Ferredoxins types

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. 

The Mosshauer data performed on the as-isolated D. vulgaris [Fe] hydrogenase are in agreement with the EPR data, and further support the presence of two ferredoxin-type [4Fe-4S] clusters and that the H-cluster is in the diamagnetic state in the as-purified enzyme (169). 

C) cuboidal three-iron-four-sulfide [Fe3-S4] clusters—stable oxidation states are 0 and + 1 and (D) cubane four-iron-four-sulfide [Fe4-S4] clusters—stable oxidation states are + 1 and +2 for ferredoxin-type clusters and +2 and +3 for HIPIP clusters. Electrons can be delocalized, such that the valences of individual iron atoms lie between ferrous and ferric forms. Low-molecular-weight proteins containing the first and the last three types are referred to as rubredoxins (Rd) and ferredoxins (Fd), respectively. The protein ligands are frequently Cys residues, but a number of others are found, notably His, which replaces two of the thiol ligands in the [Fe2-S2] Rieske proteins. In addition to these, discrete Rd... 

This enzyme [EC 1.14.99.15] catalyzes the reaction of 4-methoxybenzoate with AH2 and dioxygen to produce 4-hydroxybenzoate, formaldehyde. A, and water. The bacterial enzyme consists of a ferredoxin-type protein and an iron-sulfur fiavoprotein (FMN). 4-Ethoxyben-zoate, A-methyl-4-aminobenzoate, and toluate can serve as substrates as well. The fungal enzyme acts best on ver-atrate. 

Xanthine oxidase catalyzes the oxidation of hypox-anthine and xanthine to uric acid. Xanthine oxidase is a complex metalloflavoprotein containing one molybdenum, one FAD and two iron-sulfur centers of the ferredoxine type in each of its two independent subunits. Usually, the enzyme is isolated from cow s milk. The enzyme is inhibited by allopurinol and related compounds. The production of uric acid from the substrate (xanthine) can be determined by measuring the change in optical density in the UV range. 

In the recently discovered photosynthetic bacterium Heliobacterium chlorum, which has a BChl g complex as primary donor, the primary acceptor is also a ferredoxin-type molecule with g values 2.04, 1.94 and 1.88 [40]. An earlier acceptor could be photoaccumulated at very low (< —620 mV) redox potential. It had a near-Gaussian EPR line at g = 2.0038 with AB = 15 G at X-band and 18 G at Q-band [40]. 

The primary acceptors of the two plant photosystems differ fundamentally from each other, no doubt because of their different redox midpoint potentials (about -100 to -200 mV for PS II, -705 to -730 mV for PS I [R3-R5]). In PS I two iron-sulfur (ferredoxin-type) proteins, and Fg, with characteristic EPR spectrum in the reduced state ( m between -450 and -550 mV), have been observed (Fig. 2) that function either parallel or in series (see Ref. R5 for a recent review). The shape of the spectra of the two ferredoxin-type acceptors and in particular their principal g values depend on whether one or both acceptors are reduced (Fig. 2). It is unlikely that this is due to a magnetic interaction, as the differences depend linearly on the microwave frequency, i.e. on the applied magnetic field (exchange and dipolar interactions are independent of field Table 3) [16,42], Possibly, Coulomb repulsion causes strain-induced g shifts. 

Orme-Johnson and Mtinck (1980) in unpublished data refer to transients signals with g = 2 (associated with a redox potential near — 240 mV) which are observed during the oxidation of these clusters, supporting the existence of PN in the most reduced state. However, the problem may be more complex since ferredoxin type (g < 2) and HiPIP type (g > 2) EPR signals are observed in CO inhibited conditions. Another possibility could be that the transition is + 3/+ 2 but the redox potential does not seem appropriate. 

Fe—4 S] center U.V./visible with broad bands around 400 and 300 mm, but similar to those observed for [3 Fe—xS] centers. Weak CD spectrum not easily identifiable. Extrusion and displacement reactions give characteristic products. EPR in 80% DMSO (reducing conditions) give an axial type spectrum observable below 35 K. HiPIP and ferredoxin type centers may be readily distinguished based on EPR characteristics. So far all the HiPIP known have a very positive redox potential. 

One of the two important parts of enzyme is a flavo-iron-sulfur protein with NADH-dependent oxidoreductase activity. The reductase is a monomeric 34 kDa (in case of phthalate dioxygenase reductase) flavo-iron-sulfur protein containing flavin mononucleotide (FMN) and a plant-ferredoxin-type [2Fe-2S] center in a 1 1 ratio [372]. Structure of this part of the enzyme has been studied recently by X-ray crystallographic analysis [375], low-temperature EPR [383], and kinetically [378]. 

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