Protein environment, iron-sulfur protein

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... 

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. 

Iron-sulfur proteins. In an iroinsulfiir protein, the metal center is surrounded by a group of sulfur donor atoms in a tetrahedral environment. Box 14-2 describes the roles that iron-sulfur proteins play in nitrogenase, and Figure 20-30 shows the structures about the metal in three different types of iron-sulfur redox centers. One type (Figure 20-30a l contains a single iron atom bound to four cysteine ligands. The electron transfer reactions at these centers... 

Maillard J, W Schumacher, F Vazquez, C Regeard, WR Hagen, C Holliger (2003) Characterization of the corrinoid iron-sulfur protein tetrachloroethene reductive dehalogenase of Dehalobacter restrictus. Appl Environ Microbiol 69 4628-4638. 

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). 

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... 

Some linear structures are known, as in (Et4N)3[Fe3S4(SR)4], where R=Et or Ph.752,810 The iron atoms are bridged by two sulfide ions and the terminal positions are filled by the thiol groups, to give a tetrahedral environment around each iron atom. The ESR spectra of these models do not correspond to the spectra of any known iron-sulfur protein, with the exception of unfolded aconitase, which thus appears to have a linear 3Fe core.811 The model also represents isolated sections of the polymer KFeS2. 

The first Mossbauer spectroscopic studies on this class of iron-sulfur proteins were carried out by Blomstrom et al. 174) who reported only spectra taken on the oxidized form of the protein from Clostridium pasteuranium. Evidence in the form of two partially resolved quadrupole pairs was presented to support two distinct environments for the iron in the oxidized form of the protein, the ratio of 5 2 was suggested in keeping with the then thought number of Fe atoms per protein molecule. This assignment of the number of Fe atom per site, of course, rests on the assumption of equal Lamb-Mossbauer recoil-free fractions for the two sites. 

The relationship between the structure of the peptide environment and active site chemical properties is thus of considerable interest and to a large extent awaits future studies. In this article, some examples of such specific relationships are described, particularly in the case of iron-sulfur proteins, because this class of proteins is distributed widely in living organisms ranging from bacterial cells to mammals. The major function of the proteins is now known to be electron transfer and the... 

The existence of iron-sulfur proteins in our present oxidizing environment has to be attributed to the fact that, during a... 

The most simple iron-sulfur protein is rubredoxin (Rd) which contains one Fe atom per protein. The Fe atom is coordinated by four sulfur atoms of cysteinyl residue in a tetrahedral environment. Figure V-21 shows the crystal structure of a model compound, [FefSi-o-xyl),]" (S2-o-xyl = o-xylene-of,a -diihiolate). Long et al." first obtained the RR spectrum of... 

Nitrogenase is a complex enzyme which catalyzes the reduction of a variety of substrates (Table IV) with the concomitant hydrolysis of ATP. It contains two, easily separable, iron-sulfur proteins one of which contains molybdenum. These require a reductant, a reducible substrate, an ATP-generating system, and an anaerobic environment to function. The ATP-generating system is necessary because ADP inhibits nitrogenase activity. Two important discoveries have helped to simplify the assay for nitrogenase. First, sodium dithionite proved to be an adequate reductant to replace the complex and unstable pyruvate phosphoroclastic system (Bulen et al., 1965) and, second, nitrogenase reduces acetylene to ethylene (Dilworth, 1966 Schollhorn and Burris, 1967) which can be measured at nanomolar concen-... 

An interesting aspect in the structures of iron-sulfur proteins are some amino acid sequences which are approximately the same in a number of proteins. Thus, as observed in Fig. 5.6, there are also some sequences, e.g. Cys-X-X-Cys or Cys-X-X-Cys-X-X-Cys, in which the relative positions of the cysteine residues which are known to directly bind the Fe-S are absolutely conserved in proteins from different species. Since the coordinative environment of iron sulfur cluster in different species are practically the same, the specificity of sites in a given protein are fundamentally determined by the configuration of the rest of the polypeptidic chain. This feature, besides the wide distribution of ferredoxins in numerous different species, permits us to establish philogenetic relationships as that illustrated in Fig. 5.7. 

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