Ferredoxin redox couples

The multinuclear tetrahedral iron clusters have the potential for additional formal oxidation states. Because not all of these states have been found in proteins or model compounds, it is possible that some oxidation states may be unstable. For a given Fe S protein only one redox couple is used the other possible states appear to be excluded by restrictions of the protein structure. This selection rule is illustrated with two 4Fe 4S low-molecular-weight electron transfer proteins ferredoxin and high-potential iron protein (HiPIP). The 4Fe 4S clusters in both proteins were shown by X-ray crystallography to be virtually identical. However, the redox potential and oxidation states for the two proteins are vastly... 

Studies of the catalytic or stoichiometric reactions of various 4-Fe-ferredoxin model complexes have been reported using the 3-/2— redox couple with a negative redox potential91 >. Relatively stable are the 2—/I — redox couple for [Fe(Z-cyst-Ile-Ala-OMe)J2 (Z = benzyloxycarbonyl) or [Fe4S4(tipbt)J2 (tripbt = 2,4,6-triiso-propylbenzene-thiolato) in A, A -dimethylformamide. Catalytic oxidation of benzoin by 1,4-benzoquinone in the presence of various cyst-containing peptide complexes or bulky thiolato complexes in DMF has been examined. The postulated mechanism of the catalytic oxidation of benzoin in the presence of [Fe4S2(SR)4]2 is illustrated in Fig. 7. 

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...

Reductions of acetylene, ketone, isonitrile, or nitrile have been carried out using the —3/ —2 redox couple of many model complexes (58-61), but not using the peptide model complex. A proton transfer function has been observed in studies on the ferredoxin model complex, [Fe4S4(SPh)4]2, in toluene/water (62). 

The synthetic [2Fe-2S] model complex of the 20-peptide complex exhibits two LMCT absorption maxima at 423 and 461 nm in DMF, maxima which are near to those of the native plant-type ferredoxin (423 and 466 nm) (69). Two redox couples for — 3/—2 were observed at — 0.64 V versus SCE and at —0.96 V versus SCE in DMF. One of them is very close to the value (—0.64 V versus SCE) of native ferredoxin. The 20-peptide complex containing invariant sequences Cys-A-B-C-D-Cys-X-Y-Cys and Leu-Thr-Cys-Val possesses all essential factors for a model of the active site except for the peptide conformation. The positive-shifted redox potential of the 20-peptide complex in DMF is undoubtedly due to the interactions between the Fe2S22+ core and adjacent amino-acid residues, giving rise to NH--S hydrogen bonding. 

Ferredoxins and Rieske proteins employ a (Fe )2/Fe Fe redox couple for biological electron transfer reactions. Within the protein, the two iron atoms are rendered inequivalent, even in the hilly oxidized (Fe )2 state, by the surrounding protein environment Within a synthetic cluster, however, both iron atoms are typically equivalent, as may be expected from the symmetry of the overall complex. Table 4 shows reduction potentials for selected analog clusters. 

Of course, the microbially mediated aquatic redox couples must be interrelated to the proper intracellular redox couples. For examples, the NADP system, ubiquitous in living organisms and believed to play a major role in electron transport during photosynthesis, exhibits pe (W) = -1-5.5. Moreover, various ferredoxins, now widely considered to be the primary electron receptors from excited chlorophylls, show pe°(W) values in the range —7.0 to —7.5 (Table 8.6b). The coincidence of this range with the pe°(W) value for conversion of CO2 to glucose is suggestive. 

The bacterial ferredoxins and HiPIPs all possess tetracubane clusters containing thiolate ligands, yet the former utilize the - 2/ - 3 cluster redox couple, whereas the latter utilize the - 1/ —2 cluster redox couple. 

Fig. 10. Theoretical calculation of for the [Fe4S4(SR)4] redox couple in representative high- and low-potential ferredoxins. (Reprinted with permission from Ref. 169. Jensen, G. M. Warshel, A. Stephens, P. J. Biochemistry 1994, 33, 10911. Copyright 1994 American Chemical Society.)...

On the basis of structural and electronic similarities, the clusters [Fe4S4(SR)4] serve as close representations of protein sites with the iso-electronic [4Fe-4S] core. Because the proteins function as electron carriers (where their function is known), the redox chemistry of synthetic clusters has been a matter of considerable interest. As seen in Fig. 3, the analogue electron-transfer series encompasses four core oxidation levels, three of which have been detected in proteins. Analogues corresponding to these three levels have been isolated and characterized in very considerable detail. The [4Fe-4S] + + or Fdox/red redox couple is one of the more pervasive in biology. Note that the native [4Fe-4S] state has been proven only in the HP proteins, which should be considered as a special class of ferredoxins. Synthetic cluster potentials, which are strongly dependent on the R substituent, have been found to be —0.2-0.6 V more negative than the E o values for Fd x,ed in aqueous solution (76, 77). 

Ferrates, tris(oxalato)-photoreduction, 471 Ferredoxins, 142 redox potentials, 513 Ferri cyanides oxidation by, 504 redox couples, 512 Ferritin structure, 137 Ferrocene history, 3 stereochemistry nomenclature, 131 Ferrocene, l-acetyl-2-methyl-nomenclature, 131 Ferroin... 

Fig. 4 Cyclic voltammogram of a film of ferredoxin that contains one [4Fe-4S] and one [3Fe-4S] cluster. The three redox couples observable in Sulfolobus acidocaldarius 7Fe ferredoxin have been assigned on the basis of other evidences, mostly spectroscopy. See Ref [34].

Fig. 5 Reaction of a [3Fe-4S] cluster with metal ions, as observed by voltammetry of a film of ferredoxin from Desulfovibrio africanus. Only the oxidative sweep is shown. The scan rate is 200 mV s . Note the decrease in intensity (f) of the two signals from [3Fe-4S]+/° and [3Fe-4S]°/2 redox couples, while the signal due to [M3Fe-4S] +A grows (f). (J. N. Butt,...

Figure 12 Voltammetry of a film of Sulfolobus acidocaldarius ferredoxin showing the formation of a new redox couple at low potential. (Reproduced with permission from J. Am. Chem. Soc. 1998,120, 11994—11999. Copyright 1998 American Chemical Society.)...

B.K., Stout, C.D., Hirst, )., and Armstrong, F.A. (2003) Mechanisms of redox-coupled proton transfer in proteins role of the proximal proline in reactions of the [3fe-4s] cluster in azotobacter vinelandii ferredoxin i. Biochemistry, 42, 10589. 

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