Rhombic iron-sulfur

FPR studies at low temperature detect the presence of one iron-sulfur center and molybdenum. At low temperature a sample of nitrate reductase reduced by dithionite shows a rhombic signal (gm,x = 2.04, gmed = 1.94, and gnm = 1.90). This signal accounts for 0.84 spins/... 

Both stopped-flow and rapid freeze quench kinetic techniques show that the substrate reduces the flavin to its hydroquinone form at a rate faster than catalytic turnover Reoxidation of the flavin hydroquinone by the oxidized Fe4/S4 center leads to formation of a unique spin-coupled species at a rate which appears to be rate limiting in catalysis. Formation of this requires the substrate since dithionite reduction leads to flavin hydroquinone formation and a rhombic ESR spectrum typical of a reduced iron-sulfur protein . The appearance of such a spin-coupled flavin-iron sulfur species suggests the close proximity of the two redox centers and provides a valuable system for the study of flavin-iron sulfur interactions. The publication of further studies of this interesting system is looked forward to with great anticipation. 

The [4Fe-4S] cluster in AOR and FOR is paramagnetic in its reduced form and displays characteristic EPR resonances, although the spin relaxation rate is very fast and the spectra are observed only at very low temperatures (see Iron-Sulfur Proteins). Hence, at 4K, the EPR spectrum of AOR is dominated by resonances from its reduced Fe-S cluster. However, this gives rise to a rhombic signal from a S = 3/2 ground state, while reduced 4Fe-clusters typically have an S = 1/2 ground state. In fact, while mixed spin, S = 3/2 and S = 1/2 reduced [4Fe-4S] clusters are sometimes observed in some iron-sulfur proteins (see Iron-Sulfur Proteins), clusters that have exclusively S = 3 /2 ground states are so far unique to AOR. Indeed, the reduced 4Fe-cluster of FOR has a mixed spin state with S = 3/2 (80%) and S = 1/2 (20%) components. The factors that determine the spin state of a [4Fe-4S] center have yet to be elucidated. 

Iron/sulfur clusters are inorganic cofactors that are used in all cells (10). They comprise ions and iron ions in the +2 or -f3 state (Fig. 1). Iron/sulfur clusters are essential cofactors for numerous redox and nonredox enzymes, alone or in tandem with organic cofactors such as llavocoenzymes and/or pyridine nucleotides. The simplest structural type is the rhombic [2Fe-2S] cluster (3). [3Fe-4S] (4) and [4Fe S] (5) clusters are characterized by distorted cubic symmetry (10,11). Clusters can form aggregates, and other metal ions can replace iron ions or can be present additionally. 

FIGURE 4.5 Structures of the four common iron—sulfur centres (C—Cys). (a) Rubredoxin (b) rhombic two iron—two sulfide [Fc2—S2] cluster (c) cuboidal three-iron—four sulfide [Fes—S4] cluster and (d) cubane four iron—four sulfide [Fc4-S4] cluster. 

Iron—sulfur proteins contain four basic core structures which have been characterized crystallographically both in model compounds and in iron—sulfur proteins (Rao and Holm, 2004). These are (Figure 13.16), respectively, (a) rubedoxins found only in bacteria, in which the [Fe—S] cluster consists of a single Fe atom hganded to four Cys residues — the iron atom can be in the +2 or +3 valence (b) rhombic two iron—two sulfide [Fe2 S2] clusters — typical stable cluster oxidation states are +1 and +2 (the charges of the coordinating cys-teinate residues are not considered) (c) cuboidal three-iron—four sulfide [Fe3—S4] clusters — stable oxidation... 

The EPR spectrum of oxidized rubredoxin (Figure 7.6) shows characteristic peaks at g = 4.31 and 9.42 (P. oleovorans), which have been assigned to transitions within excited and ground-state Kramers doublets, respectively, of a nearly completely rhombic S = i site, with D = 1.8 and E = 0.5 cm . These values for the mononuclear Fe ion stand in sharp contrast to those for other iron-sulfur proteins, which are usually 5 = 2 (when reduced) and have g values close to 2. The even-electron Fe + state (S = 2) in reduced rubredoxin has no detectable EPR when conventional instruments are used. ... 

The protein sequence data in Table 2 show that the cysteine residues in all the proteins occur in identical positions (18, 39, 44, 47, 77) in the sequence. Thus, the ligand field produced by the cysteinyl-sulfur atoms is not likely to be different among these proteins unless there is a difference in protein conformation which causes a displacement in one or more of the cysteinyl sulfur atoms. Note that a displacement of any cysteinyl sulfur atom in the model in Fig. 15 results in rhombic distortion at the iron to which it is ligated. Since, according to the spin-coupled model, this rhombic distortion will manifest itself in the difference between gx and gx for a particular protein, the EPR data in Table 1 provide a measure of the rhombic distortion around the ferrous iron in the reduced proteins. In particular, the g-values of adrenodoxin are axially symmetric while the g-values of spinach ferredoxin show a rhombic distortion. Thus, the observation of Kimura et al. (168) that adrenodoxin and spinach ferredoxin have different protein conformations is consistent with the prediction of the above model. 

We present here the preliminary results of our attempt to develop a new method for the analysis of pyrite in coal and lignite. It is well known that sulfur in coal is present in different forms. In particular, although the iron sulfide in coal is generally pyrite ( 1), other iron sulfides are frequently present. For example, iron disulfide occurs as marcasite, a rhombic crystalline form, as well as pyrite, a cubic crystalline form. Perhaps the term disulfide sulfur should be used to replace the pyritic sulfur more commonly quoted, as recently suggested by Youh (2). Since the chemical reactivity of these two disulfides of iron is similar, our method will record them equally well. Nonetheless, we will continue to refer to the pyrite determinations here, although we are really talking about the chemical species FeS2 rather than a particular crystalline structure. 

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