# FeMoco

**FeMoco** (the iron-molybdenum cofactor, also written FeMo-co) is the metal-sulfur cluster that forms the active site of molybdenum-dependent nitrogenase, the bacterial enzyme that reduces atmospheric dinitrogen (N₂) to ammonia (NH₃) at ambient temperature and pressure. Its composition is [MoFe₇S₉C] together with an organic R-homocitrate ligand, making it one of the most complex metal cofactors known.<sup>[1](https://www.nature.com/articles/s41467-024-48271-8)</sup><sup> • </sup><sup>[2](https://www.annualreviews.org/content/journals/10.1146/annurev.micro.62.081307.162737)</sup>  Biological nitrogen fixation performed by nitrogenase supplies nearly 50% of the bioavailable nitrogen pool on Earth.<sup>[3](https://www.nature.com/articles/s41467-024-54713-0)</sup>

| Key fact | Detail |
|---|---|
| Composition | [MoFe₇S₉C] cluster plus one R-homocitrate molecule bound to Mo<sup>[1](https://www.nature.com/articles/s41467-024-48271-8)</sup> |
| Location | Active site of the MoFe protein, buried 10 Å beneath the surface of each α subunit<sup>[4](https://pubs.acs.org/doi/10.1021/acs.chemrev.9b00489)</sup> |
| Protein ligation | α-Cys275 to the terminal Fe and α-His442 to Mo in the *Azotobacter vinelandii* MoFe protein; only two residues coordinate an eight-metal cluster<sup>[4](https://pubs.acs.org/doi/10.1021/acs.chemrev.9b00489)</sup><sup> • </sup><sup>[3](https://www.nature.com/articles/s41467-024-54713-0)</sup>  |
| Central atom | A μ₆-carbide, unprecedented in biology, donated from S-adenosylmethionine and not exchanged during catalysis<sup>[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC8606229/)</sup><sup> • </sup><sup>[6](https://www.nature.com/articles/ncomms10902)</sup>  |
| Resting-state oxidation states | Fe²⁺₃Fe³⁺₄Mo³⁺, with the cofactor in an S = 3/2 ground state<sup>[7](https://pmc.ncbi.nlm.nih.gov/articles/PMC5508211/)</sup><sup> • </sup><sup>[6](https://www.nature.com/articles/ncomms10902)</sup>  |
| First crystal structure | 1992, with the MoFe protein of *A. vinelandii*<sup>[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC8606229/)</sup> |
| Biosynthetic flow | NifU → NifS → NifB → NifEN → MoFe protein, with NifV supplying homocitrate and NifQ handling molybdenum<sup>[8](https://kdl.kogistate.gov.ng/wp-content/uploads/2024/02/Biological-Inorganic-Chemistry-2nd-Ed.-R.-Crichton-Elsevier-2012.pdf)</sup><sup> • </sup><sup>[9](https://pmc.ncbi.nlm.nih.gov/articles/PMC3077758/)</sup>  |
| Catalytic role | Site of N₂ reduction in an eight-electron Lowe-Thorneley cycle (states E₀ to E₇), with N₂ binding at E₃ or E₄<sup>[1](https://www.nature.com/articles/s41467-024-48271-8)</sup> |

## Structure

FeMoco is a variation on the iron-sulfur cluster theme, but it differs from all other members of that family. It contains one more acid-labile sulfide than it has metal ions, and a central μ₆-carbide holds a position that fuses two cubane-like half clusters, which are additionally connected by three μ²-bridging sulfides.<sup>[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC8606229/)</sup> The homocitrate molecule, synthesized by the [homocitrate synthase](https://www.edgepedia.org/homocitrate-synthase) NifV from acetyl-coenzyme A and 2-oxoglutarate, serves as a ligand to the molybdenum atom.<sup>[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC8606229/)</sup>

**Protein coordination.** Despite carrying eight metal sites, the cluster is held in the protein through the side chains of only two residues, α-Cys275 and α-His442, which ligate the terminal Fe1 and the Mo respectively. All seven iron ions are tetrahedrally four-coordinate, while molybdenum is octahedrally coordinated through three sulfide ligands from the cluster, the homocitrate, and the histidine.<sup>[4](https://pubs.acs.org/doi/10.1021/acs.chemrev.9b00489)</sup><sup> • </sup><sup>[3](https://www.nature.com/articles/s41467-024-54713-0)</sup>  This sparse coordination contrasts with the P-cluster, the [8Fe:7S] electron-transfer relay of the same protein, where every metal has a protein ligand.<sup>[3](https://www.nature.com/articles/s41467-024-54713-0)</sup> The environment around the cofactor is mostly hydrophilic, and the homocitrate site is surrounded by a pool of water molecules participating in a hydrogen-bonding network.<sup>[4](https://pubs.acs.org/doi/10.1021/acs.chemrev.9b00489)</sup>

**The central carbide.** Early crystal structures showed a "void" at the cluster center, and the identity of the light atom there was debated, with candidates including nitrogen, oxygen, and carbon. The atom was eventually identified as a carbon species that originates from S-adenosylmethionine and does not exchange during catalysis.<sup>[6](https://www.nature.com/articles/ncomms10902)</sup> A 1.21-Å structure of a hyperthermophilic archaeon nitrogenase later corroborated the carbide assignment and showed the FeMoco chemical structure to be essentially identical across organisms.<sup>[10](https://www.nature.com/articles/s41467-026-77173-0)</sup>

## Discovery and structural history

FeMoco was first detected as an "activating factor" capable of reconstituting inactive nitrogenase in *A. vinelandii* strain UW45, which carries a mutation in the nifB gene and accumulates active Fe protein but inactive MoFe protein. The factor could be extracted from acid-denatured MoFe protein by a method analogous to the extraction of the molybdenum cofactor from xanthine oxidase, and it could equally be obtained from *Clostridium pasteurianum*, *Klebsiella pneumoniae*, or *Rhodospirillum rubrum* MoFe proteins.<sup>[4](https://pubs.acs.org/doi/10.1021/acs.chemrev.9b00489)</sup>

The first crystal structure of Mo-nitrogenase, reported in 1992, defied expectations: the unusual cluster could not be convincingly modeled by synthetic chemistry beforehand, and its central light atom was masked by [Fourier series](https://www.edgepedia.org/fourier-series) termination artifacts in early maps.<sup>[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC8606229/)</sup><sup> • </sup><sup>[6](https://www.nature.com/articles/ncomms10902)</sup>  The intact cofactor can be extracted from the protein after acid denaturation into N-methylformamide and reconstituted into apo-nitrogenase, yielding active protein, which shows that the cluster retains structural integrity outside the enzyme.<sup>[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC8606229/)</sup>

## Function in nitrogenase catalysis

The Mo nitrogenase consists of two metalloproteins: the Fe protein, a reductase with a [4Fe-4S] cluster and two ATP-binding sites, and the α₂β₂ MoFe protein, which carries the P-cluster and FeMoco. Electrons flow from the Fe protein [4Fe-4S] cluster through the surface-located P-cluster to FeMoco, with each electron transfer costing two ATP hydrolyzed at the Fe protein.<sup>[4](https://pubs.acs.org/doi/10.1021/acs.chemrev.9b00489)</sup><sup> • </sup><sup>[3](https://www.nature.com/articles/s41467-024-54713-0)</sup>

The Lowe-Thorneley kinetic scheme describes the reaction as an eight-electron process proceeding through eight one-electron reduced states, E₀ through E₇, with alternating electron and proton transfer. N₂ binding occurs at the E₃ or E₄ states, and exchange of N₂ for H₂ is a mechanistic requirement of the cycle.<sup>[1](https://www.nature.com/articles/s41467-024-48271-8)</sup> The resting-state cofactor does not bind N₂ before activation by a 3- or 4-electron reduction.<sup>[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC8606229/)</sup> Spectroscopic and mutation studies indicate that the belt iron sites are the location of N₂ binding and reduction.<sup>[7](https://pmc.ncbi.nlm.nih.gov/articles/PMC5508211/)</sup>

**Lability of the belt sulfurs.** The three belt sulfurs (S2B, S3A, and S5A) are notably labile. [Carbon monoxide](https://www.edgepedia.org/carbon-monoxide) specifically displaces S2B, turnover with selenocyanate replaces all three belt sulfurs with selenium to varying extents, and time-resolved cryoEM structures under alkaline turnover show progressive depletion of S2B density and distortions of the cluster framework, including elongation of the Fe7-carbide bond from a typical 2.0 Å to over 2.5 Å in one state.<sup>[3](https://www.nature.com/articles/s41467-024-54713-0)</sup> [Selenium](https://www.edgepedia.org/selenium) substitution has also been exploited as a spectroscopic probe: high-energy resolution fluorescence detected X-ray absorption on Se-substituted FeMoco revealed a significant asymmetry in the electronic distribution, consistent with Fe2/Fe6 forming an antiferromagnetically coupled diferric pair and supporting a more localized electronic structure than is typically assumed for iron-sulfur clusters.<sup>[11](https://pubs.acs.org/doi/10.1021/jacs.9b06988)</sup>

## Biosynthesis

FeMoco is assembled stepwise on external scaffolds rather than in situ, involving molecular scaffolds, metallochaperones, radical chemistry, and unique biosynthetic intermediates.<sup>[4](https://pubs.acs.org/doi/10.1021/acs.chemrev.9b00489)</sup> The established flow runs NifU → NifS → NifB → NifEN → MoFe protein: NifU and NifS handle iron-sulfur cluster assembly, NifB builds a precursor, and NifEN, a scaffold structurally homologous to the MoFe protein, receives and matures the cluster before insertion.<sup>[8](https://kdl.kogistate.gov.ng/wp-content/uploads/2024/02/Biological-Inorganic-Chemistry-2nd-Ed.-R.-Crichton-Elsevier-2012.pdf)</sup><sup> • </sup><sup>[9](https://pmc.ncbi.nlm.nih.gov/articles/PMC3077758/)</sup>  Spectroscopic characterization of intermediates suggests that the complete Fe-S core structure is assembled before the incorporation of Mo and homocitrate, rather than coupling two prebuilt sub-clusters.<sup>[9](https://pmc.ncbi.nlm.nih.gov/articles/PMC3077758/)</sup>

Other required factors include NifV, the homocitrate synthase; NifQ, likely involved in the storage or mobilization of molybdenum; and the Fe protein itself, which serves as catalytic electron donor for the MoFe protein and is also crucial for FeMoco maturation.<sup>[9](https://pmc.ncbi.nlm.nih.gov/articles/PMC3077758/)</sup> [In vitro](https://www.edgepedia.org/in-vitro) synthesis of FeMoco from iron, sulfur, molybdenum, and homocitrate using purified proteins has been demonstrated.<sup>[2](https://www.annualreviews.org/content/journals/10.1146/annurev.micro.62.081307.162737)</sup>

## The nitrogenase cofactor family

Nitrogenase occurs in three isoforms defined by the heterometal of the active-site cofactor: FeMoco in Mo-dependent nitrogenase, FeVco in V-dependent nitrogenase, and FeFeco in Fe-only nitrogenase, all following the general composition [MFe₇S₉C-(R)-homocitrate].<sup>[12](https://doi.org/10.1038/s41594-023-01124-2)</sup> The structures of FeMoco, FeVco, and, more recently, FeFeco have been confirmed by [X-ray crystallography](https://www.edgepedia.org/x-ray-crystallography). In FeVco one belt sulfur is replaced by a carbonate, giving a [VFe₇S₈C(CO₃²⁻)(R)-homocitrate] cluster, while the FeFeco appears almost identical to FeMoco except that Mo is replaced by another Fe.<sup>[12](https://doi.org/10.1038/s41594-023-01124-2)</sup><sup> • </sup><sup>[13](https://pmc.ncbi.nlm.nih.gov/articles/PMC10803253/)</sup>

## Electronic structure and computation

FeMoco's electronic structure is a benchmark problem in quantum chemistry because of strong electron correlation among its many metal centers. Active spaces ranging from [54, 54] to [65, 57] orbitals and electrons have been predicted as necessary for a quantitative description.<sup>[14](https://arxiv.org/abs/1805.08746)</sup> This complexity, together with the enzyme's ability to fix nitrogen at ambient conditions where the industrial Haber-[Bosch process](https://www.edgepedia.org/bosch-process) requires high temperatures and pressures, has made FeMoco a target for synthetic model complexes that reproduce aspects of its iron coordination environment.<sup>[3](https://www.nature.com/articles/s41467-024-54713-0)</sup><sup> • </sup><sup>[7](https://pmc.ncbi.nlm.nih.gov/articles/PMC5508211/)</sup>

## References

1. [Analysis of early intermediate states of the nitrogenase reaction by regularization of EPR spectra, Nature Communications (2024)](https://www.nature.com/articles/s41467-024-48271-8)
2. [Rubio LM, Ludden PW (2008). Biosynthesis of the Iron-Molybdenum Cofactor of Nitrogenase. Annual Review of Microbiology 62:93-111](https://www.annualreviews.org/content/journals/10.1146/annurev.micro.62.081307.162737)
3. [Structural evolution of nitrogenase states under alkaline turnover, Nature Communications 15, 10472 (2024)](https://www.nature.com/articles/s41467-024-54713-0)
4. [Biosynthesis of Nitrogenase Cofactors, Chemical Reviews (2020) 120:4921-4968](https://pubs.acs.org/doi/10.1021/acs.chemrev.9b00489)
5. [Structural Enzymology of Nitrogenase Enzymes, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC8606229/)
6. [Nitrogenase FeMoco investigated by spatially resolved anomalous dispersion refinement, Nature Communications (2016)](https://www.nature.com/articles/ncomms10902)
7. [Insight into the FeMoco of nitrogenase from synthetic iron complexes with sulfur, carbon, and hydride ligands, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC5508211/)
8. [Biological Inorganic Chemistry, 2nd ed., R. Crichton, Elsevier (2012)](https://kdl.kogistate.gov.ng/wp-content/uploads/2024/02/Biological-Inorganic-Chemistry-2nd-Ed.-R.-Crichton-Elsevier-2012.pdf)
9. [Biosynthesis of Nitrogenase FeMoco, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC3077758/)
10. [Molecular basis of N2 fixation in a hyperthermophilic archaeon, Nature Communications](https://www.nature.com/articles/s41467-026-77173-0)
11. [Localized Electronic Structure of Nitrogenase FeMoco Revealed by Selenium K-Edge HERFD XAS, JACS (2019)](https://pubs.acs.org/doi/10.1021/jacs.9b06988)
12. [Structural insights into the iron nitrogenase complex, Nature Structural & Molecular Biology (2023)](https://doi.org/10.1038/s41594-023-01124-2)
13. [Structural insights into the iron nitrogenase complex, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC10803253/)
14. [Strong Electron Correlation in Nitrogenase Cofactor, FeMoco, arXiv:1805.08746](https://arxiv.org/abs/1805.08746)

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*Topic: Encyclopedia › Life and health › Biological foundations › Biochemistry and metabolism › Metabolites, cofactors, and biomolecules › Coenzymes and cofactors › Metal and inorganic cofactors › Molybdenum cofactor*

*Initially written Sep 17, 2026 · Reviewed: Oct 21, 2026 · Edited: Oct 15, 2026; Oct 21, 2026 · Last review: Oct 21, 2026*

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