Structural Conversions of Synthetic and Protein-Bound Iron–Sulfur Clusters

Insight into the photodynamic mechanism and protein binding of a nitrosyl iron-sulfur [Fe2S2(NO)4]2- cluster

Iron-sulfur cluster conversion and nitrosyl modification are involved in regulating their functions and play critical roles in signaling for biological systems. Hereby, the photo-induced dynamic process of (Me4N)2[Fe2S2(NO)4] was monitored using time-resolved electron paramagnetic resonance (EPR) spectra, MS spectra and cellular imaging methods. Photo-irradiation and the solvent affect the reaction rates and products. Spectroscopic and kinetic studies have shown that the process involves at least three intermediates: spin-trapped NO free radical species with a gav at 2.040, and two other iron nitrosyl species, dinitrosyl iron units (DNICs) and mononitrosyl iron units (MNICs) with gav values at 2.031 and 2.024, respectively. Moreover, the [Fe2S2(NO)4]2- cluster could bind with ferritin and decompose gradually, and a binding state of dinitrosyl iron coordinated with Cys102 of the recombinant human heavy chain ferritin (rHuHF) was finally formed. This study provides insight into the photodynamic mechanism of nitrosyl iron - sulfur clusters to improve the understanding of physiological activity.

Nonheme binuclear transition metal complexes with hydrosulfide and polychalcogenides

The intriguing chemistry of chalcogen (S, Se)-containing ligands and their capability to bridge multiple metal centres have resulted in a plethora of reports on transition metal complexes featuring hydrosulfide (HS-) and polychalcogenides (En2-, E = S, Se). While a large number of such molecules are strictly organometallic complexes, examples of non-organometallic complexes featuring HS- and En2- with N-/O-donor ligands are relatively rare. The general synthetic procedure for the transition metal-hydrosulfido complexes involves the reaction of the corresponding metal salts with HS-/H2S and this is prone to generate sulfido bridged oligomers in the absence of sterically demanding ligands. On the other hand, the synthetic methods for the preparation of transition metal-polychalcogenido complexes include the reaction of the corresponding metal salts with En2- or the two electron oxidation of low-valent metals with elemental chalcogen, often at an elevated temperature and/or for a long time. Recently, we have developed new synthetic methods for the preparation of two new classes of binuclear transition metal complexes featuring either HS-, or Sn2- and Sen2- ligands. The new method for the synthesis of transition metal-hydrosulfido complexes involved transition metal-mediated hydrolysis of thiolates at room temperature (RT), while the method for the synthesis of transition metal-polychalcogenido complexes involved redox reaction of coordinated thiolates and exogenous elemental chalcogens at RT. An overview of the synthetic aspects, structural properties and intriguing reactivity of these two new classes of transition metal complexes is presented.

Redox Processes Involving Oxygen: The Surprising Influence of Redox-Inactive Lewis Acids

Metalloenzymes with heteromultimetallic active sites perform chemical reactions that control several biogeochemical cycles. Transformations catalyzed by such enzymes include dioxygen generation and reduction, dinitrogen reduction, and carbon dioxide reduction-instrumental transformations for progress in the context of artificial photosynthesis and sustainable fertilizer production. While the roles of the respective metals are of interest in all these enzymatic transformations, they share a common factor in the transfer of one or multiple redox equivalents. In light of this feature, it is surprising to find that incorporation of redox-inactive metals into the active site of such an enzyme is critical to its function. To illustrate, the presence of a redox-inactive Ca2+ center is crucial in the Oxygen Evolving Complex, and yet particularly intriguing given that the transformation catalyzed by this cluster is a redox process involving four electrons. Therefore, the effects of redox inactive metals on redox processes-electron transfer, oxygen- and hydrogen-atom transfer, and O-O bond cleavage and formation reactions-mediated by transition metals have been studied extensively. Significant effects of redox inactive metals have been observed on these redox transformations; linear free energy correlations between Lewis acidity and the redox properties of synthetic model complexes are observed for several reactions. In this Perspective, these effects and their relevance to multielectron processes will be discussed.

Three-Coordinate Nickel and Metal-Metal Interactions in a Heterometallic Iron-Sulfur Cluster

Biological multielectron reactions often are performed by metalloenzymes with heterometallic sites, such as anaerobic carbon monoxide dehydrogenase (CODH), which has a nickel-iron-sulfide cubane with a possible three-coordinate nickel site. Here, we isolate the first synthetic iron-sulfur clusters having a nickel atom with only three donors, showing that this structural feature is feasible. These have a core with two tetrahedral irons, one octahedral tungsten, and a three-coordinate nickel connected by sulfide and thiolate bridges. Electron paramagnetic resonance (EPR), Mössbauer, and superconducting quantum interference device (SQUID) data are combined with density functional theory (DFT) computations to show how the electronic structure of the cluster arises from strong magnetic coupling between the Ni, Fe, and W sites. X-ray absorption spectroscopy, together with spectroscopically validated DFT analysis, suggests that the electronic structure can be described with a formal Ni1+ atom participating in a nonpolar Ni-W σ-bond. This metal-metal bond, which minimizes spin density at Ni1+, is conserved in two cluster oxidation states. Fe-W bonding is found in all clusters, in one case stabilizing a local non-Hund state at tungsten. Based on these results, we compare different M-M interactions and speculate that other heterometallic clusters, including metalloenzyme active sites, could likewise store redox equivalents and stabilize low-valent metal centers through metal-metal bonding.

Expression of clMagR/clCry4 protein in mBMSCs provides T2-contrast enhancement of MRI

Here, we propose for the first time the evaluation of magnetosensitive clMagR/clCry4 as a magnetic resonance imaging (MRI) reporter gene that imparts sensitivity to endogenous contrast in eukaryotic organisms. Using a lentiviral vector, we introduced clMagR/clCry4 into C57BL/6 mice-derived bone marrow mesenchymal stem cells (mBMSCs), which could specifically bind with iron, significantly affected MRI transverse relaxation, and generated readily detectable contrast without adverse effects in vivo. Specifically, clMagR/clCry4 makes mBMSCs beneficial for enhancing the sensitivity of MRI-R2 for iron-bearing granules, in which cells recruit exogenous iron and convert these stores into an MRI-detectable contrast; this is not achievable with control cells. Additionally, Prussian blue staining was performed together with ultrathin cell slices to provide direct evidence of natural iron-bearing granules being detectable on MRI. Hence, it was inferred that the sensitivity of MRI detection should be correlated with clMagR/clCry4 and exogenous iron. Taken together, the clMagR/clCry4 has great potential as an MRI reporter gene. STATEMENT OF SIGNIFICANCE: In this study, we propose the evaluation of magnetosensitive clMagR/clCry4 as an MRI reporter gene, imparting detection sensitivity to eukaryotic mBMSCs for endogenous contrast. At this point, the clMagR and clCry4 were located within the cytoplasm and possibly influence each other. The clMagR/clCry4 makes mBMSCs beneficial for enhancing the sensitivity of MRI-R2 for iron-bearing granules, in which protein could specifically bind with iron and convert these stores into MRI-detectable contrast; this is not achieved by control cells. The viewpoint was speculated that the clMagR/clCry4 and exogenous iron were complementary to each other. Additionally, Prussian blue staining was performed together with TEM observations to provide direct evidence that the iron-bearing granules were sensitive to MRI.

An Iron-Sulfur Cluster with a Highly Pyramidalized Three-Coordinate Iron Center and a Negligible Affinity for Dinitrogen

Attempts to generate open coordination sites for N2 binding at synthetic Fe-S clusters often instead result in cluster oligomerization. Recently, it was shown for Mo-Fe-S clusters that such oligomerization reactions can be prevented through the use of sterically protective supporting ligands, thereby enabling N2 complex formation. Here, this strategy is extended to Fe-only Fe-S clusters. One-electron reduction of (IMes)3Fe4S4Cl (IMes = 1,3-dimesitylimidazol-2-ylidene) forms the transiently stable edge-bridged double cubane (IMes)6Fe8S8, which loses two IMes ligands to form the face-bridged double-cubane, (IMes)4Fe8S8. The finding that the three supporting IMes ligands do not confer sufficient protection to curtail cluster oligomerization prompted the design of a new N-heterocyclic carbene, SIArMe,iPr (1,3-bis(3,5-diisopropyl-2,6-dimethylphenyl)-2-imidazolidinylidene; abbreviated as SIAr), that features bulky groups strategically placed in remote positions. When the reduction of (SIAr)3Fe4S4Cl or [(SIAr)3Fe4S4(THF)]+ is conducted in the presence of SIAr, the formation of (SIAr)4Fe8S8 is indeed suppressed, permitting characterization of the reduced [Fe4S4]0 product. Surprisingly, rather than being an N2 complex, the product is simply (SIAr)3Fe4S4: a cluster with a three-coordinate Fe site that adopts an unusually pyramidalized geometry. Although (SIAr)3Fe4S4 does not coordinate N2 to any appreciable extent under the surveyed conditions, it does bind CO to form (SIAr)3Fe4S4(CO). This finding demonstates that the binding pocket at the unique Fe is not too small for N2; instead, the exceptionally weak affinity for N2 can be attributed to weak Fe-N2 bonding. The differences in the N2 coordination chemistry between sterically protected Mo-Fe-S clusters and Fe-only Fe-S clusters are discussed.

Bioinspired Framework Catalysts: From Enzyme Immobilization to Biomimetic Catalysis

Enzymatic catalysis has fueled considerable interest from chemists due to its high efficiency and selectivity. However, the structural complexity and vulnerability hamper the application potentials of enzymes. Driven by the practical demand for chemical conversion, there is a long-sought quest for bioinspired catalysts reproducing and even surpassing the functions of natural enzymes. As nanoporous materials with high surface areas and crystallinity, metal-organic frameworks (MOFs) represent an exquisite case of how natural enzymes and their active sites are integrated into porous solids, affording bioinspired heterogeneous catalysts with superior stability and customizable structures. In this review, we comprehensively summarize the advances of bioinspired MOFs for catalysis, discuss the design principle of various MOF-based catalysts, such as MOF-enzyme composites and MOFs embedded with active sites, and explore the utility of these catalysts in different reactions. The advantages of MOFs as enzyme mimetics are also highlighted, including confinement, templating effects, and functionality, in comparison with homogeneous supramolecular catalysts. A perspective is provided to discuss potential solutions addressing current challenges in MOF catalysis.

Cubane-type tungsten-iron-sulfur clusters with a nitrogen atom in the core: terminal ligand substitutions and redox behaviors

The lack of M-Fe-S (M = Mo or W) clusters incorporating a second period (2p) atom in the core has resulted in limited investigations and poor understanding of the physical and chemical properties of the M-Fe-S clusters closely related to the FeMo cofactor. In this work, systematic studies have been carried out to explore the chemical reactivities at the terminal ligand sites and the redox properties of a series of clusters comprising a [WFe₃S₃N] cubane core, based on the previously developed cluster [(Tp*)WFe₃S₃(μ₃-NSiMe₃)Cl₃]^1-. Substitutions of the terminal chlorides with ethanethiolate, methanethiolate, thiophenolate, p-thiocresolate and azide occurred smoothly, while the replacement of the chlorides with carbene ligands required the reduction of the precursor into [(Tp*)WFe₃S₃(μ₃-NSiMe₃)Cl₃]^2- first. The reduced cluster core could also be supported by thiophenolates as terminal ligands, but not thiolates or azides. It is remarkable that the thiophenolate ligated reduced cluster can be synthesized from the precursor [(Tp*)WFe₃S₃(μ₃-NSiMe₃)Cl₃]^1-via different synthetic routes, either reduction followed by substitution or substitution followed by reduction, either in situ or stepwise. This work indicates that terminal ligands contribute significantly to determine the chemical and physical properties of the clusters, even though they might affect the cluster core to a limited extent from a structural point of view, which raises the possibility of delicate control in regulating the physical/chemical properties of M-Fe-S clusters with a heteroleptic core incorporating 2p atom(s).

A Cut-to-Link Strategy for Cubane-Based Heterometallic Sulfide Clusters with Giant Third-Order Nonlinear Optical Response

Although the synthesis of low-dimensional metal sulfides by assembling cluster-based units is expected to promote the development of optical materials and models of enzyme active centers such as dinitrogenase, it is faced with limited assembly methodology. Herein we present a cut-to-link strategy to generate high-nuclearity assemblies, inspired by the formation of a Z-type dimer of the W-S-Cu analogues of P^N cluster through in situ release of active linkers. Four new compounds with structures based on the same {Tp*WS₃Cu₃} incomplete cubane-like units were obtained using varied combinations of mild reagents. Open-aperture Z-scan measurements demonstrated the highest-nuclearity complex has the largest nonlinear optical absorption coefficient among discrete cluster-based materials reported to date. This approach enables building high-nuclearity metal sulfide clusters through cluster-based building blocks and opens a way to the design and exploration of materials based on well-identified building blocks.

Preparation of Iron-Based Sulfides and Their Applications in Biomedical Fields

Recently, iron-based sulfides, including iron sulfide minerals and biological iron sulfide clusters, have attracted widespread interest, owing to their excellent biocompatibility and multi-functionality in biomedical applications. As such, controlled synthesized iron sulfide nanomaterials with elaborate designs, enhanced functionality and unique electronic structures show numerous advantages. Furthermore, iron sulfide clusters produced through biological metabolism are thought to possess magnetic properties and play a crucial role in balancing the concentration of iron in cells, thereby affecting ferroptosis processes. The electrons in the Fenton reaction constantly transfer between Fe²⁺ and Fe³⁺, participating in the production and reaction process of reactive oxygen species (ROS). This mechanism is considered to confer advantages in various biomedical fields such as the antibacterial field, tumor treatment, biosensing and the treatment of neurodegenerative diseases. Thus, we aim to systematically introduce recent advances in common iron-based sulfides.

Evidence for [2Fe-2S]2+ and Linear [3Fe-4S]1+ Clusters in a Unique Family of Glycine/Cysteine-Rich Fe-S Proteins from Megavirinae Giant Viruses

We have discovered a protein with an amino acid composition exceptionally rich in glycine and cysteine residues in the giant virus mimivirus. This small 6 kDa protein is among the most abundant proteins in the icosahedral 0.75 μm viral particles; it has no predicted function but is probably essential for infection. The aerobically purified red-brownish protein overproduced inEscherichia coli contained both iron and inorganic sulfide. UV/vis, EPR, and Mössbauer studies revealed that the viral protein, coined GciS, accommodated two distinct Fe-S clusters: a diamagnetic S = 0 [2Fe-2S]²⁺ cluster and a paramagnetic S = 5/2 linear [3Fe-4S]¹⁺ cluster, a geometry rarely stabilized in native proteins. Orthologs of mimivirus GciS were identified within all clades of Megavirinae, a Mimiviridae subfamily infecting Acanthamoeba, including the distantly related tupanviruses, and displayed the same spectroscopic features. Thus, these glycine/cysteine-rich proteins form a new family of viral Fe-S proteins sharing unique Fe-S cluster binding properties.

Structural insight into halide-coordinated [Fe4S4XnY4-n]2- clusters (X, Y = Cl, Br, I) by XRD and Mössbauer spectroscopy

Iron sulphur halide clusters [Fe₄S₄Br₄]^2- and [Fe₄S₄X₂Y₂]^2- (X, Y = Cl, Br, I) were obtained in excellent yields (77 to 78%) and purity from [Fe(CO)₅], elemental sulphur, I₂ and benzyltrimethylammonium (BTMA⁺) iodide, bromide and chloride. Single crystals of (BTMA)₂[Fe₄S₄Br₄] (1), (BTMA)₂[Fe₄S₄Br₂Cl₂] (2), (BTMA)₂[Fe₄S₄Cl₂I₂] (3), and (BTMA)₂[Fe₄S₄Br₂I₂] (4) were isostructural to the previously reported (BTMA)₂[Fe₄S₄I₄] (5) (monoclinic, Cc). Instead of the chloride cubane cluster [Fe₄S₄Cl₄]^2-, we found the prismane-shaped cluster (BTMA)₃[Fe₆S₆Cl₆] (6) (P1̄). ⁵⁷Fe Mössbauer spectroscopy indicates complete delocalisation with Fe^2.5+ oxidation states for all iron atoms. Magnetic measurements showed small χ_MT values at 298 K ranging from 1.12 to 1.54 cm³ K mol^-1, indicating the dominant antiferromagnetic exchange interactions. With decreasing temperature, the χ_MT values decreased to reach a plateau at around 100 K. From about 20 K, the values drop significantly. Fitting the data in the Heisenberg-Dirac-van Vleck (HDvV) as well as the Heisenberg Double Exchange (HDE) formalism confirmed the delocalisation and antiferromagnetic coupling assumed from Mössbauer spectroscopy.

Platonic and Archimedean solids in discrete metal-containing clusters

Metal-containing clusters have attracted increasing attention over the past 2-3 decades. This intense interest can be attributed to the fact that these discrete metal aggregates, whose atomically precise structures are resolved by single-crystal X-ray diffraction (SCXRD), often possess intriguing geometrical features (high symmetry, aesthetically pleasing shapes and architectures) and fascinating physical properties, providing invaluable opportunities for the intersection of different disciplines including chemistry, physics, mathematical geometry and materials science. In this review, we attempt to reinterpret and connect these fascinating clusters from the perspective of Platonic and Archimedean solid characteristics, focusing on highly symmetrical and complex metal-containing (metal = Al, Ti, V, Mo, W, U, Mn, Fe, Co, Ni, Pd, Pt, Cu, Ag, Au, lanthanoids (Ln), and actinoids) high-nuclearity clusters, including metal-oxo/hydroxide/chalcogenide clusters and metal clusters (with metal-metal binding) protected by surface organic ligands, such as thiolate, phosphine, alkynyl, carbonyl and nitrogen/oxygen donor ligands. Furthermore, we present the symmetrical beauty of metal cluster structures and the geometrical similarity of different types of clusters and provide a large number of examples to show how to accurately describe the metal clusters from the perspective of highly symmetrical polyhedra. Finally, knowledge and further insights into the design and synthesis of unknown metal clusters are put forward by summarizing these "star" molecules.

Transfection of clMagR/clCry4 imparts MR-T2 imaging contrast properties to living organisms (E. coli) in the presence of Fe3+ by endogenous formation of iron oxide nanoparticles

Rapid development of medical imaging, such as cellular tracking, has increased the demand for "live" contrast agents. This study provides the first experimental evidence demonstrating that transfection of the clMagR/clCry4 gene can impart magnetic resonance imaging (MRI) T₂-contrast properties to living prokaryotic Escherichia coli (E. coli) in the presence of Fe³⁺ through the endogenous formation of iron oxide nanoparticles. The transfected clMagR/clCry4 gene markedly promoted uptake of exogenous iron by E. coli, achieving an intracellular co-precipitation condition and formation of iron oxide nanoparticles. This study will stimulate further exploration of the biological applications of clMagR/clCry4 in imaging studies.

Selenocyanate derived Se-incorporation into the Nitrogenase Fe protein cluster

The nitrogenase Fe protein mediates ATP-dependent electron transfer to the nitrogenase MoFe protein during nitrogen fixation, in addition to catalyzing MoFe protein-independent substrate (CO₂) reduction and facilitating MoFe protein metallocluster biosynthesis. The precise role(s) of the Fe protein Fe₄S₄ cluster in some of these processes remains ill-defined. Herein, we report crystallographic data demonstrating ATP-dependent chalcogenide exchange at the Fe₄S₄ cluster of the nitrogenase Fe protein when potassium selenocyanate is used as the selenium source, an unexpected result as the Fe protein cluster is not traditionally perceived as a site of substrate binding within nitrogenase. The observed chalcogenide exchange illustrates that this Fe₄S₄ cluster is capable of core substitution reactions under certain conditions, adding to the Fe protein’s repertoire of unique properties.

Many of the molecules that form the building blocks of life contain nitrogen. This element makes up most of the gas in the atmosphere, but in this form, it does not easily react, and most organisms cannot incorporate atmospheric nitrogen into biological molecules. To get around this problem, some species of bacteria produce an enzyme complex called nitrogenase that can transform nitrogen from the air into ammonia. This process is called nitrogen fixation, and it converts nitrogen into a form that can be used to sustain life.

The nitrogenase complex is made up of two proteins: the MoFe protein, which contains the active site that binds nitrogen, turning it into ammonia; and the Fe protein, which drives the reaction. Besides the nitrogen fixation reaction, the Fe protein is involved in other biological processes, but it was not thought to bind directly to nitrogen, or to any of the other small molecules that the nitrogenase complex acts on. The Fe protein contains a cluster of iron and sulfur ions that is required to drive the nitrogen fixation reaction, but the role of this cluster in the other reactions performed by the Fe protein remains unclear.

To better understand the role of this iron sulfur cluster, Buscagan, Kaiser and Rees used X-ray crystallography, a technique that can determine the structure of molecules. This approach revealed for the first time that when nitrogenase reacts with a small molecule called selenocyanate, the selenium in this molecule can replace the sulfur ions of the iron sulfur cluster in the Fe protein. Buscagan, Kaiser and Rees also demonstrated that the Fe protein could still incorporate selenium ions in the absence of the MoFe protein, which has traditionally been thought to provide the site essential for transforming small molecules.

These results indicate that the iron sulfur cluster in the Fe protein may bind directly to small molecules that react with nitrogenase. In the future, these findings could lead to the development of new molecules that artificially produce ammonia from nitrogen, an important process for fertilizer manufacturing. In addition, the iron sulfur cluster found in the Fe protein is also present in many other proteins, so Buscagan, Kaiser and Rees’ experiments may shed light on the factors that control other biological reactions.

S K-edge XAS of CuII, CuI, and ZnII oxidized Dithiolene complexes: Covalent contributions to structure and the Jahn-Teller effect

Reduced dithiolene ligands are bound to high valent Mo centers in the active site of the oxotransferase family of enzymes. Related model complexes have been studied with great insight by Prof. Holm and his colleagues. This study focuses on the other limit of dithiolene chemistry: an investigation of the 2-electron oxidized dithiolene bound to low-valent late transition metal (TM) ions (ZnII, CuI, and CuII). The bonding descriptions of the oxidized dithiolene [N,N-dimethyl piperazine 2,3-dithione (Me2Dt0)] complexes are probed using S K-edge X-ray absorption spectroscopy (XAS) and the results are correlated to density functional theory (DFT) calculations. These experimentally supported calculations are then extended to explain the different geometric structures of the three complexes. The ZnII(Me2Dt0)2 complex has only ligand-ligand repulsion so it is stabilized at the D2d symmetry limit. The CuI(Me2Dt0)2 complex has additional weak backbonding thus distorts somewhat from D2d toward D2h symmetry. The CuII(Me2Dt0)2 complex has a strong σ donor bond that leads to both a large Jahn-Teller stabilization to D2h and an additional covalent contribution to the geometry. The combined strong stabilization results in the square planar, D2h structure. This study quantifies the competition between the ligand-ligand repulsion and the change in electronic structures in determining the final geometric structures of the oxidized dithiolene complexes, and provides quantitative insights into the Jahn-Teller stabilization energy and its origin.

The Intriguing Role of Iron-Sulfur Clusters in the CIAPIN1 Protein Family

Iron-sulfur (Fe/S) clusters are protein cofactors that play a crucial role in essential cellular functions. Their ability to rapidly exchange electrons with several redox active acceptors makes them an efficient system for fulfilling diverse cellular needs. They include the formation of a relay for long-range electron transfer in enzymes, the biosynthesis of small molecules required for several metabolic pathways and the sensing of cellular levels of reactive oxygen or nitrogen species to activate appropriate cellular responses. An emerging family of iron-sulfur cluster binding proteins is CIAPIN1, which is characterized by a C-terminal domain of about 100 residues. This domain contains two highly conserved cysteine-rich motifs, which are both involved in Fe/S cluster binding. The CIAPIN1 proteins have been described so far to be involved in electron transfer pathways, providing electrons required for the biosynthesis of important protein cofactors, such as Fe/S clusters and the diferric-tyrosyl radical, as well as in the regulation of cell death. Here, we have first investigated the occurrence of CIAPIN1 proteins in different organisms spanning the entire tree of life. Then, we discussed the function of this family of proteins, focusing specifically on the role that the Fe/S clusters play. Finally, we describe the nature of the Fe/S clusters bound to CIAPIN1 proteins and which are the cellular pathways inserting the Fe/S clusters in the two cysteine-rich motifs.

Radical SAM-dependent formation of a nitrogenase cofactor core on NifB

Nitrogenase is a versatile metalloenzyme that reduces N₂, CO and CO₂ at its cofactor site. Designated the M-cluster, this complex cofactor has a composition of [(R-homocitrate)MoFe₇S₉C], and it is assembled through the generation of a unique [Fe₈S₉C] core prior to the insertion of Mo and homocitrate. NifB is a radical S-adenosyl-L-methionine (SAM) enzyme that is essential for nitrogenase cofactor assembly. This review focuses on the recent work that sheds light on the role of NifB in the formation of the [Fe₈S₉C] core of the nitrogenase cofactor, highlighting the structure, function and mechanism of this unique radical SAM methyltransferase.

Interconversion between [2Fe-2S] and [4Fe-4S] cluster glutathione complexes

We present here how different iron-sulfide-glutathione ratios, applied in in vitro conditions comparable to those present in the mitochondrial matrix, affect the speciation of iron-sulfur cluster glutathione complexes. An excess of sulfide with respect to iron ions promotes the formation of a tetranuclear [Fe^II₂Fe^III₂S₄(GS)₄]^2- complex, while an excess of iron ions favors the formation of a dinuclear [Fe^IIFe^IIIS₂(GS)₄]^3- complex. These two complexes establish an interconversion equilibrium. The latter might play a role in the composition of the mitochondrial labile iron pool potentially contributing to the regulation of cellular iron homeostasis.

Structural diversity of cysteine desulfurases involved in iron-sulfur cluster biosynthesis

Cysteine desulfurases are pyridoxal-5'-phosphate (PLP)-dependent enzymes that mobilize sulfur derived from the l-cysteine substrate to the partner sulfur acceptor proteins. Three cysteine desulfurases, IscS, NifS, and SufS, have been identified in ISC, NIF, and SUF/SUF-like systems for iron-sulfur (Fe-S) cluster biosynthesis, respectively. These cysteine desulfurases have been investigated over decades, providing insights into shared/distinct catalytic processes based on two types of enzymes (type I: IscS and NifS, type II: SufS). This review summarizes the insights into the structural/functional varieties of bacterial and eukaryotic cysteine desulfurases involved in Fe-S cluster biosynthetic systems. In addition, an inactive cysteine desulfurase IscS paralog, which contains pyridoxamine-5'-phosphate (PMP), instead of PLP, is also described to account for its hypothetical function in Fe-S cluster biosynthesis involving this paralog. The structural basis for cysteine desulfurase functions will be a stepping stone towards understanding the diversity and evolution of Fe-S cluster biosynthesis.

Sulfide and transition metals - A partnership for life

Sulfide and transition metals often came together in Biology. The variety of possible structural combinations enabled living organisms to evolve an array of highly versatile metal-sulfide centers to fulfill different physiological roles. The ubiquitous iron‑sulfur centers, with their structural, redox, and functional diversity, are certainly the best-known partners, but other metal-sulfide centers, involving copper, nickel, molybdenum or tungsten, are equally crucial for Life. This review provides a concise overview of the exclusive sulfide properties as a metal ligand, with emphasis on the structural aspects and biosynthesis. Sulfide as catalyst and as a substrate is discussed. Different enzymes are considered, including xanthine oxidase, formate dehydrogenases, nitrogenases and carbon monoxide dehydrogenases. The sulfide effect on the activity and function of iron‑sulfur, heme and zinc proteins is also addressed.

Controlled Protonation of [2Fe-2S] Leading to MitoNEET Analogues and Concurrent Cluster Modification

MitoNEET, a key regulatory protein in mitochondrial energy metabolism, exhibits a uniquely ligated [2Fe-2S] cluster with one histidine and three cysteines. This unique cluster has been postulated to sense the redox environment and release Fe-S cofactors under acidic pH. Reported herein is a synthetic system that shows how [2Fe-2S] clusters react with protons and rearrange their coordination geometry. The low-temperature stable, site-differentiated clusters [Fe₂S₂(SPh)₃(CF₃COO)]^2- and [Fe₂S₂(SPh)₃(py)]^- have been prepared via controlled protonation below -35 °C and characterized by NMR, UV-vis, and X-ray absorption spectroscopy. Both complexes exhibit anodically shifted redox potentials compared to [Fe₂S₂(SPh)₄]^2- and convert to [Fe₄S₄(SPh)₄]^2- upon warming to room temperature. The current study provides insight into how mitoNEET releases its [2Fe-2S] in response to highly tuned acidic conditions, the chemistry of which may have further implications in Fe-S biogenesis.

Theoretical Study on Redox Potential Control of Iron-Sulfur Cluster by Hydrogen Bonds: A Possibility of Redox Potential Programming

The effect of hydrogen bonds around the active site of Anabaena [2Fe-2S] ferredoxin (Fd) on a vertical ionization potential of the reduced state (IP(red)) is examined based on the density functional theory (DFT) calculations. The results indicate that a single hydrogen bond increases the relative stability of the reduced state, and shifts IP(red) to a reductive side by 0.31–0.33 eV, regardless of the attached sulfur atoms. In addition, the IP(red) value can be changed by the number of hydrogen bonds around the active site. The results also suggest that the redox potential of [2Fe-2S] Fd is controlled by the number of hydrogen bonds because IP(red) is considered to be a major factor in the redox potential. Furthermore, there is a possibility that the redox potentials of artificial iron-sulfur clusters can be finely controlled by the number of the hydrogen bonds attached to the sulfur atoms of the cluster.

On the origins of the mechanistic variants in the thermal reactions of Sx+ (x = 1-3) with benzene

The S-π interaction between sulfur atom(s) and aromatic ring prevails in chemical and biochemical processes. The thermal gas-phase reactions of the S_n⁺ (n = 1-3) ions with benzene have been explored by using Quadrupole-Ion Trap (Q-IT) mass spectrometry complemented by quantum chemical calculations. Charge transfer was found to be the only reaction channel for S₂⁺/C₆H₆, while both charge transfer and bond activation are available for the S⁺/C₆H₆ and S₃⁺/C₆H₆ couples. Upon interrogating the associated electronic origins, multiple factors were found to matter for these processes. In contrast to the σ-type two-center three-electron (2c-3e) S-π hemibond as reported previously, unusual S-π hemibonds were addressed for the S_n⁺/C₆H₆ couples, i.e. the 2c-3e π(S061Eπ) and the three-center three-electron (3c-3e) σ(S₂061Eπ) hemibonds. Such S-π interaction was found to be responsible for the charge transfer processes in S⁺/C₆H₆ and S₂⁺/C₆H₆, but uninvolved in any transformation for S₃⁺/C₆H₆.

Dinitrogen binding and activation at a molybdenum-iron-sulfur cluster

The Fe-S clusters of nitrogenases carry out the life-sustaining conversion of N₂ to NH₃. Although progress continues to be made in modelling the structural features of nitrogenase cofactors, no synthetic Fe-S cluster has been shown to form a well-defined coordination complex with N₂. Here we report that embedding an [MoFe₃S₄] cluster in a protective ligand environment enables N₂ binding at Fe. The bridging [MoFe₃S₄]₂(μ-η¹:η¹-N₂) complex thus prepared features a substantially weakened N-N bond despite the relatively high formal oxidation states of the metal centres. Substitution of one of the [MoFe₃S₄] cubanes with an electropositive Ti metalloradical induces additional charge transfer to the N₂ ligand with generation of Fe-N multiple-bond character. Structural and spectroscopic analyses demonstrate that N₂ activation is accompanied by shortened Fe-S distances and charge transfer from each Fe site, including those not directly bound to N₂. These findings indicate that covalent interactions within the cluster play a critical role in N₂ binding and activation.

Synthesis of a Nitrogenase PN -Cluster Model with [Fe8 S7 (μ-Sthiolate )2 ] Core from the All-Ferric [Fe4 S4 (Sthiolate )4 ] Cubane Synthon

Constructing synthetic models of the nitrogenase P^N -cluster has been a long-standing synthetic challenge. Here, we report an optimal nitrogenase P^N -cluster model [{(TbtS)(OEt₂ )Fe₄ S₃ }₂ (μ-STbt)₂ (μ₆ -S)] (2) [Tbt=2,4,6-tris{bis(trimethylsilyl)methyl}phenyl] that is the closest synthetic mimic constructed to date. Of note is that two thiolate ligands and one hexacoordinated sulfide are connecting the two Fe₄ S₃ incomplete cubanes similar to the native P^N -cluster, which has never been achieved. Cluster 2 has been characterized by X-ray crystallography and relevant physico-chemical methods. The variable temperature magnetic moments of 2 indicate a singlet ground state (S=0). The Mössbauer spectrum of 2 exhibits two doublets with an intensity ratio of 3:1, which suggests the presence of two types of iron sites. The synthetic pathway of the cluster 2 could indicate the native P^N -cluster maturation process as it has been achieved from the Fe₄ S₄ cubane Fe₄ S₄ (STbt)₄ (1).

Spectral decomposition of iron-sulfur clusters

The near universal availability of UV-Visible spectrophotometers makes this instrument a highly exploited tool for the inexpensive, rapid examination of iron-sulfur clusters. Yet, the analysis of iron-sulfur cluster reconstitution experiments by UV-Vis spectroscopy is notoriously difficult due to the presence of broad, ill-defined peaks. Other types of spectroscopies, such as electron paramagnetic resonance spectroscopy and Mössbauer spectroscopy, are superior in characterizing the type of cluster present and their associated electronic transitions but require expensive, less readily available equipment. Here, we describe a tool that utilizes the accessible and convenient platform of Microsoft Excel to allow for the semi-quantitative analysis of iron-sulfur clusters by UV-Vis spectroscopy. This tool, which we call Fit-FeS, could potentially be used to additionally decompose spectra of solutions containing chromophores other than iron-sulfur clusters.

Nitride-incorporated W-Fe-S double cubane clusters: terminal ligand substitutions and redox behaviors

Structural mimicking of the nitrogenase FeMo cofactor has long been a challenge in synthetic inorganic chemistry and bioinorganic chemistry. This already very tough task had become even harder after the discovery of an interstitial light atom, which was later evidenced to be carbide. From a synthetic point of view, to introduce such a 2p atom into the core of a Fe-S cluster would have to overcome the coordination competition from overwhelming sulfide ligands. Recently, we have reported a controlled synthetic strategy named redox metathesis based on template-assisted structure design, and have successfully synthesized a couple of nitride-incorporated edge-bridged double cubane (N-EBDC) W-Fe-S clusters. In this work, we have systematically studied the terminal ligand substitutions of heteroleptic N-EBDC clusters, utilizing ethanethiolate, thiophenolate, p-thiocresolate, azide, and methoxide to replace the terminally bound chloride ligands. Structural analysis of this family of N-EBDC clusters reveals that different terminal ligands affect the fine structures of the cluster cores at different levels. Further studies by cyclic voltammetry indicate that these N-EBDC clusters with distinct terminal ligands exhibit different redox behaviors, furnishing in-depth information on the electronic structure of these clusters potentially related to their reactivity. This study provided useful information for the investigation of nitrogenase related Fe-S clusters toward structural and functional mimicking of the nitrogenase FeMo cofactor.

[2Fe-2S] Cluster Supported by Redox-Active o-Phenylenediamide Ligands and Its Application toward Dinitrogen Reduction

As prevalent cofactors in living organisms, iron-sulfur clusters participate in not only the electron-transfer processes but also the biosynthesis of other cofactors. Many synthetic iron-sulfur clusters have been used in model studies, aiming to mimic their biological functions and to gain mechanistic insight into the related biological systems. The smallest [2Fe-2S] clusters are typically used for one-electron processes because of their limited capacity. Our group is interested in functionalizing small iron-sulfur clusters with redox-active ligands to enhance their electron storage capacity, because such functionalized clusters can potentially mediate multielectron chemical transformations. Herein we report the synthesis, structural characterization, and catalytic activity of a diferric [2Fe-2S] cluster functionalized with two o-phenylenediamide ligands. The electrochemical and chemical reductions of such a cluster revealed rich redox chemistry. The functionalized diferric cluster can store up to four electrons reversibly, where the first two reduction events are ligand-based and the remainder metal-based. The diferric [2Fe-2S] cluster displays catalytic activity toward silylation of dinitrogen, affording up to 88 equiv of the amine product per iron center.

The curious case of peptide-coordinated iron-sulfur clusters: prebiotic and biomimetic insights

Iron-sulfur clusters are among the most ancient biological cofactors and are thought to have had an ancient role in mediating the chemical reactions that led to life. Two different, yet complementary approaches, based on bioinorganic chemistry and prebiotic chemistry, have already provided important clues for the formation and activity of biomimetic iron-sulfur analogues in aqueous solution. This frontier article discusses the efforts spent in the last 50 years in the context of peptide-coordinated iron-sulfur clusters, with a particular emphasis on insightful contributions from recent prebiotic chemistry research.

Thiolate Coordination vs C-S Bond Cleavage of Thiolates in Dinickel(II) Complexes

A detailed study for the synthesis of dinickel(II)-thiolate and dinickel(II)-hydrosulfide complexes and the complete characterization of the relevant intermediates involved in the C-S bond cleavage of thiolates are presented. Hydrated Ni(II) salts mediate the hydrolytic C-S bond cleavage of thiolates (NaSR/RSH; R = Me, Et, ⁿBu, ^tBu), albeit inefficiently, to yield a mixture of a dinickel(II)-hydrosulfide complex, [Ni₂(BPMP)(μ-SH)(DMF)₂]²⁺ (1), and the corresponding dinickel(II)-thiolate complexes, such as [Ni₂(BPMP)(μ-SEt)(ClO₄)]¹⁺ (2) (HBPMP is 2,6-bis[[bis(2-pyridylmethyl)amino]methyl]-4-methylphenol). A systematic study for the reactivity of thiolates with Ni(II) was therefore pursued which finally yielded 1 as a pure product which has been characterized in comparison with the dinickel(II)-dichloride complex, [Ni₂(BPMP)(Cl)₂(MeOH)₂]¹⁺ (3). While the reaction of thiolates with anhydrous Ni(OTf)₂ in dry conditions could only yield [Ni₂(BPMP)(OTf)₂]¹⁺ (5) instead of the expected dinickel(II)-thiolate compound, the C-S bond cleavage could be suppressed by the use of a chelating thiol, such as PhCOSH, to yield [Ni₂(BPMP)(SCOPh)₂]¹⁺ (6). Finally, with the suitable choice of a monodentate thiol, a dinickel(II)-monothiolate complex, [Ni₂(BPMP)(SPh)(DMF)(MeOH)(H₂O)]²⁺ (7), was isolated as a pure product within 1 h of reaction, which after a longer time of reaction yielded 1 and PhOH. Complex 7 may thus be regarded as the intermediate that precedes the C-S bond cleavage and is generated by the reaction of a thiolate with an initially formed dinickel(II)-solvento complex, [Ni₂(BPMP)(MeOH)₂(H₂O)₂]³⁺(4). Selected dinickel(II) complexes were explored further for the scope of substitution reactions, and the results include the isolation of a dinickel(II)-bis(thiolate) complex, [Ni₂(BPMP)(μ-SPh)₂]¹⁺ (8).

Synthesis and redox reactions of binuclear zinc(ii)–thiolate complexes with elemental sulfur

The synthesis, characterization and reactivity of a series of binuclear zinc(ii) complexes are described featuring the redox reaction of coordinated thiolates with elemental sulfur.

Quantifying Mineral-Ligand Structural Similarities: Bridging the Geological World of Minerals with the Biological World of Enzymes

Metal compounds abundant on Early Earth are thought to play an important role in the origins of life. Certain iron-sulfur minerals for example, are proposed to have served as primitive metalloenzyme cofactors due to their ability to catalyze organic synthesis processes and facilitate electron transfer reactions. An inherent difficulty with studying the catalytic potential of many metal compounds is the wide range of data and parameters to consider when searching for individual minerals and ligands of interest. Detecting mineral-ligand pairs that are structurally analogous enables more relevant selections of data to study, since structural affinity is a key indicator of comparable catalytic function. However, current structure-oriented approaches tend to be subjective and localized, and do not quantify observations or compare them with other potential targets. Here, we present a mathematical approach that compares structural similarities between various minerals and ligands using molecular similarity metrics. We use an iterative substructure search in the crystal lattice, paired with benchmark structural similarity methods. This structural comparison may be considered as a first stage in a more advanced analysis tool that will include a range of chemical and physical factors when computing mineral-ligand similarity. This approach will seek relationships between the mineral and enzyme worlds, with applications to the origins of life, ecology, catalysis, and astrobiology.

Exploring the Limits of Dative Boratrane Bonding: Iron as a Strong Lewis Base in Low-Valent Non-Heme Iron-Nitrosyl Complexes

We previously reported the synthesis and preliminary characterization of a unique series of low-spin (ls) {FeNO}8-10 complexes supported by an ambiphilic trisphosphineborane ligand, [Fe(TPB)(NO)]+/0/-. Herein, we use advanced spectroscopic techniques and density functional theory (DFT) calculations to extract detailed information as to how the bonding changes across the redox series. We find that, in spite of the highly reduced nature of these complexes, they feature an NO+ ligand throughout with strong Fe-NO π-backbonding and essentially closed-shell electronic structures of their FeNO units. This is enabled by an Fe-B interaction that is present throughout the series. In particular, the most reduced [Fe(TPB)(NO)]- complex, an example of a ls-{FeNO}10 species, features a true reverse dative Fe → B bond where the Fe center acts as a strong Lewis-base. Hence, this complex is in fact electronically similar to the ls-{FeNO}8 system, with two additional electrons "stored" on site in an Fe-B single bond. The outlier in this series is the ls-{FeNO}9 complex, due to spin polarization (quantified by pulse EPR spectroscopy), which weakens the Fe-NO bond. These data are further contextualized by comparison with a related N2 complex, [Fe(TPB)(N2)]-, which is a key intermediate in Fe(TPB)-catalyzed N2 fixation. Our present study finds that the Fe → B interaction is key for storing the electrons needed to achieve a highly reduced state in these systems, and highlights the pitfalls associated with using geometric parameters to try to evaluate reverse dative interactions, a finding with broader implications to the study of transition metal complexes with boratrane and related ligands.

Iron Sulphur Cluster [Fe4S4(SPh)4]2– Catalyzed Electrochemical Reduction of CO2 on Carbon Electrodes in [Bu4N][BF4]-DMF Mixture

Background:

An efficient, selective and durable electrocatalytic carbon dioxide (CO2) reduction system is a prerequisite to tackle energy and pollution-related issues. In this context, both organic and inorganic materials have gained a significant interest worldwide.

Objective:

In the present work, the electrocatalytic reduction activity of an iron-sulphur (Fe-S) cluster, [Fe4S4(SPh)4]2– for CO2 → carbon monoxide (CO) conversion has been investigated. The effect of catalyst concentration on the yield of CO and H2 was determined. Besides, the influence of reaction conditions (presence or absence of a Brønsted acid, electrolysis time etc.) on faradaic yield and product selectivity was also investigated.

Methods:

Cyclic voltammetry (CV) was carried out on vitreous carbon electrode in 0.1 M [Bu4N] [BF4]-DMF electrolyte. At the end of electrolysis, products were collected by tight-gas syringe and analyzed by gas chromatography (GC) system coupled with a thermal conductivity detector.

Results:

The Fe-S cluster was found to efficiently catalyse the process on carbon electrode in 0.1 M [Bu4N][BF4]-DMF electrolyte. Moreover, the presence of cluster shifted the reduction potential by ~ 200 mV towards the positive. GC analysis confirmed the formation of CO with a current efficiency of ca. 70%. On the other hand, 12% H2 was observed at the end of electrocatalysis.

Conclusion:

In summary, Fe-S cluster was used for the electrocatalytic reduction of CO2 in 0.1 M [Bu4N][BF4]-DMF electrolyte. The use of cluster (catalyst) was found to be important for CO2 reduction as no CO was detected in the absence of the catalyst. This study highlights the potential application of Fe-S cluster for CO2 reduction.

Isolated Mixed-Valence Iron Vanadium Malate and Its Metal Hydrates (M = Fe2+, Cu2+, Zn2+) with Reversible and Irreversible Adsorptions for Oxygen

Isolated octanuclear iron-vanadium malate (NH₄)₃(CH₃NH₃)₃[Fe^III₂V^IV₂V^V₄O₁₁(mal)₆]·7.5H₂O (1; H₃mal = malic acid) and its family of metal hydrates M'_3n[M^II(H₂O)₂]_1.5n[Fe^III₂V^IV₂V^V₄O₁₁(mal)₆]_n·xnH₂O (2 or 2-Fe, M' = NH₄⁺, M = Fe, x = 7.5; 3 or 3-Cu, M' = K⁺, M = Cu, x = 10; 4 or 4-Zn, M' = K⁺, M = Zn, x = 6.5) have been obtained by self-assembly in water. The cluster anion [Fe₂V₆O₁₁(mal)₆]^6- (1a) shows an interesting iron bicapped-triangular-prismatic structure, which is bridged by M²⁺ hydrates (M = Fe, Cu, Zn) to construct isostructural metal organic frameworks (MOFs) 2-4. The mixed-valence vanadium systems in 1-4 were determined by theoretical bond valence calculations (BVS) and charge balance. The magnetic susceptibilities are further elucidated as high spin for Fe³⁺ in 1a and bridging Fe²⁺ in 2-Fe, respectively. A strong ferromagnetic interaction was also observed for 2-Fe at 3 K. 2-Fe, 3-Cu, and 4-Zn have similar hydrophilic channels with diameters of 6.8, 6.5, and 6.6 Å, respectively, which show obvious affinity for O₂ in comparison with no adsorption of N₂, H₂, CO₂, and CH₄ at room temperature under different pressures. Moreover, 2-Fe and 4-Zn exhibit irreversible O₂ absorptions, which may be attributed to charge transfer between O₂ and open metal sites (OMSs) formed during vacuum heating pretreatment. UV-vis and EPR spectra show a change in electronic structure of 2-Fe after O₂ adsorption. The reversible adsorption observed in 3-Cu suggests a weak interaction between O₂ and Cu²⁺ due to the Jahn-Teller effect. The properties of gas adsorption provide an insight into the performances of small molecules in the channels constructed by synthetic octanuclear model compounds, which are related to the interactions between the gas substrate and the heterometal cluster in biology.

An [Fe4S4]3+-Alkyl Cluster Stabilized by an Expanded Scorpionate Ligand

Alkyl-ligated iron-sulfur clusters in the [Fe₄S₄]³⁺ charge state have been proposed as short-lived intermediates in a number of enzymatic reactions. To better understand the properties of these intermediates, we have prepared and characterized the first synthetic [Fe₄S₄]³⁺-alkyl cluster. Isolation of this highly reactive species was made possible by the development of an expanded scorpionate ligand suited to the encapsulation of cuboidal clusters. Like the proposed enzymatic intermediates, this synthetic [Fe₄S₄]³⁺-alkyl cluster adopts an S = ¹/₂ ground state with g_iso > 2. Mössbauer spectroscopic studies reveal that the alkylated Fe has an unusually low isomer shift, which reflects the highly covalent Fe-C bond and the localization of Fe³⁺ at the alkylated site in the solid state. Paramagnetic ¹H NMR studies establish that this valence localization persists in solution at physiologically relevant temperatures, an effect that has not been observed for [Fe₄S₄]³⁺ clusters outside of a protein. These findings establish the unusual electronic-structure effects imparted by the strong-field alkyl ligand and lay the foundation for understanding the electronic structures of [Fe₄S₄]³⁺-alkyl intermediates in biology.

Heavy chalcogenide-transition metal clusters as coordination polymer nodes

While metal-oxygen clusters are widely used as secondary building units in the construction of coordination polymers or metal-organic frameworks, multimetallic nodes with heavier chalcogenide atoms (S, Se, and Te) are comparatively untapped. The lower electronegativity of heavy chalcogenides means that transition metal clusters of these elements generally exhibit enhanced coupling, delocalization, and redox-flexibility. Leveraging these features in coordination polymers provides these materials with extraordinary properties in catalysis, conductivity, magnetism, and photoactivity. In this perspective, we summarize common transition metal heavy chalcogenide building blocks including polynuclear metal nodes with organothiolate/selenolate or anionic heavy chalcogenide atoms. Based on recent discoveries, we also outline potential challenges and opportunities for applications in this field.

Structural Properties and Catalytic Implications of the SPASM Domain Iron-Sulfur Clusters in Methylorubrum extorquens PqqE

Understanding the relationship between the metallocofactor and its protein environment is the key to uncovering the mechanism of metalloenzymes. PqqE, a radical S-adenosylmethionine enzyme in pyrroloquinoline quinone (PQQ) biosynthesis, contains three iron-sulfur cluster binding sites. Two auxiliary iron-sulfur cluster binding sites, designated as AuxI and AuxII, use distinctive ligands compared to other proteins in the family while their functions remain unclear. Here, we investigate the electronic properties of these iron-sulfur clusters and compare the catalytic efficiency of wild-type (WT) Methylorubrum extorquens AM1 PqqE to a range of mutated constructs. Using native mass spectrometry, protein film electrochemistry, and electron paramagnetic resonance spectroscopy, we confirm the previously proposed incorporation of a mixture of [2Fe-2S] and [4Fe-4S] clusters at the AuxI site and are able to assign redox potentials to each of the three iron-sulfur clusters. Significantly, a conservative mutation at AuxI, C268H, shown to selectively incorporate a [4Fe-4S] cluster, catalyzes an enhancement of uncoupled S-adenosylmethionine cleavage relative to WT, together with the elimination of detectable peptide cross-linked product. While a [4Fe-4S] cluster can be tolerated at the AuxI site, the aggregate findings suggest a functional [2Fe-2S] configuration within the AuxI site. PqqE variants with nondestructive ligand replacements at AuxII also show that the reduction potential at this site can be manipulated by changing the electronegativity of the unique aspartate ligand. A number of novel mechanistic features are proposed based on the kinetic and spectroscopic data. Additionally, bioinformatic analyses suggest that the unique ligand environment of PqqE may be relevant to its role in PQQ biosynthesis within an oxygen-dependent biosynthetic pathway.

The Solvent-Exposed Fe-S D-Cluster Contributes to Oxygen-Resistance in Desulfovibrio vulgaris Ni-Fe Carbon Monoxide Dehydrogenase

Ni-Fe CO-dehydrogenases (CODHs) catalyze the conversion between CO and CO₂ using a chain of Fe-S clusters to mediate long-range electron transfer. One of these clusters, the D-cluster, is surface-exposed and serves to transfer electrons between CODH and external redox partners. These enzymes tend to be extremely O₂-sensitive and are always manipulated under strictly anaerobic conditions. However, the CODH from Desulfovibrio vulgaris (Dv) appears unique: exposure to micromolar concentrations of O₂ on the minutes-time scale only reversibly inhibits the enzyme, and full activity is recovered after reduction. Here, we examine whether this unusual property of Dv CODH results from the nature of its D-cluster, which is a [2Fe-2S] cluster, instead of the [4Fe-4S] cluster observed in all other characterized CODHs. To this aim, we produced and characterized a Dv CODH variant where the [2Fe-2S] D-cluster is replaced with a [4Fe-4S] D-cluster through mutagenesis of the D-cluster-binding sequence motif. We determined the crystal structure of this CODH variant to 1.83-Å resolution and confirmed the incorporation of a [4Fe-4S] D-cluster. We show that upon long-term O₂-exposure, the [4Fe-4S] D-cluster degrades, whereas the [2Fe-2S] D-cluster remains intact. Crystal structures of the Dv CODH variant exposed to O₂ for increasing periods of time provide snapshots of [4Fe-4S] D-cluster degradation. We further show that the WT enzyme purified under aerobic conditions retains 30% activity relative to a fully anaerobic purification, compared to 10% for the variant, and the WT enzyme loses activity more slowly than the variant upon prolonged aerobic storage. The D-cluster is therefore a key site of irreversible oxidative damage in Dv CODH, and the presence of a [2Fe-2S] D-cluster contributes to the O₂-tolerance of this enzyme. Together, these results relate O₂-sensitivity with the details of the protein structure in this family of enzymes.

Special Issue on Nitrogenases and Homologous Systems

The nitrogenase superfamily comprises homologous enzyme systems that carry out fundamentally important processes, including the reduction of N₂ and CO, and the biosynthesis of bacteriochlorophyll and coenzyme F430. This special issue provides a cross-disciplinary overview of the ongoing research in this highly diverse and unique research area of metalloprotein biochemistry.