Iron-sulphur clusters Ferredoxins

While the protein chain C surroimding the iron-sulphur clusters (Fig. 2.28) is similar in structure to ferredoxin, it needs a real ferredoxin protein docked on the stroma side to transfer the electron from FeS-B away from PS I. The subsystems C, D and E on the stroma side of PS I (Fig. 2.27) allow docking not only of the fairly small ferredoxin system (an example of which is shown in Fig. 2.30 - there are variations in structure between species such as plants and cyanobacteria), but also of the larger flavodoxin protein (Fromme et al, 2003), an example of which is shown in Fig. 2.31. 

The proposed catalytic mechanism of the ferredoxin oxidoreductase [32] is shown in Fig. 4, a similar mechanism existing for the analogous citric acid cycle enzyme, 2-oxoglutarate oxidoreductase. In outline, the 2-oxoacid is decarboxylated in a TPP-dependent reaction to give an hydroxyalkyl-TPP. From this, one electron is abstracted and transferred to the enzyme-bound iron-sulphur cluster, generating a free-radical-TPP species. This intermediate can then interact direct with coenzyme-A to form acyl-CoA, the iron-cluster receiving the second electron. In each case, ferredoxin serves to re-oxidise the enzyme s redox centre. 

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]. 

Synthetic iron-sulphur cluster compounds of general structure (11) serve as analogues of the active sites of (Fe4S4) ferredoxins and high-potential iron proteins... 

Several models have been proposed for the active center of iron and sulphur in Clostridial ferredoxin in which the cysteine residues in the peptide chain participate in the sulphur bridging. Fig 9 166). Unfortunately X-ray analysis of crystals of these proteins has not been completed. It is difficult to confirm that all the irons are clustered in a single linear array 167, 168). X-ray studies of another non-heme iron protein, the high potential iron protein, hipip, from chromatium, carried out by J. Kraut (personal communication), indicate that the four irons of this molecule may be clustered in a tetrahedral array. 

Figure 10.3 Crystallographic structures of the most important iron-sulphur centres in proteins (a) dimeric centre of ferrodoxin from Spinacia oleracea [54], (b) trimeric centre of ferredoxin from Bacillus schlegelii [55], (c) cubane cluster of nitrogenase reductase from Azotobacter vinelandii [56], (d) nitrogenase octameric cluster from Azotobacter vinelandii [57], (e) nitrogenase octameric cluster from Clostridium pasteurianum [58], (f) MoFe cluster of nitrogenase from Azotobacter vinelandii [59], and (g) active centre of sulphite reductase from Escherichia coli [60]...

The persistent role of Fe-S molecules in electron transfer is quite remarkable. In photosystem I (Figs. 2.28-2.29), in ferredoxin (Fig. 2.30) and now in the nitrogenase and hydrogenase enzymes, similar but not identical clusters of typically 2-5 iron-sulphur molecules are performing the energy transfer processes of a variety of organisms. 

The iron-sulphur centre is probably of the 2Fe-2S type [191,223]. It is a one-electron donor/acceptor with of approx. 280 mV in mitochondria (pH independent below pH 8 [224,225]). It exhibits an EPR spectrum in the reduced state that is somewhat anomalous for 2Fe-2S clusters (see Ref. 221). This, as well as the high midpoint redox potential, suggest that the iron ligands may be less electronegative than the four cysteine sulphurs of the plant ferredoxin model (see Ref. 226). The EPR spectrum of the FeS cluster is affected by the redox state of ubiquinone... 

FIGURE 9.8. Structures of iron-sulphur centers of rubredoxin and ferredoxin. Rubredoxins are the simplest iron-sulphur proteins, and these contain one Fe-S center with the iron in a tetrahedral environment. Ferredoxins contain clusters of two [2Fe-2S] or four [4Fe-4S] iron atoms. 

Much effort has been devoted to the synthesis of iron - sulphur and heterometallic clusters with the aim of modelling the spectroscopic, electron - transfer and catalytic properties of clusters within proteins such hydrogenase, the ferredoxins, the high potential iron proteins and, not least, the components of nitrogenase. 

Clostridial ferredoxin, which is characterized by two clusters containing four sulphur-linked iron atoms each, has been studied by Mossbauer and e.s,r. spectroscopy. The iron of oxidized ferredoxin is high-spin Fe and reduction leads to an uptake of one electron per cluster. The two Fe centres in oxidized spinach ferredoxin are not equivalent and it seems that the metal co-ordination geometry at the non-reducible site is basically tetrahedral. The first well-defined synthetic analogue of the active sites of two-iron ferredoxins has been prepared and charac-... 

Ferredoxins (Fds) are widespread in the three domains of life and an abundance of sequence data and structural information are available for Fds isolated from several sources. In particular, the bacterial type Fds are small electron-transfer proteins that posses cubane xFe-yS clusters attached to the protein matrix by Fe ligation of Cys via a conserved consensus ligating sequence. The archaeal type ferredoxins are water-soluble electron acceptors for the acyl-coenzyme A forming 2-oxoacid/ferredoxin oxidoreductase, a key enzyme involved in the central archaeal metabolic pathways. Fds have been distinguished according to the number of iron and inorganic sulphur atoms, 2Fe-2S, 4Fe-4S/3Fe-4S (Fig. Ib-d) and Zn-containing Fds. 

The cluster found in certain bacterial ferredoxins involved in anaerobic metabolism. It consists of a cubane-like cluster of four iron atoms, four labile sulphur atoms, thus Fe4S4, and four cysteine ligands... 

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