7Fe ferredoxin

The last class of iron-sulfur proteins to be considered is that of the bacterial 7Fe ferredoxins, which contain both an Fe3S4 and an Fe4S4 cluster. Figure 28 shows the molecular structure of that from Azotobacter vinelandii (FW — 12 700).53... 

Figure 28 X-Ray structure of the active site of the 7Fe ferredoxin of Azotobacter vinelandii...

Figure 29 Cyclic voltammogram recorded at an edge-orientedpyrolitic graphite electrode in an aqueous solution (pH 6.4) of the 7Fe ferredoxin of Sulfolabus acidocaldarius in the presence of neomicin sulfate...

In confirmation of the chemical and electrochemical reversibility of these processes, the one-electron reduced complex of the 7Fe ferredoxin of Azotobacter vinelandii has been structurally characterized. The Fe3S4 cluster, on which the first reduction is centred, maintains a quasi-cuboidal geometry essentially unaltered with respect to the oxidized form some minor structural variation is observed on the peripheral amino acids.55... 

Figure 4.40 Voltammograms of films of different 7Fe ferredoxins on a pyrolitic graphite (edge) electrode, obtained at 0 °C (a) Azotobacter mnelandii (Fd I), at pH 7.0, with a scan rate of 20 mV s 1 (b) Desulfovibrio africanus (Fd III), at pH 7.0, with a scan rate of 191 mV s 1 (c) Sulfolobus acidocal-darius (Fd), at pH 7.4, with a scan rate of 10 mV s-1. In each case, an electroactive coverage of approximately one monolayer is obtained in the presence of polymyxin as the co-adsorbate. The signals, A, B and C refer to the redox couples [3FE-4S]+/0, [4FE-4S]2+/+ and [3FE-4S]0/,2+, respectively. Reprinted from Electrochim. Acta, 45, F.A. Armstrong and G.S. Wilson, Recent developments in faradaic bioelectrochemistry, 2623-2645, (Copyright) 2000, with permission from Elsevier Science...

An obvious application of direct electrochemistry of proteins is the determination of redox potentials. In cases of thermodynamically inaccessible or kinetically reactive species for which traditional potentio-metric methods are inappropriate, direct electrochemistry offers an alternative approach. A good example is the 7Fe ferredoxin from Azo-tobacter chroococcum, which contains one [4Fe-4S] and one [3Fe-4S] cluster. The [4Fe-4S] + potential obtained (7i) by cyclic voltamme-... 

Armstrong, F.A., George, S.J., Thomson, A.J., and Yates, M.G. (1988) Direct electrochemistry in the characterization of redox proteins-novel properties of Azobacter 7Fe ferredoxin. FEES Letters, 234 (1), 107-110. 

The first point 1) is very important since complex formation that is strong enough to cause the protein to interact intimately with the electrode surface may alter intrinsic properties and change the reduction potential. This is particularly relevant since the promoters are free in solution. One example in which two promoters yield different reduction potentials is in the voltammetry [105] of Azotobacter chroococcum 7Fe ferredoxin, which is discussed later. 

The investigations outlined above demonstrate the utility of electrochemical techniques in probing cluster reactions. The titrimetric bulk electrolysis procedure allows new cluster types to be prepared easily, with quantitative information on stoichiometry. The cyclic voltammetry approach permitted a systematic study of other ferredoxins to determine the factors that allow rapid reactions of this type to occur. It could be shown, for example, that the presence of Asp in place of Cys in the sequence was not sufficient to confer this striking ambivalence between cluster types. The 7Fe ferredoxin from Sulfolobus acidocaldarius, which also has the... 

Figure 5 Voltammograms of solutions of 7Fe ferredoxins at a PGE electrode (A) Azo-tobacter vinelandii, (B) Sulfolobus acidocaldarius. In each case, proteins are 80 pM in 20 mM Mes, 0.1 M NaCl, 2mM neomycin, 0.1 mM EOT A, pH 6.4. Scan rate 5 mV s", temperature 0°C.

Figure 6 Cyclic voltammograms of 7Fe ferredoxins immobilized at aPGE electrode with polymyxin as co-adsorbate, pH 7.6, temperature 0°C. Conditions (not optimized to reveal couples in clearest detail) are as follows. Top Azotobacter vinelandii Fdl scan rate 100 mV s Center Desulfovibrio qfricanus Fdlll scan rate 10 mV s Bottom Sulfolobus acido-calarius Fd scan rate 100 mV s. For A.v. Fdl, signals B interfere significantly with C at this scan rate. (Reproduced with permission from J. Am. Chem. Soc. 1996,118, 8593-8603. Copyright 1996 American Chemical Society).

Figure 9 Voltammograms of Azotobacter vinelandii 7Fe ferredoxin D15N and native forms, each commenced from the oxidized state, showing how the oxidation reaction is gated in different time domains ( ) by proton transfer. Proton transfer is retarded in the D15N mutant, thus re-oxidation is not observed at 1 V s but is observed again at 100 V s because the proton does not arrive at the cluster during the cycle. For the native protein, re-oxidation is not observed at 100 V s. ...

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