Proteins iron sulfur

Iron-sulfur proteins are found in a variety of organisms, bacteria, plants, and animals, and serve as electron transfer agent.s via one-electron oxidation-reduction step [redox potential --0.43V in chloroplasts to 1-0.35 V in 

Johnson et al. and Czemuszewicz et al. measured the RR spectra of Ihese proteins and their mode] compound as sh6wn in Fig. V-25. The bands near 385, 335, 298, 270, and 250 cm are assigned to the ridging whereas those near 395 and 362 cm are attributed to the terminal p(Fe-S) vibraiions. 

Vibrational Mode Symmetry SpFd, (R) [FeaSzCSj-w-xyOz]  

The number tn parentheses indicates the due to i lFe-S ) (S, = bridging sulfur). S- S isotope shift of a band primarily  

Czernuszewicz et al. also carried out normal coordinate analysis on the model compound mentioned above, and proposed the band assignments shown in Table V-3. It is seen that oxidized Clostridium Pasieitrianum (Cp Fd) and the model compound exhibit marked spliiiings of ihe triply degenerate modes 

The peptides cystein and methionine contain sulfur in their side groups. Sulfides bind easily to metal ions, such as Fe U Cu +, Zn +, and Pb . In the case of Fe, Fe-S complexes are formed, which are capable of easy reduction or oxidation. Pb + is poisonous and binds strongly at Zn + sites, but the structure is very different from the tetrahedral zinc complex. 

FIGURE 11.9 (a) Heme A (b) heme B (c) heme C. The isoprenoid chain of heme A is 

4Fe-4S (c) complex. The Fe atoms are connected to the sulfurs of the cys peptide. All iron atoms are surrounded by four sulfur ions in a tetrahedral geometry. Other iron-sulfur proteins have three iron ions. The oxidation state is varying. Iron and sulfur are strongly coupled and form a unit that can easily accept or donate electrons without great structural changes. 

Complex I is a very large enzyme, also called NADH dehydrogenase or NADH quinone reductase. The electrons of NADH and two protons form FMNH2 from FMN. The electrons are transferred through iron-sulfur proteins and then further on to ubiquinone. Many ET steps in Complex I have been studied down to atomic detail by Stuchebrukhov et al. Other detailed studies of iron-sulfur proteins have been carried out by L. Noodleman. 

Iron-sulfur proteins are also found in Complex III and Complex IV and in fer-rodoxins, cyt c reductase, nitrogenase, and a number of other systems. In most cases, they have a role in mitochondrial redox reactions. The iron-sulfur complex is easily oxidized or reduced. In fact, the HOMO-LUMO gap is small, so that Fe-S complexes work as small pieces of metal. Solid iron sulfide may become a metal if high pressure is applied. 

The classification of iron-sulfur proteins typically uses the number of irons contained in the coordination. Although at least two irons are necessary to accommodate the inorganic sulfur, the simple Fe(S)4 coordination with cysteinyl sulfur also counts to the group of iron-sulfur proteins. This motif is rare and realized only in few proteins, e.g. rubredoxin or desulforedoxin. 

An excellent, still up-to-date survey on chemical properties, structures and biological functions of iron-sulfur clusters has been given by Beinert.174 As a more general survey of EPR properties of iron-sulfur proteins we recommend a reference which was also quoted in our earlier report.175 

An unusual cysteine sequence motif together with atypical EPR and Mossbauer properties has been reported earlier for a [Fe2S2] protein denoted FhuF. It is an iron-regulated E. coli protein which is probably involved in the reduction of ferric iron in ferrioxamine B.191 In a recent study the authors could provide evidence for the mixed valence state in the FhuF protein to be capable of 

An interesting new experimental approach has been taken in order to separate overlapping EPR spectra as they appear e.g. in the multi Fe/S centre containing complex I. Inversion- and saturation-recovery measurements which allow to measure Ti relaxation times are used in a inversion-recovery filter which is subsequently applied to separate EPR signals on account of their Trdifferences. In addition, this filter can be used in conjunction with high-resolution hyperfine measurements e.g. by ESEEM and thus the separated centres can be characterized in depth.211 

Two [Fe4S4] clusters have been identified and analyzed in bacterial and archaeal adenylylsulfate reductases. These enzymes are of importance in the sulfur cycle. The role of the iron-sulfur centres in relation to the third cofactor, FAD, has been studied. It was shown that the [Fe4S4] clusters function were electron transport guiding two electrons to the FAD catalytic site.234 A novel [Fe4S4] cluster with a high spin ground state (S = 3/2) was observed in the catalytic site of E. coli nitrate reductase A.235 

Many proteins contain a peculiar unit that is made of iron atoms and sulfur (S) atoms. The most widely occurring such unit is a cube in which four iron atoms and four sulfur atoms are occupying alternate positions (see Fig. 6.3). An alternative is a unit that consists of two iron atoms and two sulfur atoms (Fig. 6.3). The proteins that contain such iron-sulfur unit(s) are called iron-sulfur protein. Both of these units act as an electron-transfer unit, by changing its iron between Fe(II) and Fe(ni) as in cytochromes. In particular, iron-sulfur proteins that are involved in photosynthesis are called ferredoxins. Obviously, ferredoxins are not found in humans or other animals they do not perform photosynthesis. However, there are a number of other proteins that contain iron-sulfur units even in human beings. Some of the important iron-sulfur proteins are involved in the electron-transport process in the respiration mentioned above. 

Synthetic Analogues of the Active Sites of Iron-Sulfur Proteins 

Examples of ferredoxin reactions can be observed in the scheme reproduced in Fig. 5.1. Some of these reactions such as the reduction of NAD to NADH by Hj, 

Two centers Rb pasteurianum) Aerobe photosynthetic bacteria Pseudomonas oleovorans 19 

Ferredoxins (Fd) 2Fe-2S Ferredoxins Plants Ferredoxins Photosynthetic organisms. Spinach 11.5 

Molybdoproteins Xanthine oxidase 2Mo, 8Fe-8S Mammals (milk, liver) 275 

A number of poorly resolved proton ENDOR peaks have been observed between 10-20 MHz269-271. Only small changes of the overall shape of the ENDOR spectrum were detected when the sample was freeze-dried and redissolved in D20, i.e. no strongly coupled exchangeable protons were present. From comparison with the proton ENDOR spectrum in an anhydrous powder, it was assumed that the signals arose from the methylene protons of the cysteine ligands and that the iron-sulfur chromophore was not exposed to solvent water270/. 

The 8-Fe proteins contain two four-iron-sulfur cubes which are separated (center-to-center) by about 12 A. The EPR spectra from the fully reduced proteins are typical for a 4-Fe center with intercluster spin-spin coupling. An ENDOR study on the 8-Fe ferredo-xin from C. pasteurianum in the fully reduced state has been reported by Anderson et al.273. Since the 57Fe-56Fe difference spectra are only poorly resolved, analysis of the ENDOR data turned out to be difficult. All eight iron atoms are assumed to have similar AFe tensors with principal values Afe = 25 MHz, AFe = 29 MHz and AFe = 33 MHz. These coupling parameters suggest that the reducing electrons are delocalized over the four irons of each of the two nearly identical 4-Fe clusters. 

The relevant structural and chemical information on iron-sulfur proteins provided by EPR, ENDOR and Mossbauer techniques serves as an instructive example for the success of combining different types of spectroscopic methods. 

In most paramagnetic sandwich complexes the proton hf structure is not resolved in low temperature powder or single crystal EPR spectra. In vanadium dibenzene, a typical example of this type of compound, the poor resolution is due to the fact that the aromatic rings are rigidly frozen at T 50 K and thus the proton hfs tensors of the benzene rings are no longer magnetically equivalent. 

To evaluate the full proton hfs tensors for the rigid molecule and to study the temperature dependence of the dynamics of the ring rotation, ENDOR and EI-EPR spectroscopy has been applied to powder samples of these two systems37,78). EPR, ENDOR and EI-EPR data of V(bz)2 diluted into Fe(cp)2 are summarized in Table 18. 

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The lUBMB Commission on Nomenclature has issued a number of recommendations dealing with areas of a more biochemical nature (72), such as peptide hormones (86), conformation of polypeptide chains (87), abbreviations for nucleic acids and polynucleotides (88), iron—sulfur proteins (89), enzyme units (90), etc. The Commission has also produced rules and recommendations for naming enzymes (91,92). 

PJ Stephens, DR Jollie, A Warshel. Protein control of redox potentials of iron-sulfur proteins. Chem Rev 96 2491-2513, 1996. 

PD Swartz, T Ichiye. Protein contributions to redox potentials of iron-sulfur proteins An energy minimization study. Biophys J 73 2733-2741, 1997. 

PD Swartz, BW Beck, T Ichiye. Stiaictural origins of redox potential m iron-sulfur proteins Electrostatic potentials of crystal structures. Biophys 1 71 2958-2969, 1996. 

Wachtershanser has also suggested that early metabolic processes first occurred on the surface of pyrite and other related mineral materials. The iron-sulfur chemistry that prevailed on these mineral surfaces may have influenced the evolution of the iron-sulfur proteins that control and catalyze many reactions in modern pathways (including the succinate dehydrogenase and aconitase reactions of the TCA cycle). 

All these intermediates except for cytochrome c are membrane-associated (either in the mitochondrial inner membrane of eukaryotes or in the plasma membrane of prokaryotes). All three types of proteins involved in this chain— flavoproteins, cytochromes, and iron-sulfur proteins—possess electron-transferring prosthetic groups. 

Three protein complexes have been isolated, including the flavoprotein (FP), iron-sulfur protein (IP), and hydrophobic protein (HP). FP contains three peptides (of mass 51, 24, and 10 kD) and bound FMN and has 2 Fe-S centers (a 2Fe-2S center and a 4Fe-4S center). IP contains six peptides and at least 3 Fe-S centers. HP contains at least seven peptides and one Fe-S center. 

Other non-haem proteins, distinct from the above iron-sulfur proteins are involved in the roles of iron transport and storage. Iron is absorbed as Fe" in the human duodenum and passes into the blood as the Fe protein, transferrin, The Fe is in a distorted octahedral environment consisting of 1 x N, 3x0 and a chelating carbonate ion which... 

Cytochromes and Iron Sulfur Proteins in Bacterial Sulfur Metabolism (U. 

Capozzi F, Ciurli S, Luchinat C (1998) Coordination Sphere Versus Protein Environment as Determinants of Electronic and Functional Properties of Iron-Sulfur Proteins 90 127-160... 

Xavier AV, Moura JJG, Moura I (1981) Novel Structures in Iron-Sulfur Proteins. 43 187-213 Xavier AV, see Pereira lAC (1998) 91 65-90... 

Teo BK, Shuhnan RG (1982) In Spiro T (ed) Iron-sulfur proteins. Wiley, New York... 

In 1964, Rieske and co-workers reported the observation of an EPR signal around g = 1.90 in the cytochrome bci complex (1). They succeeded in the isolation of the iron sulfur protein that gave rise to the EPR signal and showed that it contained a [2Fe-2S] cluster. Over the... 

Rieske proteins are constituents of the be complexes that are hydro-quinone-oxidizing multisubunit membrane proteins. All be complexes, that is, bci complexes in mitochondria and bacteria, b f complexes in chloroplasts, and corresponding complexes in menaquinone-oxidizing bacteria, contain three subunits cytochrome b (cytochrome 6e in b f complexes), cytochrome Ci (cytochrome f in b(,f complexes), and the Rieske iron sulfur protein. Cytochrome 6 is a membrane protein, whereas the Rieske protein, cytochrome Ci, and cytochrome f consist of water-soluble catalytic domains that are bound to cytochrome b through a membrane anchor. In Rieske proteins, the membrane anchor can be identified as an N-terminal hydrophobic sequence (13). 

Resonance Raman (RR) spectroscopy provides information about the vibrational characteristics of a chromophore, for example, a metal center, within the complex environment of a protein. In RR spectra, those vibrational transitions are observed selectively that are coupled to electronic transitions. In iron sulfur proteins, this technique has been used to resolve the complex electronic absorption spectra and to identify both vibrational and electronic transitions. 

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 ... 

Since their discovery, Rieske proteins have been the object of numerous studies aimed at gaining insight into the molecular basis of their unique properties. These studies not only have shed light on Rieske and Rieske-type clusters, but also have contributed to the understanding of iron sulfur proteins in general. 

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