Production and isolation of vanadium nitrogenase from Azotobacter vinelandii by molybdenum depletion

Structural design of thiadiazole-based donor-acceptor COF/Fe-doped N vacancy g-C3Nx nanosheets for photocatalytic nitrogen fixation under visible light

The rational design of efficient photocatalysts to achieve artificial nitrogen fixation is an urgent challenge. Herein, we combined donor-acceptor covalent organic framework with iron-doped nitrogen vacancy graphitized carbon nitride (D-A COF/Fe-g-C3Nx) for photocatalytic nitrogen fixation. The photocatalyst exhibited good crystallinity, high porosity, and a large specific surface area. Without a sacrificial agent, the optimal 40 % D-A COF/Fe-g-C3Nx exhibited an excellent rate of ammonia production (646 μmol h-1 g-1) at 420 nm, and durable stability after successive cycling. Exhaustive experimental research and theory calculations verified that the D-A unit and Fe doping redistributed the distribution of the charge, which enhanced the visible light utilization and provided chemisorption sites for further polarization. Besides N-vacancies can serve as electron-trapping active sites to promote the directional migration of carriers. The reaction mechanism demonstrated that superoxide radical and hydrogen peroxide were formed by electron and hole, respectively, which promote the reduction of nitrogen to ammonia. This work provides a new idea for the rationalizing design of efficient catalysts for photocatalytic nitrogen fixation under mild conditions.

On the Shoulders of Giants-Reaching for Nitrogenase

Only a single enzyme system-nitrogenase-carries out the conversion of atmospheric N2 into bioavailable ammonium, an essential prerequisite for all organismic life. The reduction of this inert substrate at ambient conditions poses unique catalytic challenges that strain our mechanistic understanding even after decades of intense research. Structural biology has added its part to this greater tapestry, and in this review, I provide a personal (and highly biased) summary of the parts of the story to which I had the privilege to contribute. It focuses on the crystallographic analysis of the three isoforms of nitrogenases at high resolution and the binding of ligands and inhibitors to the active-site cofactors of the enzyme. In conjunction with the wealth of available biochemical, biophysical, and spectroscopic data on the protein, this has led us to a mechanistic hypothesis based on an elementary mechanism of repetitive hydride formation and insertion.

Interaction of Vanadium Complexes with Proteins: Revisiting the Reported Structures in the Protein Data Bank (PDB) since 2015

The structural determination and characterization of molecules, namely proteins and enzymes, is crucial to gaining a better understanding of their role in different chemical and biological processes. The continuous technical developments in the experimental and computational resources of X-ray diffraction (XRD) and, more recently, cryogenic Electron Microscopy (cryo-EM) led to an enormous growth in the number of structures deposited in the Protein Data Bank (PDB). Bioinorganic chemistry arose as a relevant discipline in biology and therapeutics, with a massive number of studies reporting the effects of metal complexes on biological systems, with vanadium complexes being one of the relevant systems addressed. In this review, we focus on the interactions of vanadium compounds (VCs) with proteins. Several types of binding are established between VCs and proteins/enzymes. Considering that the V-species that bind may differ from those initially added, the mentioned structural techniques are pivotal to clarifying the nature and variety of interactions of VCs with proteins and to proposing the mechanisms involved either in enzymatic inhibition or catalysis. As such, we provide an account of the available structural information of VCs bound to proteins obtained by both XRD and/or cryo-EM, mainly exploring the more recent structures, particularly those containing organic-based vanadium complexes.

Overview of physiological, biochemical, and regulatory aspects of nitrogen fixation in Azotobacter vinelandii

Understanding how Nature accomplishes the reduction of inert nitrogen gas to form metabolically tractable ammonia at ambient temperature and pressure has challenged scientists for more than a century. Such an understanding is a key aspect toward accomplishing the transfer of the genetic determinants of biological nitrogen fixation to crop plants as well as for the development of improved synthetic catalysts based on the biological mechanism. Over the past 30 years, the free-living nitrogen-fixing bacterium Azotobacter vinelandii emerged as a preferred model organism for mechanistic, structural, genetic, and physiological studies aimed at understanding biological nitrogen fixation. This review provides a contemporary overview of these studies and places them within the context of their historical development.

Rnf and Fix Have Specific Roles during Aerobic Nitrogen Fixation in Azotobacter vinelandii

Biological nitrogen fixation requires large amounts of energy in the form of ATP and low potential electrons to overcome the high activation barrier for cleavage of the dinitrogen triple bond. The model aerobic nitrogen-fixing bacteria, Azotobacter vinelandii, generates low potential electrons in the form of reduced ferredoxin (Fd) and flavodoxin (Fld) using two distinct mechanisms via the enzyme complexes Rnf and Fix. Both Rnf and Fix are expressed during nitrogen fixation, but deleting either rnf1 or fix genes has little effect on diazotrophic growth. However, deleting both rnf1 and fix eliminates the ability to grow diazotrophically. Rnf and Fix both use NADH as a source of electrons, but overcoming the energetics of NADH's endergonic reduction of Fd/Fld is accomplished through different mechanisms. Rnf harnesses free energy from the chemiosmotic potential, whereas Fix uses electron bifurcation to effectively couple the endergonic reduction of Fd/Fld to the exergonic reduction of quinone. Different reaction stoichiometries and condition-specific differential gene expression indicate specific roles for the two reactions. This work's complementary physiological studies and thermodynamic modeling reveal how Rnf and Fix balance redox homeostasis in various conditions. Specifically, the Fix complex is required for efficient growth under low oxygen concentrations, while Rnf is presumed to maintain reduced Fd/Fld production for nitrogenase under standard conditions. This work provides a framework for understanding how the production of low potential electrons sustains robust nitrogen fixation in various conditions. IMPORTANCE The availability of fixed nitrogen is critical for life in many ecosystems, from extreme environments to agriculture. Due to the energy demands of biological nitrogen fixation, organisms must tailor their metabolism during diazotrophic growth to deliver the energy requirements to nitrogenase in the form of ATP and low potential electrons. Therefore, a complete understanding of diazotrophic energy metabolism and redox homeostasis is required to understand the impact on ecological communities or to promote crop growth in agriculture through engineered diazotrophs.

Structural analysis of the reductase component AnfH of iron-only nitrogenase from Azotobacter vinelandii

Biological nitrogen fixation, the conversion of atmospheric dinitrogen into bioavailable ammonium, is exclusively catalyzed by the enzyme nitrogenase that is present in nitrogen-fixing organisms, the diazotrophs. So far, three different nitrogenase variants, encoded in their corresponding, distinct gene clusters, have been found in nature. Each one of these consists of a catalytic dinitrogenase component and a unique, ATP-dependent reductase, the Fe protein. The three variant nitrogenases differ in the composition of the active site and contain either molybdenum, vanadium or only iron in the dinitrogenase component. Here we present the 2.0 Å resolution crystal structure of the ADP-bound reductase component AnfH of the iron-only nitrogenase from the model diazotroph Azotobacter vinelandii. A comparison of this structure with the ones reported for the two other Fe protein homologs NifH and VnfH in the ADP-bound state shows that all are adopting the same conformation. However, cross-reactivity assays with the three nitrogenase homologs revealed AnfH to be compatible with iron-only nitrogenase and to a lesser degree with the vanadium-containing enzyme, but not with molybdenum nitrogenase.

Challenge of diabetes mellitus and researchers’ contributions to its control

The aim of this review study was to assess the past significant events on diabetes mellitus, transformations that took place over the years in the medical records of treatment, countries involved, and the researchers who brought about the revolutions. This study used the content analysis to report the existence of diabetes mellitus and the treatments provided by researchers to control it. The focus was mainly on three main types of diabetes (type 1, type 2, and type 3 diabetes). Ethical consideration has also helped to boost diabetic studies globally. The research has a history path from pharmaceuticals of organic-based drugs to metal-based drugs with their nanoparticles in addition to the impacts of nanomedicine, biosensors, and telemedicine. Ongoing and future studies in alternative medicine such as vanadium nanoparticles (metal nanoparticles) are promising.

Two ligand-binding sites in CO-reducing V nitrogenase reveal a general mechanistic principle

Besides its role in biological nitrogen fixation, vanadium-containing nitrogenase also reduces carbon monoxide (CO) to hydrocarbons, in analogy to the industrial Fischer-Tropsch process. The protein yields 93% of ethylene (C₂H₄), implying a C-C coupling step that mandates the simultaneous binding of two CO at the active site FeV cofactor. Spectroscopic data indicated multiple CO binding events, but structural analyses of Mo and V nitrogenase only confirmed a single site. Here, we report the structure of a two CO-bound state of V nitrogenase at 1.05 Å resolution, with one μ-bridging and one terminal CO molecule. This additional, specific ligand binding site suggests a mechanistic route for CO reduction and hydrocarbon formation, as well as a second access pathway for protons required during the reaction. Moreover, carbonyls are strong-field ligands that are chemically similar to mechanistically relevant hydrides that may be formed and used in a fully analogous fashion.

The electronic structure of FeV-cofactor in vanadium-dependent nitrogenase

The electronic structure of the active-site metal cofactor (FeV-cofactor) of resting-state V-dependent nitrogenase has been an open question, with earlier studies indicating that it exhibits a broad S = 3/2 EPR signal (Kramers state) having g values of ∼4.3 and 3.8, along with suggestions that it contains metal-ions with valencies [1V3+, 3Fe3+, 4Fe2+]. In the present work, genetic, biochemical, and spectroscopic approaches were combined to reveal that the EPR signals previously assigned to FeV-cofactor do not correlate with active VFe-protein, and thus cannot arise from the resting-state of catalytically relevant FeV-cofactor. It, instead, appears resting-state FeV-cofactor is either diamagnetic, S = 0, or non-Kramers, integer-spin (S = 1, 2 etc.). When VFe-protein is freeze-trapped during high-flux turnover with its natural electron-donating partner Fe protein, conditions which populate reduced states of the FeV-cofactor, a new rhombic S = 1/2 EPR signal from such a reduced state is observed, with g = [2.18, 2.12, 2.09] and showing well-defined 51V (I = 7/2) hyperfine splitting, a iso = 110 MHz. These findings indicate a different assignment for the electronic structure of the resting state of FeV-cofactor: S = 0 (or integer-spin non-Kramers state) with metal-ion valencies, [1V3+, 4Fe3+, 3Fe2+]. Our findings suggest that the V3+ does not change valency throughout the catalytic cycle.

Expression, Isolation, and Characterization of Vanadium Nitrogenase from Azotobacter vinelandii

Nitrogenases are the sole enzymes known to mediate biological nitrogen fixation, an essential process for sustaining life on earth. Among the three known variants, molybdenum nitrogenase is the best-studied to date. Recent work on the alternative vanadium nitrogenase provided important insights into the mechanism of nitrogen fixation since this enzyme differs from its molybdenum counterpart in some important aspects. Here, we present a protocol to obtain unmodified vanadium nitrogenase in high yield and purity from the paradigmatic diazotroph Azotobacter vinelandii, including procedures for cell cultivation, purification, and protein characterization.

Revealing a role for the G subunit in mediating interactions between the nitrogenase component proteins

Azotobacter vinelandii contains three forms of nitrogenase known as the Mo-, V-, and Fe-nitrogenases. They are all two-component enzyme systems, where the catalytic component, referred to as NifDK, VnfDGK, and AnfDGK, associates with the reductase component, the Fe protein or NifH, VnfH, and AnfH respectively. AnfDGK and VnfDGK have an additional subunit compared to NifDK, termed gamma or AnfG and VnfG, whose role is unknown. The expression of each nitrogenase is tightly regulated by metal availability, however it is known that there is crosstalk between the Mo- and V‑nitrogenases but the Fe‑nitrogenase components cannot support substrate reduction with its Mo‑nitrogenase counterparts. Here, docking models for the nitrogenase complexes were generated in ClusPro 2.0 based on the crystal structure of the Mo‑nitrogenase and refined using the HADDOCK 2.2 refinement interface to identify structural determinants that enable crosstalk between the Mo- and V‑nitrogenase but not the Fe‑nitrogenase. Differing salt bridge interactions were identified at the binding interface of each complex. Specifically, positively charged residues of VnfG enable complementary interactions with NifH and VnfH but not AnfH. Similarly, negatively charged residues of AnfG enable interactions with AnfH but not NifH or VnfH. A role for the G subunit is revealed where VnfG could be mediating crosstalk between the Mo- and V‑nitrogenases while the AnfG subunit on AnfDGK makes interactions with NifH and VnfH unfavorable, reducing competition with NifDK and funneling electrons to the most efficient nitrogenase.

The Spectroscopy of Nitrogenases

Nitrogenases are responsible for biological nitrogen fixation, a crucial step in the biogeochemical nitrogen cycle. These enzymes utilize a two-component protein system and a series of iron-sulfur clusters to perform this reaction, culminating at the FeMco active site (M = Mo, V, Fe), which is capable of binding and reducing N2 to 2NH3. In this review, we summarize how different spectroscopic approaches have shed light on various aspects of these enzymes, including their structure, mechanism, alternative reactivity, and maturation. Synthetic model chemistry and theory have also played significant roles in developing our present understanding of these systems and are discussed in the context of their contributions to interpreting the nature of nitrogenases. Despite years of significant progress, there is still much to be learned from these enzymes through spectroscopic means, and we highlight where further spectroscopic investigations are needed.

Modeling of nitrogen fixation and polymer production in the heterotrophic diazotroph Azotobacter vinelandii DJ

Nitrogen fixation is an important metabolic process carried out by microorganisms, which converts molecular nitrogen into inorganic nitrogenous compounds such as ammonia (NH₃). These nitrogenous compounds are crucial for biogeochemical cycles and for the synthesis of essential biomolecules, i.e. nucleic acids, amino acids and proteins. Azotobacter vinelandii is a bacterial non-photosynthetic model organism to study aerobic nitrogen fixation (diazotrophy) and hydrogen production. Moreover, the diazotroph can produce biopolymers like alginate and polyhydroxybutyrate (PHB) that have important industrial applications. However, many metabolic processes such as partitioning of carbon and nitrogen metabolism in A. vinelandii remain unknown to date.

Genome-scale metabolic models (M-models) represent reliable tools to unravel and optimize metabolic functions at genome-scale. M-models are mathematical representations that contain information about genes, reactions, metabolites and their associations. M-models can simulate optimal reaction fluxes under a wide variety of conditions using experimentally determined constraints. Here we report on the development of a M-model of the wild type bacterium A. vinelandii DJ (iDT1278) which consists of 2,003 metabolites, 2,469 reactions, and 1,278 genes. We validated the model using high-throughput phenotypic and physiological data, testing 180 carbon sources and 95 nitrogen sources. iDT1278 was able to achieve an accuracy of 89% and 91% for growth with carbon sources and nitrogen source, respectively. This comprehensive M-model will help to comprehend metabolic processes associated with nitrogen fixation, ammonium assimilation, and production of organic nitrogen in an environmentally important microorganism.

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Genome-scale metabolic model of Azotobacter vinelandii DJ achives over 90% accuracy.

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iDT1278 is the most comprehensive model to simulate diazotrophy.

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Determining the most suitable culture conditions to produce polymers A. vinelandii.

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Constraint-based modeling unravels links among nitrogen fixation and production of organic nitrogen.

Reduction of Substrates by Nitrogenases

Nitrogenase is the enzyme that catalyzes biological N₂ reduction to NH₃. This enzyme achieves an impressive rate enhancement over the uncatalyzed reaction. Given the high demand for N₂ fixation to support food and chemical production and the heavy reliance of the industrial Haber-Bosch nitrogen fixation reaction on fossil fuels, there is a strong need to elucidate how nitrogenase achieves this difficult reaction under benign conditions as a means of informing the design of next generation synthetic catalysts. This Review summarizes recent progress in addressing how nitrogenase catalyzes the reduction of an array of substrates. New insights into the mechanism of N₂ and proton reduction are first considered. This is followed by a summary of recent gains in understanding the reduction of a number of other nitrogenous compounds not considered to be physiological substrates. Progress in understanding the reduction of a wide range of C-based substrates, including CO and CO₂, is also discussed, and remaining challenges in understanding nitrogenase substrate reduction are considered.

Reactivity, Mechanism, and Assembly of the Alternative Nitrogenases

Biological nitrogen fixation is catalyzed by the enzyme nitrogenase, which facilitates the cleavage of the relatively inert triple bond of N₂. Nitrogenase is most commonly associated with the molybdenum-iron cofactor called FeMoco or the M-cluster, and it has been the subject of extensive structural and spectroscopic characterization over the past 60 years. In the late 1980s and early 1990s, two "alternative nitrogenase" systems were discovered, isolated, and found to incorporate V or Fe in place of Mo. These systems are regulated by separate gene clusters; however, there is a high degree of structural and functional similarity between each nitrogenase. Limited studies with the V- and Fe-nitrogenases initially demonstrated that these enzymes were analogously active as the Mo-nitrogenase, but more recent investigations have found capabilities that are unique to the alternative systems. In this review, we will discuss the reactivity, biosynthetic, and mechanistic proposals for the alternative nitrogenases as well as their electronic and structural properties in comparison to the well-characterized Mo-dependent system. Studies over the past 10 years have been particularly fruitful, though key aspects about V- and Fe-nitrogenases remain unexplored.

Carbon substrate re-orders relative growth of a bacterium using Mo-, V-, or Fe-nitrogenase for nitrogen fixation

Biological nitrogen fixation is catalyzed by the molybdenum (Mo), vanadium (V) and iron (Fe)‐only nitrogenase metalloenzymes. Studies with purified enzymes have found that the ‘alternative’ V‐ and Fe‐nitrogenases generally reduce N₂ more slowly and produce more byproduct H₂ than the Mo‐nitrogenase, leading to an assumption that their usage results in slower growth. Here we show that, in the metabolically versatile photoheterotroph Rhodopseudomonas palustris, the type of carbon substrate influences the relative rates of diazotrophic growth based on different nitrogenase isoforms. The V‐nitrogenase supports growth as fast as the Mo‐nitrogenase on acetate but not on the more oxidized substrate succinate. Our data suggest that this is due to insufficient electron flux to the V‐nitrogenase isoform on succinate compared with acetate. Despite slightly faster growth based on the V‐nitrogenase on acetate, the wild‐type strain uses exclusively the Mo‐nitrogenase on both carbon substrates. Notably, the differences in H₂:N₂ stoichiometry by alternative nitrogenases (~1.5 for V‐nitrogenase, ~4–7 for Fe‐nitrogenase) and Mo‐nitrogenase (~1) measured here are lower than prior in vitro estimates. These results indicate that the metabolic costs of V‐based nitrogen fixation could be less significant for growth than previously assumed, helping explain why alternative nitrogenase genes persist in diverse diazotroph lineages and are broadly distributed in the environment.

Nitrogenases

Biological nitrogen fixation, the conversion of dinitrogen (N₂) into ammonia (NH₃), stands as a particularly challenging chemical process. As the entry point into a bioavailable form of nitrogen, biological nitrogen fixation is a critical step in the global nitrogen cycle. In Nature, only one enzyme, nitrogenase, is competent in performing this reaction. Study of this complex metalloenzyme has revealed a potent substrate reduction system that utilizes some of the most sophisticated metalloclusters known. This chapter discusses the structure and function of nitrogenase, covers methods that have proven useful in the elucidation of enzyme properties, and provides an overview of the three known nitrogenase variants.

Azotobacter vinelandii Nitrogenase Activity, Hydrogen Production, and Response to Oxygen Exposure

Azotobacter vinelandii selectively utilizes three types of nitrogenase (molybdenum, vanadium, and iron only) to fix N₂, with their expression regulated by the presence or absence of different metal cofactors in its environment. Each alternative nitrogenase isoenzyme is predicted to have different electron flux requirements based on in vitro measurements, with the molybdenum nitrogenase requiring the lowest flux and the iron-only nitrogenase requiring the highest. Here, prior characterized strains, derepressed in nitrogenase synthesis and also deficient in uptake hydrogenase, were further modified to generate new mutants lacking the ability to produce poly-β-hydroxybutyrate (PHB). PHB is a storage polymer generated under oxygen-limiting conditions and can represent up to 70% of the cells' dry weight. The absence of such granules facilitated the study of relationships between catalytic biomass and product molar yields across different adaptive respiration conditions. The released hydrogen gas observed during growth, due to the inability of the mutants to recapture hydrogen, allowed for direct monitoring of in vivo nitrogenase activity for each isoenzyme. The data presented here show that increasing oxygen exposure limits equally the in vivo activities of all nitrogenase isoenzymes, while under comparative conditions, the Mo nitrogenase enzyme evolves more hydrogen per unit of biomass than the alternative isoenzymes.IMPORTANCEA. vinelandii has been a focus of intense research for over 100 years. It has been investigated for a variety of functions, including agricultural fertilization and hydrogen production. All of these endeavors are centered around A. vinelandii's ability to fix nitrogen aerobically using three nitrogenase isoenzymes. The majority of research up to this point has targeted in vitro measurements of the molybdenum nitrogenase, and robust data contrasting how oxygen impacts the in vivo activity of each nitrogenase isoenzyme are lacking. This article aims to provide in vivo nitrogenase activity data using a real-time evaluation of hydrogen gas released by derepressed nitrogenase mutants lacking an uptake hydrogenase and PHB accumulation.

The Critical E4 State of Nitrogenase Catalysis

The reaction catalyzed by the nitrogenase enzyme involves breaking the stable triple bond of the dinitrogen molecule and is consequently considered among the most challenging reactions in biology. While many aspects regarding its atomic mechanism remain to be elucidated, a kinetic scheme established by David Lowe and Roger Thorneley has remained a gold standard for functional studies of the enzyme for more than 30 years. Recent three-dimensional structures of ligand-bound states of molybdenum- and vanadium-dependent nitrogenases have revealed the actual site of substrate binding on the large active site cofactors of this class of enzymes. The binding mode of an inhibitor and a reaction intermediate further substantiate a hypothesis by Seefeldt, Hoffman, and Dean that the activation of N₂ is made possible by a reductive elimination of H₂ that leaves the cofactor in a super-reduced state that can bind and reduce the inert N₂ molecule. Here we discuss the immediate implications of the structurally observed mode of binding of small molecules to the enzyme with respect to the early stages of the Thorneley-Lowe mechanism of nitrogenase. Four consecutive single-electron reductions give rise to two bridging hydrides at the cluster surface that can recombine to eliminate H₂ and enable the reduced cluster to bind its substrate in a bridging mode.

A bound reaction intermediate sheds light on the mechanism of nitrogenase

Reduction of N₂ by nitrogenases occurs at an organometallic iron cofactor that commonly also contains either molybdenum or vanadium. The well-characterized resting state of the cofactor does not bind substrate, so its mode of action remains enigmatic. Carbon monoxide was recently found to replace a bridging sulfide, but the mechanistic relevance was unclear. Here we report the structural analysis of vanadium nitrogenase with a bound intermediate, interpreted as a μ²-bridging, protonated nitrogen that implies the site and mode of substrate binding to the cofactor. Binding results in a flip of amino acid glutamine 176, which hydrogen-bonds the ligand and creates a holding position for the displaced sulfide. The intermediate likely represents state E₆ or E₇ of the Thorneley-Lowe model and provides clues to the remainder of the catalytic cycle.

The structure of vanadium nitrogenase reveals an unusual bridging ligand

Nitrogenases catalyze the reduction of N₂ gas to ammonium at a complex heterometallic cofactor. Most commonly this is the FeMo cofactor (FeMoco), a [Mo:7Fe:9S:C] cluster whose exact reactivity and substrate binding mode remain unknown. Alternative nitrogenases replace molybdenum with either vanadium or iron and differ in reactivity, prominently in the ability of vanadium nitrogenase to reduce CO to hydrocarbons. Here we report the 1.35 Å structure of vanadium nitrogenase from Azotobacter vinelandii. The 240 kDa protein contains an additional α-helical subunit not present in molybdenum nitrogenase. The FeV cofactor (FeVco) is a [V:7Fe:8S:C] cluster with a homocitrate ligand to vanadium. Unexpectedly, it lacks one sulfide ion compared to FeMoco that is replaced by a bridging ligand, likely a μ-1,3-carbonate. The anion fits into a pocket within the protein that is obstructed in molybdenum nitrogenase, and its different chemical character helps to rationalize the altered chemical properties of this unique N₂- and CO-fixing enzyme.

Comparative electronic structures of nitrogenase FeMoco and FeVco

An investigation of the active site cofactors of the molybdenum and vanadium nitrogenases (FeMoco and FeVco) was performed using high-resolution X-ray spectroscopy. Synthetic heterometallic iron-sulfur cluster models and density functional theory calculations complement the study of the MoFe and VFe holoproteins using both non-resonant and resonant X-ray emission spectroscopy. Spectroscopic data show the presence of direct iron-heterometal bonds, which are found to be weaker in FeVco. Furthermore, the interstitial carbide is found to perturb the electronic structures of the cofactors through highly covalent Fe-C bonding. The implications of these conclusions are discussed in light of the differential reactivity of the molybdenum and vanadium nitrogenases towards various substrates. Possible functional roles for both the heterometal and the interstitial carbide are detailed.