Ferredoxin reduction potentials

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. 

The catalytic significance of this observation is not known since no deviation from a two-electron Nemst plot is observed with NADH as reductant and no kinetic studies have been done to compare the rate of the NAD -facilitated comproportionation reaction with the rate of catalytic turnover. No comparable studies on the effect of NADP on the oxidation-reduction potential of ferredoxin-NADP reductase have been, to our knowledge, published. Inasmuch as the physiological role for this enzyme is reduction of the pyridine nucleotide rather than its oxidation, the potential of the enzyme should be significantly lower than that of the pyridine nucleotide couple. Indeed, a value of —445 mV has been determined for this flavoenzyme with the driving force for its reduction being due to a decrease of 90 mV in the one-electron potential of the ferredoxin reductant. This increase... 

FIGURE 19-5 Iron-sulfur centers. The Fe-S centers of iron-sulfur proteins may be as simple as (a), with a single Fe ion surrounded by the S atoms of four Cys residues. Other centers include both inorganic and Cys S atoms, as in (b) 2Fe-2S or (c) 4Fe-4S centers, (d) The ferredoxin of the cyanobacterium Anabaena 7120 has one 2Fe-2S center (PDB ID 1 FRD) Fe is red, inorganic S2 is yellow, and the S of Cys is orange. (Note that in these designations only the inorganic S atoms are counted. For example, in the 2Fe-2S center (b), each Fe ion is actually surrounded by four S atoms.) The exact standard reduction potential of the iron in these centers depends on the type of center and its interaction with the associated protein. 

In crystal field theory calculations the direction of the axial distortion is along the z-axis. Therefore, the dz2 orbitals in iron atoms in Fig. 15 are along the line adjoining the two iron atoms. Remembering that the dz2 orbital lies lowest in this symmetry, the effect of reducing the complex is to add electron density to the dz% orbitals of the iron atoms. Since the dz2 iron-orbitals in Fig. 15 overlap, this structure results in an electron repulsion term between the iron atoms which increases as the iron atoms in these proteins are reduced. Thus, the negative reduction potentials (Table 1) of the plant-type ferredoxins can be accounted for by this model. 

The direct energetic comparison of different cycles is sometimes questionable, as the reducing power of various electron donors is not the same. For example, the reduction potential of reduced ferredoxin is stronger than that of the reduced pyridine nucleotides. In growing cells, ferredoxin is often reduced by a hydroge-... 

K. Tagawa and D. F. Arnon, Oxidation-reduction potentials and stoichiometry of electron transfer in ferredoxins, Biochim. Biophys. Acta, 153, 602-613 (1968). 

Because of this low oxidation-reduction potential, the number of methods available for reducing ferredoxin is limited. Apart from hydrogen gas, ferredoxin may be reduced with organic reductants, such as pyruvate or hypoxanthine in the presence of the appropriate enzymes. Ferredoxin can be reduced nonenzymically with sodium hydrosulfite (dithionite) (Tagawa and Arnon (99) Fry et al. (45)), potassium borohydride (D Eustachio and Hardy (40)), and formamidine sulfinic acid (Shashoua (90)). It can be reduced also by illuminated chloroplasts (Whatley, Tagawa, and Arnon (114)) and, under these conditions, the reduction of ferredoxin is most complete (Bachofen and Arnon (12)). 

The data of Table 11 compare the properties of certain non-heme iron proteins with ferredoxin. While there are certain similarities and differences between these proteins, it is stressed that the main feature which uniquely distinguishes ferredoxin from the others and from spectrally similar proteins from mammalian sources Kimura and Suzuki 60) Omura et al. (77)) is its low oxidation-reduction potential. This feature of ferredoxin renders it capable of fulfilling its recently recognized roles in cellular metabolism. These are dealt with in the final section of this chapter. 

Figure 3.7. The energy profile of a nitrogenase reaction. Eo is the standard redox potential of the reactants, intermediates and products of the reaction Fd = ferredoxin FeP = Fe protein FeMo = FeMo protein. The arrow indicates the increase of the reduction potential upon ATP hydrolysis (Likhtenshtein 1988a). Reproduced with permission.

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. 

In all oxygen-evolving organisms, the PS I reaction centres finally reduce a water-soluble ferredoxin. This small protein of around 10 kDa has a (2Fe-2S) cluster and a rather low midpoint reduction potential of -400 mV. Ferredoxin binds to the PS I centre after reduction it participates both in linear electron flow to NADP, via ferredoxin-NADP reductase, and in cyclic electron flow around the PS I centre. Two membrane-bound iron-sulfur centres, designated Centre A (or F, ) and Centre B (or Fg), appear to be the terminal acceptors in the reaction centre. Their mode of functioning is not clearly established and their structure is not well known, mainly because they cannot be extracted without their complete denaturation. F and Fq can be photoreduced at low temperature in cells or in purified PS I centres. Characteristic EPR spectra are thus obtained with g values of 1.86, 1.94, 2.05 for F, and 1.89, 1.92, 2.05 for F -. 

Given that the reduction potentials for plastocyanin and ferredoxin are +0.37 V and -0.45 V, respectively, the standard free energy for the oxidation of reduced plastocyanin by oxidized ferredoxin is +18.9 kcal mob (+79.1 kJ mofi). This uphill reaction is driven by the absorption of a 700-nm photon which has an energy of 40.9 kcal mofi (171 kJ mob )... 

Iron-sulphur clusters are the third type of the widely available electron-transfer sites in biology. They consist of iron ions surrounded by four sulphur ions, either thiolate groups from cysteine residues or inorganic sulphide ions. Regular clusters with one (rubredoxins), two, three, or four (ferredoxins) iron ions are known, as well as a number of more irregular clusters, also with other ligands than cysteine [112,181]. Their reduction potentials vary between -700 and +400 mV [112]. 

The overall process (outlined in Figure 3-7) is based on the following considerations, (a) The electrons from the pigment system must have a reduction potential more negative than - 0.55 v in order to reduce methylviologen. A likely estimate would be -0.60v, which would be sufficient to reduce a natural electron acceptor, such as ferredoxin, and, in turn, NADP". (b) Two einsteins of 700 nm light contains 81,572 cal. If 60% is... 

Plastoquinone in turn is a reductant for excited P700 of photosystem PS I, which operates similarly to the system PS II and has a reduction potential sufficient for an electron transfer to the iron-sulfur complex of ferredoxin and finally to NADP , producing NADP -H,. 

---