The Iron-Sulfur Clusters

Iron-sulfur clusters are important co-factors in electron-transfer. Type I reaction centres contain [4Fe-4S] clusters as final electron acceptors mediating ET to soluble electron carriers like ferredoxin or flavodoxin (reviewed in references 188, 224, 314, 315) In PS I three clusters (F FA and FB) have been clearly identified and spectroscopically characterized. The PsaA and PsaB subunits carry most of the ET cofactors in PS I.18178-316 Each of them provides two Cys ligands to the binding site of the interpolypeptide [4Fe-4S] cluster Fx. This binding site is identical on both core PS I subunits.317 Both [4Fe-4S] clusters FA and Fb are bound to the PS I stromal subunit PsaC. It contains two identical [4Fe-4S] consensus binding sites CxxCxxCxxxCP (C = cysteine, P = proline). 

The oxidized state [4Fe-4S]2+ is diamagnetic in its ground state, due to antiferromagnetic coupling of the two ferric and the two ferrous ions. However, at room temperature excited paramagnetic states of the cluster can be populated. Due to effective relaxation EPR signals of these paramagnetic clusters are usually only detectable at T 35 K. In the reduced state the [4Fe-4S]l+ cluster 

The unique spectra of either Aj or Aj- could be detected in unbound PsaC where cysteine II in the consensus iron-sulfur binding site was changed to Asp, Gly, Ala, 88 314324. in this case the altered cluster is suggested to be in a high spin state (S 3/2) and is therefore not detectable in the g = 2 region of the EPR spectra, where pure spectra of unchanged iron sulfur clusters in a low spin state (S = 1/2) could be observed. The unique spectra of FA- and FB could also be observed in the fully assembled PS I, which was dark-frozen in the presence of ascorbate and illuminated under cryogenic temperatures inside the EPR cavity (for review see reference 188). 

The dilferent EPR properties (g) values, relaxation behaviour) of Fj and Fj-together with site-directed mutations allowed the assignment of the 2 clusters to specific sites in the PS I protein.325-326 This assignment was found to be the same in PS I from different species.188 

In recent work Shen et al.336 inactivated a gene in Synechococcus sp. PCC 7002 

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FIGURE 20.7 (a) The aconitase reaction converts citrate to cis-aconitate and then to isocitrate. Aconitase is stereospecific and removes the pro-/ hydrogen from the pro-/ arm of citrate, (b) The active site of aconitase. The iron-sulfur cluster (red) is coordinated by cysteines (yellow) and isocitrate (white). 

Plus 241 distance constraints for the unassigned residues close to the iron—sulfur cluster derived from the X-ray structure... 

In 1987, the iron-sulfur clusters Fa and Fb acting as terminal electron acceptors in photosystem I have been shown to be located on a... 

The mimicking of the iron-sulfur clusters by synthetic chemistry has been quite successful over the years. One of the last synthetic clusters to be obtained was the [3Fe-4S] cluster (109, 110). This new synthetic compound was useful for the demonstration of interconversion pathways, as well as for the formation of different heterometal clusters beyond those produced in proteins (111). The [3Fe-4S] core... 

Hagen, W.R., Vanoni, M.A., Rosenbaum, K., and Schnackerz, K.D. 2000. On the iron-sulfur clusters in the complex redox enzyme dihydropyrimidine dehydrogenase. European Journal of Biochemistry 267 3640-3646. 

Verhagen, M.F.J.M., Kooter, I.M., Wolbert, R.B.G., and Hagen, W.R. 1993. On the iron-sulfur cluster of adenosine phosphosulfate reductase from Desulfovibrio vulgaris (Hildenborough). European Journal of Biochemistry 221 831-837. 

Core extrusion studies—removal of the iron-sulfur cluster intact from the enzyme surroundings—have been carried out and the iron-cluster types in proteins identified through the process shown in equation 6.10.18 DMS0/H20 is the protein unfolding solvent for this process. By this method, Fe-protein and MoFe-protein metal-sulfur clusters have been removed from the holoenzyme for separate analysis by many instrumental techniques. 

The interactions of NO with the iron-sulfur cluster moieties of several enzymes generate iron-sulfur-nitrosyl cluster compounds [156]. However, synthetic nitrosyl clusters such as Roussins black salt (RBS), Roussins red salt (RRS), Roussins red ester (RRE) and [FeNOS]4 (Fig. 5.3) are well known [129, 157] and can be synthesized easily [158-164]. For example, the RBS can be prepared by mixing FeS04 with NaN03 and (NH4)2S in aqueous solution [158]. RRE salts are generally insoluble in water, but recently the water soluble sulfonated derivative, Na2[Fe2(SCH2CH2S03)2(N0)4], has been prepared [165]. 

Fig. 38. Stereo drawing of the polypeptide backbone of high-potential iron protein. Tight turns are shown with their central peptide as a dark line. The box in the center represents the iron-sulfur cluster.

For the cytochrome c-plastocyanin complex, the kinetic effects of cross-linking are much more drastic while the rate of the intracomplex transfer is equal to 1000 s in the noncovalent complex where the iron-to-copper distance is expected to be about 18 A, it is estimated to be lower than 0.2 s in the corresponding covalent complex [155]. This result is all the more remarkable in that the spectroscopic and thermodynamic properties of the two redox centers appear weakly affected by the cross-linking process, and suggests that an essential segment of the electron transfer path has been lost in the covalent complex. Another system in which such conformational effects could be studied is the physiological complex between tetraheme cytochrome and ferredoxin I from Desulfovibrio desulfuricans Norway the spectral and redox properties of the hemes and of the iron-sulfur cluster are found essentially identical in the covalent and noncovalent complexes and an intracomplex transfer, whose rate has not yet been measured, takes place in the covalent species [156]. 

Erbes, D. L., Burris, R. H. and Orme-Johnson, W. H. (1975) On the iron-sulfur cluster in hydrogenase from Clostridium pasteurianum W5. Proc. Natl. Acad. Sci. USA, 72, 4795-9. 

Pereira AS, Tavares P, Moura I, et al. 2001. Mossbauer characterization of the iron-sulfur clusters in Desulfovibrio vulgaris hydrogenase. I Am Chem Soc 123 2771-82. 

Hoff KG, Silberg JJ, Vickery LE. 2000. Interaction of the iron-sulfur cluster assembly protein IscU with the Hsc66/Hsc20 molecular chaperone system of Escherichia coli. Proc Natl Acad Sci USA 97 7790-5. 

Hoff KG, Ta DT, Tap ley TL, et al. 2002. Hsc66 substrate specificity is directed toward a discrete region of the iron-sulfur cluster template protein IscU. J Biol Chem 277 27353-9. 

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