Simulation of the electrochemical behavior of multi-redox systems. Current potential studies on multiheme cytochromes

Direct electrochemical reduction of carbon dioxide by a molybdenum-containing formate dehydrogenase

Formate dehydrogenase enzymes catalyse the reversible two-electron oxidation of formate to carbon dioxide. The class of metal-dependent formate dehydrogenases comprises prokaryotic enzymes holding redox-active centres and a catalytic site, containing either molybdenum or tungsten ion, that mediates the formate/carbon dioxide interconversion. The carbon dioxide reduction is of a particular interest, since it may be a route for its atmospheric mitigation with the simultaneous production of added-value products, as formate-derived compounds. Recently, the periplasmic formate dehydrogenase from Desulfovibrio desulfuricans, a molybdenum-containing enzyme, was proven to be an efficient enzyme for the CO₂ reduction to formate. In this work, the immobilized formate dehydrogenase isolated from Desulfovibrio desulfuricans direct electrochemical behaviour was attained in the presence and absence of substrates and the formal potentials associated with the catalytic centre transitions were determined in non-turnover conditions. The enzyme catalytic activity towards carbon dioxide reduction was observed using direct electrochemical methods.

Thermodynamic and kinetic characterisation of individual haems in multicentre cytochromes c3

The characterisation of individual centres in multihaem proteins is difficult due to the similarities in the redox and spectroscopic properties of the centres. NMR has been used successfully to distinguish redox centres and allow the determination of the microscopic thermodynamic parameters in several multihaem cytochromes c(3) isolated from different sulphate-reducing bacteria. In this article we show that it is also possible to discriminate the kinetic properties of individual centres in multihaem proteins, if the complete microscopic thermodynamic characterisation is available and the system displays fast intramolecular equilibration in the time scale of the kinetic experiment. The deconvolution of the kinetic traces using a model of thermodynamic control provides a reference rate constant for each haem that does not depend on driving force and can be related to structural factors. The thermodynamic characterisation of three tetrahaem cytochromes and their kinetics of reduction by sodium dithionite are reported in this paper. Thermodynamic and kinetic data were fitted simultaneously to a model to obtain microscopic reduction potentials, haem-haem and haem-proton interacting potentials, and reference rate constants for the haems. The kinetic information obtained for these cytochromes and recently published data for other multihaem cytochromes is discussed with respect to the structural factors that determine the reference rates. The accessibility for the reducing agent seems to play an important role in controlling the kinetic rates, although is clearly not the only factor.

Degradation of 2,4,6-trinitrotoluene by immobilized horseradish peroxidase and electrogenerated peroxide

This paper presents horseradish peroxidase (HRP)-catalyzed removal of 2,4,6-trinitrotoluene (TNT) by an electrochemical packed-bed flow reactor operated in a circulating batch mode with the help of in situ generated hydrogen peroxide. HRP immobilized on the reticulated vitreous carbon electrode was prepared for the cyclic voltammetry of 2,4,6-TNT. Effects of pH and temperature on the TNT electroreduction in 0.2M phosphate buffer saturated with oxygen were examined. HRP immobilized carbon electrode was capable of catalyzing the oxidation and detoxification of 44 microM TNT in aqueous solution under optimized conditions. The removal rate of TNT for the electroenzymatic method was much greater than for electrochemical and biochemical methods. Stoichiometric and kinetic studies indicated that the hydrogen peroxide was utilized more effectively in the electroenzymatic method. Denitrification as intermediate reaction was also investigated.

Evaluation of the role of specific acidic amino acid residues in electron transfer between the flavodoxin and cytochrome c3 from Desulfovibrio vulgaris

A hypothetical model for electron transfer complex between cytochrome c3 and the flavodoxin from the sulfate-reducing bacteria Desulfovibrio vulgaris has been proposed, based on electrostatic potential field calculations and NMR data [Stewart, D. E., LeGall, J. , Moura, I., Moura, J. J. G., Peck, H. D., Jr., Xavier, A. V., Weiner, P. K., & Wampler, J. E. (1988) Biochemistry 27, 2444-2450]. This modeled complex relies primarily on the formation of five ion pairs between lysine residues of the cytochrome and acidic residues surrounding the flavin mononucleotide cofactor of the flavodoxin. In this study, the role of several acidic residues of the flavodoxin in the formation of this complex and in electron transfer between these two proteins was evaluated. A total of 17 flavodoxin mutants were studied in which 10 acidic amino acids--Asp62, Asp63, Glu66, Asp69, Asp70, Asp95, Glu99, Asp106, Asp127, and Asp129--had been permanently neutralized either individually or in various combinations by substitution with their amide amino acid equivalent (i.e., asparate to asparagine, glutamate to glutamine) through site-directed mutagenesis. The kinetic data for the transfer of electrons from reduced cytochrome c3 to the various flavodoxin mutants do not conform well to a simple bimolecular mechanism involving the formation of an intermediate electron transfer complex. Instead, a minimal electron transfer mechanism is proposed in which an initial complex is formed that is stabilized by intermolecular electrostatic interactions but is relatively inefficient in terms of electron transfer. This step is followed by a rate-limiting reorganization of that complex leading to efficient electron transfer. The apparent rate of this reorganization step was enhanced by the disruption of the initial electrostatic interactions through the neutralization of certain acidic amino acid residues leading to faster overall observed electron transfer rates at low ionic strengths. Of the five acidic residues involved in ion pairing in the modeled complex proposed by Stewart et al. (1988), the kinetic data strongly implicate Asp62, Glu66, and Asp95 in the formation of the electrostatic interactions that control electron transfer. Less certainty is provided by this study for the involvement of Asp69 and Asp129, although the data do not exclude their participation. It was not possible to determine whether the modeled complex represents the optimal configuration for electron transfer obtained after the reorganization step or actually represents the initial complex. The data do provide evidence for the importance of electrostatic interactions in electron transfer between these two proteins and for the existence of alternative binding modes involving acidic residues on the surface of the flavodoxin other than those proposed in that model.

Electron transfer in tetrahemic cytochromes c3: spectroelectrochemical evidence for a conformational change triggered by heme IV reduction

Electron transfer in tetrahemic cytochromes c3 from Desulfovibrio vulgaris Hildenborough (D.v.H.) and Desulfovibrio desulfuricans Norway (D.d.N.) strains has been investigated by thin layer spectroelectrochemistry with visible absorption, CD, and resonance Raman (RR) monitoring. The observed splitting of the isosbestic point in the Soret absorption band indicates that the electron transfer from the (FeIII)4 state to the (FeII)4 state proceeds via an intermediate species, which corresponds to 25 and 50% reduction for the D.v.H. cyt.c3 and the D.d.N. cyt.c3, respectively. For the latter, a specific CD signal is observed at half-reduction. RR monitoring of the redox process does not reveal multiple splitting of the high-frequency RR bands, at variance with previously published results on the enzymatic reduction of cyt.c3 from Desulfovibrio vulgaris Miyazaki, a cytochrome highly homologous to D.v.H. cyt.c3 [Verma, A.L., Kimura, A., Nakamura, A., Yagi, T., Inoguchi, H., & Kitagawa, T. (1988) J. Am. Chem. Soc. 110, 6617-6623]. The low-frequency RR spectra of the intermediate species differ significantly from the ones calculated from a linear combination of the all-ferric and all-ferrous states, for the same reduction ratio. Frequency shifts of the bending modes of the cysteine and propionate heme substituents are observed, as well as changes specific to each cytochrome; most notable is the activation of two torsional modes in the case of D.d.N. cyt.c3. Comparison of the results obtained for the two cytochromes leads to the conclusion that reduction of heme IV triggers the observed conformational change. This conclusion is supported by the spectroelectrochemical investigation of the mutant D.v.H. cyt.c3 H25M, in which the sixth ligand of heme III, histidine, is replaced by a methionine.

Kinetic studies on the electron-transfer reaction between cytochrome c3 and flavodoxin from Desulfovibrio vulgaris strain Hildenborough

The kinetic properties of the electron-transfer process between reduced Desulfovibrio vulgaris cytochrome c3 and D. vulgaris flavodoxin have been studied by anaerobic stopped-flow techniques. Anaerobic titrations of reduced cytochrome c3 with oxidized flavodoxin show a stoichiometry of 4 mol of flavodoxin required to oxidize the tetraheme cytochrome. Flavodoxin neutral semiquinone and oxidized cytochrome c3 are the only observable products of the reaction. At pH 7.5, the four-electron-transfer reaction is biphasic. Both the rapid and the slow phases exhibit limiting rates as the flavodoxin concentration is increased with respective rates of 73.4 and 18.5 s-1 and respective Kd values of 65.9 +/- 9.4 microM and 54.5 +/- 13 microM. A biphasic electron-transfer rate is observed when the ionic strength is increased to 100 mM KCl; however, the observed rate is no longer saturable, and relative second-order rate constants of 5.3 x 10(5) and 8.5 x 10(4) M-1 s-1 are calculated. The magnitude of the rapid phase of electron transfer diminishes with the level of heme reduction when varying reduced levels of the cytochrome are mixed with oxidized flavodoxin. No rapid phase is observed when 0.66e(-)-reduced cytochrome c3 reacts with an approximately 25-fold molar excess of flavodoxin. At pH 6.0, the electron-transfer reaction is monophasic with a limiting rate of 42 +/- 1.4 s-1 and a Kd value of approximately 8 microM. Increasing the ionic strength of the pH 6.0 solution to 100 microM KCl results in a biphasic reaction with relative second-order rate constants of 5.3 x 10(5) and 1.1 x 10(4) M-1 s-1. Azotobacter vinelandii flavodoxin reacts with reduced D. vulgaris cytochrome c3 in a slow, monophasic manner with limiting rate of electron transfer of 1.2 +/- 0.06 s-1 and a Kd value of 80.9 +/- 10.7 microM. These results are discussed in terms of two equilibrium conformational states for the cytochrome which are dependent on the pH of the medium and the level of heme reduction [Catarino et al. (1991) Eur. J. Biochem. 207, 1107-1113].

Redox properties of Desulfovibrio gigas [Fe3S4] and [Fe4S4] ferredoxins and heterometal cubane-type clusters formed within the [Fe3S4] core. Square wave voltammetric studies

The same polypeptide chain (58 amino acids, 6 cysteines) is used to build up two ferredoxins in Desulfovibrio gigas a sulfate reducing organism. Ferredoxin II (FdII) contains a single [Fe3S4] core and ferredoxin I (FdI) mainly a [Fe4S4] core. The [Fe3S4] core can readily be interconverted into a [Fe4S4] complex (J.J.G. Moura, I. Moura, T.A. Kent, J.D. Lipscomb, B.H. Huynh, J. LeGall, A.V. Xavier, and E. Munck, J. Biol. Chem. 257, 6259 (1982)). This interconversion process suggested that the [Fe3S4] core could be used as a synthetic precursor for the formation of heterometal clusters. Co, Zn, Cd, and Ni derivatives were produced (I. Moura, J.J.G. Moura, E. Munck, V. Papaephthymiou, and J. LeGall, J. Am. Chem. Soc. 108, 349 (1986), K. Sureurs, E. Munck, I. Moura, J.J.G. Moura, and J. LeGall, J. Am. Chem. Soc. 109, 3805 (1986), and A.L. Macedo, I. Moura, J.J.G. Moura, K. Surerus, and E. Munck, unpublished results). The redox properties of a series of heterometal clusters (MFe3S4] are assessed using direct electrochemistry (square wave voltammetry--SWV) promoted by Mg(II) at a glassy carbon electrode (derivatives: Cd (-495 mV), Fe (-420 mV), Ni (-360 mV), and Co (-245 mV) vs normal hydrogen electrode (NHE)). In parallel, the electrochemical behavior (cyclic voltammetry--CV, differential pulse voltammetry--DPV and SWV) of FdI and FdII were investigated as well as the cluster interconversion process. In addition to the +1/0 (3Fe cluster) and +2/+1 (4Fe cluster) redox transitions, a very negative redox step, at -690 mV, was detected for the 3Fe core, reminiscent of a postulated further 2e- reduction step, as proposed for D. africanus ferredoxin III by F.A. Armstrong, S.J. George, R. Cammack, E.C. Hatchikian, and A.J. Thomson, Biochem. J. 264, 265 (1989). The electrochemical redox potential values are compared with those determined by independent methods (namely by electron paramagnetic resonance (EPR) and visible spectroscopy).

Recent progress in the electrochemistry of c-type cytochromes

C-type cytochromes are classified into two main groups: i) cytochromes which give fast electrochemical responses at the conventional electrodes in the absence of any promoter (eg multi-heme cytochromes c3); ii) cytochromes which need the presence of promoters or the use of modified electrodes to exhibit fast electrochemical responses (eg one-heme mitochondrial cytochrome c). In the latter case, careful design of electrode surface and composition of the solution are required for the attainment of rapid and reversible electron-exchange reactions. Some general considerations are given on the 'electrochemical model'. In particular, binding interactions between the electrode and the protein can take place in a similar manner to that occurring between physiological partner proteins. Electrochemistry when coupled to other physical techniques can give more complete insights in the relationship between the redox properties, structure and function of c-type cytochromes. In particular, in the case of polyheme cytochromes, promising results are expected from the study of site-directed mutagenesis-modified cytochrome c3.

Voltammetric studies of the catalytic electron-transfer process between the Desulfovibrio gigas hydrogenase and small proteins isolated from the same genus

The kinetics of electron transfer between the Desulfovibrio gigas hydrogenase and several electron-transfer proteins from Desulfovibrio species were investigated by cyclic voltammetry, square-wave voltammetry and chronoamperometry. The cytochrome c3 from Desulfovibrio vulgaris (Hildenborough), Desulfovibrio desulfuricans (Norway 4), Desulfovibrio desulfuricans (American Type Culture Collection 27774) and D. gigas (NCIB 9332) were used as redox carriers. They differ in their redox potentials and isoelectric point. Depending on the pH, all the reduced forms of these cytochromes were effective in electron exchange with hydrogenase. Other small electron-transfer proteins such as ferredoxin I, ferredoxin II and rubredoxin from D. gigas were tentatively used as redox carriers. Only ferredoxin II was effective in mediating electron exchange between hydrogenase and the working electrode. The second-order rate constants k for the reaction between reduced proteins and hydrogenase were calculated based on the theory of the simplest electrocatalytic mechanism [Moreno, C., Costa, C., Moura, I., Le Gall, J., Liu, M. Y., Payne, W. J., van Dijk, C. & Moura, J. J. G. (1993) Eur. J. Biochem. 212, 79-86] and the results obtained by cyclic voltammetry were compared with those obtained by chronoamperometry. Values for k of 10(5)-10(6) M-1 s-1 (cytochrome c3 as electron carrier) and 10(4) M-1 s-1 (ferredoxin II as the electron carrier) were determined. The rate-constant values are discussed in terms of the existence of an electrostatic interaction between the electrode surface and the redox carrier and between the redox carrier and a positively charged part of the enzyme.

Electrochemical studies of the hexaheme nitrite reductase from Desulfovibrio desulfuricans ATCC 27774

The electron-transfer kinetics between three different mediators and the hexahemic enzyme nitrite reductase isolated from Desulfovibrio desulfuricans (ATCC 27774) were investigated by cyclic voltammetry and by chronoamperometry. The mediators, methyl viologen, Desulfovibrio vulgaris (Hildenborough) cytochrome c3 and D. desulfuricans (ATCC 27774) cytochrome c3 differ in structure, redox potential and charge. The reduced form of each mediator exchanged electrons with nitrite reductase. Second-order rate constants, k, were calculated on the basis of the theory for a simple catalytic mechanism and the results, obtained by cyclic voltammetry, were compared with those obtained by chronoamperometry. Values for k are in the range 10(6)-10(8) M-1 s-1 and increase in the direction D. desulfuricans cytochrome c3-->D. vulgaris cytochrome c3-->methyl viologen. An explanation is advanced on the basis of electrostatic interactions and relative orientation between the partners involved. Chronoamperometry (computer controlled) offers advantages over cyclic voltammetry in the determination of homogeneous rate constants (faster, more accurate and better reproducibility). Direct, unmediated electrochemical responses of the hexaheme nitrite reductase were also reported.