Ferredoxins redox potentials

Key Words Ferredoxin redox potential transgenic technology. 

P700, the primary electron donor of PSI, and Ao, the electron acceptor, are chlorophylls, P700 having a redox potential of around -490 mV. Ai is possibly a quinone, and FeSx, FeSA, and FeSe are iron-sulfur centers with potentials of -705, -530, and -580 mV, respectively. FeSA and FeSe probably operate in parallel and denote electrons to ferredoxin (redox potential, -420 mV) (Figure 1.7). 

A second example is that of an Ala-to-Cys mutation, which causes the fonnation of a rare SH S hydrogen bond between the cysteine and a redox site sulfur and a 50 mV decrease in redox potential (and vice versa) in the bacterial ferredoxins [73]. Here, the side chain contribution of the cysteine is significant however, a backbone shift can also contribute depending on whether the nearby residues allow it to happen. Site-specific mutants have confirmed the redox potential shift [76,77] and the side chain conformation of cysteine but not the backbone shift in the case with crystal structures of both the native and mutant species [78] the latter can be attributed to the specific sequence of the ferre-doxin studied [73]. 

While the redox potentials of Rieske clusters are above -1-100 mV at pH 7, values between 100 and 150 mV have been determined for the redox potentials of Rieske-type clusters (Table XI). Several 4-cysteine coordinated [2Fe-2S] clusters have redox potentials similar to those of Rieske-type clusters, for example, the [2Fe-2S] clusters of the dioxygenase reductases [compilation in (104)]-, therefore, the redox potential is not useful for distinguishing between Rieske-type and ferredoxin-type clusters. 

Although the redox potential of Rieske-type clusters is approximately 400 mV lower than that of Rieske clusters, it is 300 mV more positive than the redox potential of plant-type ferredoxins (approximately -400 mV). Multiple factors have been considered to be essential for the redox potential of iron sulfur proteins ... 

While the oxidation reduction potential of the ferredoxins is —0.2 V to —0.4 V and that of the rubredoxins is about —0.05 V, a protein from the photosynthetic bacterium Chromatium has a redox potential of +0.35 V. This is the high potential iron protein, or HIPIP. 

Studies of ferredoxin [152] and a photosynthetic reaction center [151] have analyzed further the protein s dielectric response to electron transfer, and the protein s role in reducing the reorganization free energy so as to accelerate electron transfer [152], Different force fields were compared, including a polarizable and a non-polarizable force field [151]. One very recent study considered the effect of point mutations on the redox potential of the protein azurin [56]. Structural relaxation along the simulated reaction pathway was analyzed in detail. Similar to the Cyt c study above, several slow relaxation channels were found, which limited the ability to obtain very precise free energy estimates. Only semiquantitative values were... 

More simple is to account for the difference between the relatively high redox potentials of Rieske ferredoxins and the low potential values of plant 2Fe ferredoxins. In fact, the dianionic form of plant ferredoxins [(Cys)2Fem(/i2-S)2Feni(Cys)2]2- certainly electrostatically disfavours electron addition with respect to the neutral charge of Rieske ferredoxins [(Cys)2FeIIWS)2FeII,(His)2]°... 

Complete sequences of ca. 50 different plant-type ferredoxins(Fd) are known. The invariant sequences nearest to the 2-Fe core are confirmed to be Pro-Tyr-Ser-Cys-Arg-Ala-Gly-Ala-Cys-Ser-Thr-Cys-Ala-Gly and Leu-Thr-Cys-Val. 2Fe-2S complexes of oligopeptides with the Cys-X-Y-Cys sequence have been synthesized by ligand exchange reactions (7,23). We have examined the redox potentials of these model complexes, and the results are shown in Table I. The reversibility improved remarkably and the potential approached the value of the native proteins as the sequence more closely simulated that of the proteins. It is conjectured that hydrogen bonds from the peptide N-H s to thiolate and/or sulfide groups increase the stability of the reduced cluster. It is likely that peptide sequences like those found in the proteins favor the formation of such hydrogen bonds. 

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... 

---