rearrangement of the diprotein complex; the rate constant kF corresponds to this rearrangement. Isokinetic correlation of DeltaH and DeltaS parameters and Coulombic energies of the various configurations of the Zncyt/pc(II) complex consistently show that the rearrangement is a facile configurational fluctuation of the associated proteins, qualitatively the same process regardless of the mutations in plastocyanin. Correlation of kF with the orientation of the cupriplastocyanin dipole moment indicates that the reactive configuration of the diprotein complex involves the area near the residue 59, between the upper acidic cluster and the hydrophobic patch. Kinetic effects and noneffects of plastocyanin mutations show that the rearrangement from the initial (docking) configuration, which involves both acidic clusters, to the reactive configuration does not involve the lower acidic cluster and the hydrophobic patch but involves the upper acidic cluster and the area near the residue 88.

  • 4.
    Olesen, Kenneth
    et al.
    Biochemistry and Biophysics, Department of Chemistry, Göteborg University, Sweden.
    Ejdebäck, Mikael
    University of Skövde, Department of Natural Sciences. Biochemistry and Biophysics, Department of Chemistry, Göteborg University, Sweden.
    Crnogorac, Milan M.
    Department of Chemistry, Iowa State University, Ames, United States.
    Kostić, Nenad M.
    Department of Chemistry, Iowa State University, Ames, United States.
    Hansson, Örjan
    Biochemistry and Biophysics, Department of Chemistry, Göteborg University, Sweden.
    Electron transfer to photosystem 1 from spinach plastocyanin mutated in the small acidic patch: ionic strength dependence of kinetics and comparison of mechanistic models1999In: Biochemistry, ISSN 0006-2960, E-ISSN 1520-4995, Vol. 38, no 50, p. 16695-16705Article in journal (Refereed)
    Abstract [en]

    A set of plastocyanin (Pc) mutants, probing the small acidic patch (Glu59, Glu60, and Asp61) and a nearby residue, Gln88, has been constructed to provide further insight into the electron transfer process between Pc and photosystem 1. The negatively charged residues were changed into their neutral counterparts or to a positive lysine. All mutant proteins exhibited electron transfer kinetics qualitatively similar to those of the wild type protein over a wide range of Pc concentrations. The kinetics were slightly faster for the Gln88Lys mutant, while they were significantly slower for the Glu59Lys mutant. The data were analyzed with two different models: one involving a conformational change of the Pc-photosystem 1 complex that precedes the electron transfer step (assumed to be irreversible) [Bottin, H., and Mathis, P. (1985) Biochemistry 24, 6453-6460] and another where no conformational change occurs, the electron transfer step is reversible, and dissociation of products is explicitly taken into account [Drepper, F., Hippler, M., Nitschke, W., and Haehnel, W. (1996) Biochemistry 35, 1282-1295]. Both models can account for the observed kinetics in the limits of low and high Pc concentrations. To discriminate between the models, the effects of added magnesium ions on the kinetics were investigated. At a high Pc concentration (0.7 mM), the ionic strength dependence was found to be consistent with the model involving a conformational change but not with the model where the electron transfer is reversible. One residue in the small acidic patch, Glu60, seems to be responsible for the major part of the ionic strength dependence of the kinetics.

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