Iron-sulfur ferredoxin

It should be noted that non-heme oxygenases can also degrade aromatics such as biphenyls and naphthalene (Scheme 7.23). A naphthalene dioxygenase consists of a catalytic oxygenase component with a mononuclear iron site, an iron-sulfur flavoprotein reductase and an iron-sulfur ferredoxin transferring electrons from... 

Pyruvate ferredoxin oxidoreductase. Within Clostridia and other strict anaerobes this enzyme catalyzes reversible decarboxylation of pyruvate (Eq. 15-35). The oxidant used by clostridia is the low-potential iron-sulfur ferredoxin.320 3203 Clostridial ferredoxins contain two Fe-S clusters and are therefore two-electron oxidants. Ferredoxin substitutes for NAD+ in Eq. 15-33 but the Gibbs energy decrease is much less (-16.9 vs - 34.9 kj / mol. for oxidation by NAD+). 

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. 

Tedro SM, Meyer TE, Kamen MD (1979) Primary structure of a high potential, four-iron-sulfur ferredoxin from the photosynthetic bacterium, Rhodospirillum tenue. J Biol Chem 254 1495-1500... 

The following sections review 3 systems which we have developed to explore the generality of these concepts mimics of hemoglobin, cytochrome P450 and of iron-sulfur ferredoxins. While the use of zeolites as enzyme mimics is an under-studied area it is certainly not new as evidence a previous volume in this series (1). The intent here is to emphasize the general concepts of zeolite biomimicry by highlighting the key results we have obtained. Interested readers are referred to the... 

Iron Sulfur Compounds. Many molecular compounds (18—20) are known in which iron is tetrahedraHy coordinated by a combination of thiolate and sulfide donors. Of the 10 or more stmcturaHy characterized classes of Fe—S compounds, the four shown in Figure 1 are known to occur in proteins. The mononuclear iron site REPLACE occurs in the one-iron bacterial electron-transfer protein mbredoxin. The [2Fe—2S] (10) and [4Fe—4S] (12) cubane stmctures are found in the 2-, 4-, and 8-iron ferredoxins, which are also electron-transfer proteins. The [3Fe—4S] voided cubane stmcture (11) has been found in some ferredoxins and in the inactive form of aconitase, the enzyme which catalyzes the stereospecific hydration—rehydration of citrate to isocitrate in the Krebs cycle. In addition, enzymes are known that contain either other types of iron sulfur clusters or iron sulfur clusters that include other metals. Examples include nitrogenase, which reduces N2 to NH at a MoFe Sg homocitrate cluster carbon monoxide dehydrogenase, which assembles acetyl-coenzyme A (acetyl-CoA) at a FeNiS site and hydrogenases, which catalyze the reversible reduction of protons to hydrogen gas. 

Nonrepetitive but well-defined structures of this type form many important features of enzyme active sites. In some cases, a particular arrangement of coil structure providing a specific type of functional site recurs in several functionally related proteins. The peptide loop that binds iron-sulfur clusters in both ferredoxin and high potential iron protein is one example. Another is the central loop portion of the E—F hand structure that binds a calcium ion in several calcium-binding proteins, including calmodulin, carp parvalbumin, troponin C, and the intestinal calcium-binding protein. This loop, shown in Figure 6.26, connects two short a-helices. The calcium ion nestles into the pocket formed by this structure. 

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 ... 

An iron sulfur-flavoprotein that transfers electrons to the ferredoxin, as in naphthalene dioxygenase (class III)... 

During the 1960s, research on proteins containing iron—sulfur clusters was closely related to the field of photosynthesis. Whereas the first ferredoxin, a 2[4Fe-4S] protein, was obtained in 1962 from the nonphotosynthetic bacterium Clostridium pasteurianum (1), in the same year, a plant-type [2Fe-2S] ferredoxin was isolated from spinach chloroplasts (2). Despite the fact that members of this latter class of protein have been reported for eubacteria and even archaebacteria (for a review, see Ref. (3)), the name plant-type ferredoxin is often used to denote this family of iron—sulfur proteins. The two decades... 

Fig. 1. Iron-sulfur clusters basic building blocks. In most cases the iron is tetrahe-drally coordinated by sulfur from cysteinyl residues (and labile sulfur). Variability on coordination is allowed (see text). A, Rubredoxin type FeS4 (simplest cluster, no labile sulfur) B, plant-type ferredoxin [2Fe-2S] C, bacterial ferredoxin [3Fe-4S] D, bacterial ferredoxin and HiPIP [4Fe-4S] E, novel cluster [4Fe-2S, 20] ( hybrid cluster ).

In particular, the study of SRB ferredoxins enables us to survey the different properties of simple iron-sulfur proteins, including electron transfer, flexibility in coordination chemistry, and ability to undergo cluster interconversions. Most of the observations can be extrapolated to more complex situations. 

Fig. 6. Representative EPR spectra displayed by trinuclear and tetranucleEir iron-sulfur centers, (a) and (b) [3Fe-4S] + center in the NarH subunit of Escherichia coli nitrate reductase and the Ni-Fe hydrogenase fromD. gigas, respectively, (c) [4Fe-4S] + center in D. desulfuricans Norway ferredoxin I. (d) [4Fe-4S] center in Thiobacillus ferrooxidans ferredoxin. Experimental conditions temperature, 15 K microwave frequency, 9.330 GHz microwave power, (a) 100 mW, (b) 0.04 mW, (c) smd (d) 0.5 mW modulation amplitude (a), (c), (d) 0.5 mT, (b) 0.1 mT.

Fe-4S] + + clusters are certainly the most ubiquitous iron-sulfur centers in biological systems. They play the role of low potential redox centers in ferredoxins, membrane-bound complexes of the respiratory... 

The conversion of a [3Fe-4S] into a [4Fe-4S] center was achieved by restoring the second residue of the consensus motif in E. coli fu-marate reductase (181) and in D. africanus ferredoxin III (161). However, the coordination scheme of the iron-sulfur centers of A. vinelan-... 

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