Iron-sulfur clusters—new features in enzymes and synthetic models

Hydrogen atom abstraction by a high spin [FeIII=S] complex

Iron sulfur clusters are critical to a plethora of biological processes; however, little is known about the elementary unit of these clusters, namely the [Fe=S]n+ fragment. Here, we report the synthesis and characterization of a terminal iron sulfido complex. Despite its high spin (S = 5/2) ground state, structural, spectroscopic, and computational characterization provide evidence for iron sulfur multiple bond character. Intriguingly, the complex reacts with additional sulfur to afford an S = 3/2 iron(III) disulfido (S2 2-) complex. Preliminary studies reveal that the sulfido complex reacts with dihydroanthracene to afford an iron(II) hydrosulfido complex, akin to the reactivity of iron oxo complexes. By contrast, there is no reaction with the disulfido complex. These results provide important insight into the nature of the iron sulfide unit.

Bioanalytical applications of Mössbauer spectroscopy

Data on the applications of Mössbauer spectroscopy in the transmission (mainly on 57Fe nuclei) and emission (on 57Co nuclei) variants for analytical studies at the molecular level of metal-containing components in a wide range of biological objects (from biocomplexes and biomacromolecules to supramolecular structures, cells, tissues and organisms) and of objects that are participants or products of biological processes, published in the last 15 years are discussed and systematized. The prospects of the technique in its biological applications, including the developing fields (emission variant, use of synchrotron radiation), are formulated.

The bibliography includes 248 references.

Characterization of Fe-S Clusters in Proteins by Mӧssbauer Spectroscopy

⁵⁷Fe Mӧssbauer spectroscopy is unparalleled in the study of Fe-S cluster-containing proteins because of its unique ability to detect all forms of iron. Enrichment of biological samples with the ⁵⁷Fe isotope and manipulation of experimental parameters such as temperature and magnetic field allow for elucidation of the number of Fe-S clusters present in a given protein, their nuclearity, oxidation state, geometry, and ligation environment, as well as any transient states relevant to enzyme chemistry. This chapter is arranged in five sections to help navigate an experimentalist to utilize ⁵⁷Fe Mӧssbauer spectroscopy for delineating the role and structure of biological Fe-S clusters. The first section lays out the tools and technical considerations for the preparation of ⁵⁷Fe-labeled samples. The choice of experimental parameters and their effects on the Mӧssbauer spectra are presented in the following two sections. The last two sections provide a theoretical and practical guide on spectral acquisition and analysis relevant to Fe-S centers.

Structural and Electronic Rearrangements in Fe2S2, Fe3S4, and Fe4S4 Atomic Clusters under the Attack of NO, CO, and O2

We report results, based on density functional theory-generalized gradient approximation calculations, that shed light on how NO, CO, and O₂ interact with Fe₂S₂, Fe₃S₄, and Fe₄S₄ clusters and how they modify their structural and electronic properties. The interest in these small iron sulfide clusters comes from the fact that they are at the protein cores and that elucidating fundamental aspects of their interaction with those light molecules which are known to modify their functionality may help in understanding complex behaviors in biological systems. CO and NO are found to bind molecularly, leading to moderate relaxations in the clusters, but nevertheless to changes in the spin-polarized electronic structure and related properties. In contrast, dissociative chemisorption of O₂ is much more stable than molecular adsorption, giving rise to significant structural distortions, particularly in Fe₄S₄ that splits into two Fe₂S₂ subclusters. As a consequence, oxygen tends to strongly reduce the spin polarization in Fe and to weaken the Fe-Fe interaction inducing antiparallel couplings that, in the case of Fe₄S₄, clearly arise from indirect Fe-Fe exchange coupling mediated by O. The three molecules (particularly CO) enhance the stability of the iron-sulfur clusters. This increase is noticeably more pronounced for Fe₂S₂ than for the other iron-sulfur clusters of different compositions, a result that correlates with the fact that in recent experiments of CO reaction with Fe_mS_m (m = 1-4), the Fe₂S₂CO product results as a prominent one.

Planar three-coordinate iron sulfide in a synthetic [4Fe-3S] cluster with biomimetic reactivity

Iron-sulfur clusters are emerging as reactive sites for the reduction of small-molecule substrates. However, the four-coordinate iron sites of typical iron-sulfur clusters rarely react with substrates, implicating three-coordinate iron. This idea is untested because fully sulfide-coordinated three-coordinate iron is unprecedented. Here we report a new type of [4Fe-3S] cluster featuring an iron center with three bonds to sulfides. Although a high-spin electronic configuration is characteristic of other iron-sulfur clusters, the planar geometry and short Fe–S bonds lead to a surprising low-spin electronic configuration at the three-coordinate Fe center as determined by spectroscopy and ab initio calculations. In a demonstration of biomimetic reactivity, the [4Fe-3S] cluster reduces hydrazine, a natural substrate of nitrogenase. The product is the first example of NH₂ bound to an iron-sulfur cluster. Our results demonstrate that three-coordinate iron supported by sulfide donors is a plausible precursor to reactivity in iron-sulfur clusters like the FeMoco of nitrogenase.

Contribution of Mössbauer spectroscopy to the investigation of Fe/S biogenesis

Fe/S cluster biogenesis involves a complex machinery comprising several mitochondrial and cytosolic proteins. Fe/S cluster biosynthesis is closely intertwined with iron trafficking in the cell. Defects in Fe/S cluster elaboration result in severe diseases such as Friedreich ataxia. Deciphering this machinery is a challenge for the scientific community. Because iron is a key player, ⁵⁷Fe-Mössbauer spectroscopy is especially appropriate for the characterization of Fe species and monitoring the iron distribution. This minireview intends to illustrate how Mössbauer spectroscopy contributes to unravel steps in Fe/S cluster biogenesis. Studies were performed on isolated proteins that may be present in multiple protein complexes. Since a few decades, Mössbauer spectroscopy was also performed on whole cells or on isolated compartments such as mitochondria and vacuoles, affording an overview of the iron trafficking. This minireview aims at presenting selected applications of ⁵⁷Fe-Mössbauer spectroscopy to Fe/S cluster biogenesis.

Biochemical Reconstitution and Spectroscopic Analysis of Iron-Sulfur Proteins

Iron-sulfur (Fe/S) proteins are involved in numerous key biological functions such as respiration, metabolic processes, protein translation, DNA synthesis, and DNA repair. The simplest types of Fe/S clusters include [2Fe-2S], [3Fe-4S], and [4Fe-4S] forms that sometimes are present in multiple copies. De novo assembly of Fe/S cofactors and their insertion into apoproteins in living cells requires complex proteinaceous machineries that are frequently highly conserved. In eukaryotes such as yeast and mammals, the mitochondrial iron-sulfur cluster assembly machinery and the cytosolic iron-sulfur protein assembly system consist of more than 30 components that cooperate in the generation of some 50 cellular Fe/S proteins. Both the mechanistic dissection of the intracellular Fe/S protein assembly pathways and the identification and characterization of Fe/S proteins rely on tool boxes of in vitro and in vivo methods. These cell biological, biochemical, and biophysical techniques help to determine the extent, stability, and type of bound Fe/S cluster. They also serve to distinguish bona fide Fe/S proteins from other metal-binding proteins containing similar cofactor coordination motifs. Here, we present a collection of in vitro methods that have proven useful for basic biochemical and biophysical characterization of Fe/S proteins. First, we describe the chemical assembly of [2Fe-2S] or [4Fe-4S] clusters on purified apoproteins. Then, we summarize a reconstitution system reproducing the de novo synthesis of a [2Fe-2S] cluster in mitochondria. Finally, we explain the use of UV-vis, CD, electron paramagnetic resonance, and Mössbauer spectroscopy for the routine characterization of Fe/S proteins.

Halide coordinated homoleptic [Fe4S4X4](2-) and heteroleptic [Fe4S4X2Y2](2-) clusters (X, Y = Cl, Br, I)--alternative preparations, structural analogies and spectroscopic properties in solution and solid state

New facile methods to prepare iron sulphur halide clusters [Fe4S4X4](2-) from [Fe(CO)5] and elemental sulphur were elaborated. Reactions of ferrous precursors like tetrahalidoferrates(ii) or simple ferrous halides with [Fe(CO)5] and sulphur turned out to be efficient methods to prepare homoleptic [Fe4S4X4](2-) (X = Cl, Br) and heteroleptic clusters [Fe4S4X4-nYn](2-) (X = Cl, Br; Y = Br, I). Solid materials were obtained as salts of BTMA(+) (= benzyltrimethylammonium); the new compounds containing [Fe4S4Br4](2-) and [Fe4S4X2Y2](2-) (X, Y = Cl, Br, I) were all isostructural to (BTMA)2[Fe4S4I4] (monoclinic, Cc) as inferred from synchrotron X-ray powder diffraction. While the solid materials contain defined heteroleptic clusters with a halide X : Y ratio of 2 : 2, dissolving these compounds leads to rapid scrambling of the halide ligands forming mixtures of all five possible [Fe4S4X4-nYn](2-) clusters as could be shown by UHR-ESI MS. The variation of X and Y allowed assignment of the absorption bands in the visible and NIR; the long-wavelength bands around 1100 nm were tentatively assigned to intervalence charge transfer (IVCT) transitions.

Cofactor composition and function of a H2-sensing regulatory hydrogenase as revealed by Mössbauer and EPR spectroscopy

The regulatory hydrogenase (RH) from Ralstonia eutropha H16 acts as a sensor for the detection of environmental H₂ and regulates gene expression related to hydrogenase-mediated cellular metabolism. In marked contrast to prototypical energy-converting [NiFe] hydrogenases, the RH is apparently insensitive to inhibition by O₂ and CO. While the physiological function of regulatory hydrogenases is well established, little is known about the redox cycling of the [NiFe] center and the nature of the iron-sulfur (FeS) clusters acting as electron relay. The absence of any FeS cluster signals in EPR had been attributed to their particular nature, whereas the observation of essentially only two active site redox states, namely Ni-SI and Ni-C, invoked a different operant mechanism. In the present work, we employ a combination of Mössbauer, FTIR and EPR spectroscopic techniques to study the RH, and the results are consistent with the presence of three [4Fe-4S] centers in the small subunit. In the as-isolated, oxidized RH all FeS clusters reside in the EPR-silent 2+ state. Incubation with H₂ leads to reduction of two of the [4Fe-4S] clusters, whereas only strongly reducing agents lead to reduction of the third cluster, which is ascribed to be the [4Fe-4S] center in 'proximal' position to the [NiFe] center. In the two different active site redox states, the low-spin Fe^II exhibits distinct Mössbauer features attributed to changes in the electronic and geometric structure of the catalytic center. The results are discussed with regard to the spectral characteristics and physiological function of H₂-sensing regulatory hydrogenases.

Mössbauer spectroscopy of Fe/S proteins

Iron-sulfur (Fe/S) clusters are structurally and functionally diverse cofactors that are found in all domains of life. (57)Fe Mössbauer spectroscopy is a technique that provides information about the chemical nature of all chemically distinct Fe species contained in a sample, such as Fe oxidation and spin state, nuclearity of a cluster with more than one metal ion, electron spin ground state of the cluster, and delocalization properties in mixed-valent clusters. Moreover, the technique allows for quantitation of all Fe species, when it is used in conjunction with electron paramagnetic resonance (EPR) spectroscopy and analytical methods. (57)Fe-Mössbauer spectroscopy played a pivotal role in unraveling the electronic structures of the "well-established" [2Fe-2S](2+/+), [3Fe-4S](1+/0), and [4Fe-4S](3+/2+/1+/0) clusters and -more-recently- was used to characterize novel Fe/S clustsers, including the [4Fe-3S] cluster of the O2-tolerant hydrogenase from Aquifex aeolicus and the 3Fe-cluster intermediate observed during the reaction of lipoyl synthase, a member of the radical SAM enzyme superfamily.

Low temperature activation of S8, Se(red) and α-Te with [Cp(BIG)Fe(CO)2] radicals

The bulky dimeric iron complex, [Cp(BIG)Fe(CO)2]2, readily activates elemental chalcogens (S8, Se(red) and α-Te) under mild conditions at room temperature. Six compounds containing Q2(2-) ligands (Q = S, Se) and a Te(2-) ligand, respectively, were isolated and completely characterized, including by X-ray diffraction techniques.

Identification of a spin-coupled Mo(iii) in the nitrogenase iron–molybdenum cofactor

The molybdenum atom in FeMoco is imperative to the high activity of the enzyme and has been proposed to be Mo(iv). We demonstrate that only Mo(iii) fits Mo HERFD XAS data, the first example of Mo(iii) in biology. Theoretical calculations further reveal an unusual spin-coupled Mo(iii).

Generation of high-spin iron(I) in a protein environment using cryoreduction

High-spin Fe(1+) sites are potentially important in iron-sulfur proteins but are rare in synthetic compounds and unknown in metalloproteins. Here, we demonstrate a spectroscopically characterized example of high-spin non-heme Fe(1+) in a protein environment. Cryoreduction of Fe(2+)-substituted azurin at 77 K with (60)Co γ radiation generates a new species with a S = (3)/2 (high-spin) Fe(1+) center having D > 0 and E/D ~ 0.25. This transient species is stable in a glycerol-water glass only up to ~170 K. A combination of electron paramagnetic resonance and Mössbauer spectroscopies provides a powerful means of identifying a transient high-spin Fe(1+) site in a protein scaffold.