Reduction-oxidation potentials Ferredoxin

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. 

Flavodoxins are a group of flavoproteins which function as electron carriers at low potential in oxidation-reduction systems. The proteins of this group contain one molecule of FMN as their prosthetic group, but, in contrast to ferredoxins, do not contain metals such as iron. 

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... 

If an enzyme binds a flavin radical much more tightly than the fully oxidized or reduced forms, reduction of the flavoprotein will take place in two one-electron steps. In such proteins the values of E° for the two steps may be widely separated. The best known examples are the small, low-potential electron-carrying proteins known as flavodoxins.266 269a These proteins, which carry electrons between pairs of other redox proteins, have a variety of functions in anaerobic and photosynthetic bacteria, cyanobacteria, and green algae. Their functions are similar to those of the ferredoxins, iron-sulfur proteins that are considered in Chapter 16. 

V = -0.431 V. Thus, the value of E1/2 changes from -0.371 to -0.460 V as the pH is increased. In the pH range between the pKa values of 7.4 and 8.9 reduction of the protein will lead to binding of a proton from the medium and oxidation to loss of a proton. Human and other vertebrate ferredoxins also show pH-depen-dent redox potentials.282 This suggests, as with the cytochromes, a possible role of Fe-S centers in the operation of proton pumps in membranes. Nevertheless, many ferredoxins, such as that of C. pasteurianum, show a constant value of E0 from pH 6.3 to 10296 and appear to be purely electron carriers. 

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. 

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. 

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 )... 

HiPIP Formerly used abbreviation for high-potential iron-sulfur protein, now classed as a ferredoxin. An ELECTRON-TRANSFER PROTEIN from photosynthetic and other bacteria, containing a [4FE-4S] CLUSTER which undergoes oxidation-reduction between the [4Fe-4S]2+ and [4Fe-4S]3+ states. 

Tagawa, K., Arnon, D.I. Oxidation-Reduction Potentials and Stoichiometry of Electron Transfer in Ferredoxins, Biochim. Biophys. Acta 153, 602 (1968)... 

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