Iron-sulfur proteins rubredoxin

XAS data comprises both absorption edge structure and extended x-ray absorption fine structure (EXAFS). The application of XAS to systems of chemical interest has been well reviewed (4 5). Briefly, the structure superimposed on the x-ray absorption edge results from the excitation of core-electrons into high-lying vacant orbitals (, ] ) and into continuum states (8 9). The shape and intensity of the edge structure can frequently be used to determine information about the symmetry of the absorbing site. For example, the ls+3d transition in first-row transition metals is dipole forbidden in a centrosymmetric environment. In a non-centrosymmetric environment the admixture of 3d and 4p orbitals can give intensity to this transition. This has been observed, for example, in a study of the iron-sulfur protein rubredoxin, where the iron is tetrahedrally coordinated to four sulfur atoms (6). 

R)-2-METHYLMALATE DEHYDRATASE NICOTINATE DEHYDROGENASE NITRATE REDUCTASE NITRITE REDUCTASE PHENYLALANINE MONOOXYGENASE PROLYL 3-HYDROXYLASE PROLYL 4-HYDROXYLASE PROTOCATECHUATE 3,4-DIOXYGENASE PROTOCATECHUATE 4,5-DIOXYGENASE RIESKE IRON-SULFUR PROTEIN RUBREDOXIN... 

In this review we will deal with iron-sulfur proteins where the iron atoms are coordinated only by cysteine ligands and bridging sulfurs, as well as rubredoxin (Rd hereafter), which is the initial building block in all subsequent discussions. 

Although heme is absent in Clostridia, it was early recognized that anaerobic bacteria may contain substantial levels of iron (44). To date the best characterized iron compounds from this source are the iron-sulfur proteins termed ferredoxins and rubredoxins. Molecular structures of representatives of both types of protein have been worked out by Jensen and his colleagues by X-ray diffraction analysis (see below). 

Although also iron-sulfur proteins, the rubredoxins do not generate H2S on acidification since in this case the thiol groups are contributed by cysteinyl residues in the polypeptide chain. The function of clostridial rubredoxin is as yet unknown in Pseudomonas sp. a similar protein catalyzes the co-hydroxylation of alkanes, a reaction requiring molecular O2. 

All members of the class of iron-sulfur proteins contain, per mole, from 1 to 8 iron atoms and from zero (rubredoxins) to 8 labile sulfides. 

Chapter 6). Other iron-sulfur proteins, so named because they contain iron sulfur clusters of various sizes, include the rubredoxins and ferredoxins. Rubredoxins are found in anaerobic bacteria and contain iron ligated to four cysteine sulfurs. Ferredoxins are found in plant chloroplasts and mammalian tissue and contain spin-coupled [2Fe-2S] clusters. Cytochromes comprise several large classes of electron transfer metalloproteins widespread in nature. At least four cytochromes are involved in the mitrochondrial electron transfer chain, which reduces oxygen to water according to equation 1.29. Further discussion of these proteins can be found in Chapters 6 and 7 of reference 13. 

Fe-S proteins contain four basic core structures, which have been characterized crystal-lographically both in model compounds (Rao and Holm, 2004) and in iron-sulfur proteins. These are (Figure 3.6), respectively, (A) rubredoxins found only in bacteria, in which the [Fe-S] cluster consists of a single Fe atom liganded to four Cys residues—the iron atom... 

In this text, iron-sulfur clusters are discussed because they appear in proteins and enzymes (1) cytochrome b(6)f, Rieske [2Fe-2S] cluster (Section 7.5 and Figure 7.26) (2) cytochrome bci, Rieske [2Fe-2S] cluster (Section 7.6 and Figure 7.30) and (3) aconitase, [4Fe-4S] cluster (Section 7.9.2.1, and Figure 7.50). The iron-sulfur protein (ISP) component of the cytochrome b(6)f and cytochrome bci complexes, now called the Rieske ISP, was first discovered and isolated by John S. Rieske and co-workers in 1964 (in the cytochrome bci complex). More information about the RISP is found in Section 7.5.1. Section 7.9.2 briefly discusses other proteins with iron-sulfur clusters—rubredoxins, ferrodoxins, and the enzyme nitrogenase. The nitrogenase enzyme was the subject of Chapter 6 in the hrst edition of this text— see especially the first edition s Section 6.3 for a discussion of iron-sulfur clusters. In this second edition, information on iron-sulfur clusters in nitrogenase is found in Section 3.6.4. See Table 3.2 and the descriptive examples discussed in Section 3.6.4. 

One large class of non-heme iron-containing biomolecules involves proteins and enzymes containing iron-sulfur clusters. Iron-sulfur clusters are described in Sections 1.7 (Bioorganometallic Chemistry) and 1.8 (Electron Transfer) as well as in Section 3.6 (Mossbauer Spectroscopy). See especially Table 3.2 and the descriptive examples discussed in Section 3.6.4. Iron-sulfur proteins include rubredoxins, ferrodoxins, and the enzymes aconitase and nitrogenase. The nitrogenase enzyme was the subject of Chapter 6 in the hrst edition of this text—see especially Section 6.3 for a discussion of iron-sulfur clusters. In this... 

Fig. 1. Schematic representation of the four basic types of iron-sulphur clusters. The rubredoxin-type and the [2Fe-2S], [3Fe—4S] and [4Fe-4S] clusters are shown in (a) to (d), respectively. Reprint from Prog. Biophys. Mol. Biol., Vol. 70, H. Sticht and P. Rosch, The structure of iron-sulfur proteins , pp. 95-136, Copyright 1998, with permission from Elsevier Science.

Iron-sulfur proteins serve predominantly as electron carriers (28,29). The best understood examples are those proteins with IFe, 2Fe, and 4Fe centers. The environment of the mononuclear iron center, rubredoxin, is shown in structure C (17). It consists of a distorted tetrahedral array of sulfur atoms from cysteine residues at nearly equal distances from the iron atom. Crystal structures are available for 2Fe-2S ferredoxins from Spindina plantensis (19) and Aphanothece sacrum (20). A representation of the geometry of this site is given in structure D. The 2Fe-2S core is anchored to the polypeptide by ligation to 4 cysteine sulfur atoms, yielding distorted tetrahedral geometry for both iron atoms. Crystal... 

Rubredoxins. Rubredoxins are the simplest form of iron-sulfur proteins in which iron is bound to the sulfur atom of cysteine as shown in Fig. lA. One of the first rubredoxins isolated was from an anaerobic bacterium, Clostridium pasteurianum, by Lovenberg and Sobel (9). The protein is composed of 54 amino acids and has a molecular weight of 6,000. The oxidized form has absorbance maxima at 380 and 490 nm. The biological role of the rubredoxin isolated from C. pasteurianum is stiU unknown. 

The prosthetic groups of iron-sulfur proteins fall into several classes (Lov-enberg, 1977 Spiro 1982). Rubredoxins bind single iron atoms with four cys-teinyl sulfur ligands they function as electron carriers in some bacterial systems. Rubredoxins generally have two such centers per molecule in the ferric state each iron center is EPR detectable. 

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. 

Iron-sulfur proteins contain non-heme iron and inorganic (acid-labile) sulfur in their active centers as 4Fe-4S or 2Fe-2S or, in the case of rubredoxin, as one iron alone. The iron is always bonded to cysteine sulfur. They catalyze redox reactions between +350 and —600 mV (hydrogen electrode = —420 mV). They are usually of low molecular weight (6000-15,000 Daltons) but can form complex enzymes with molybdenum and flavin. They occur as soluble or membrane-bound proteins and catalyze key reactions in photosynthesis, oxidative phosphorylation, nitrogen fixation, H2 metabolism, steroid hydroxylation, carbon and sulfur metabolism, etc. They occur in all organisms so far investigated and may... 

Iron-sulfur proteins containing characteristic Cys-X-Y-Cys sequences have been found to exhibit enzymatic activity through an electron transfer. For example, rubredoxin has an Fe(II/III) ion surrounded by two Cys-X-Y-Cys sequences. Figure 4 shows a structure... 

NADH-cytochrome f>o reductase (9) and NADPH-cytochrome P-450 reductase (10, 11) are microsomal enzymes. The latter has been referred to until very recently as NADPH-cytochrome c reductase, since that is how it is assayed, but there is no cytochrome c in microsomes and its physiological acceptor seems to be cytochrome P-460. It is thus distinguished from NADH-putidaredoxin reductase (12), NADPH-adrenodoxin reductase (13), and NADH-rubredoxin reductase (14). The adrenodoxin reductase and the rubredoxin reductase, together with their respective iron-sulfur protein acceptors, each constitute a cytochrome P-460 reductase system. 

CV = cyclic voltammetry Fd = ferredoxin HP = high potential iron-sulfur protein IRP = iron regulatory protein LS3 = l,3,5-tris((4,6-dimethyl-3-mercaptophenyl)thio)-2,4,6-tris(/ -tolylthio)benzene (3-) Rd = rubredoxin SCE = standard calomel electrode tibt = 2,4,6-triisopropylphenyl Tp = tris(pyrazolyl)hydroborate (1-). 

Negative-ion gas-phase photoelectron spectroscopy has also recently been nsed to evalnate the reorganization energies of tetrahedrally coordinated Fe+ anions that serve as models for the active sites of mononuclear iron sulfur proteins such as rubredoxin that cycles between high spin Fe+ and high spin... 

W. A. Eaton and W. Lovenberg, The Iron-Sulfur Complex in Rubredoxin, in Iron Sulfur Proteins , ed. W. Lovenberg, Academic Press, New York, 1973, Vol. 12, p. 131. 

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