Definition
FeMoco (iron‑molybdenum cofactor), also known as the M‑cluster, is the primary metallocofactor of the enzyme nitrogenase. It is the catalytic site where atmospheric dinitrogen (N₂) is reduced to ammonia (NH₃) during biological nitrogen fixation.
Overview
Nitrogenase consists of two component proteins: the MoFe protein (catalytic component) and the Fe protein (electron‑donor component). FeMoco is embedded in the active site of the MoFe protein. The cofactor enables the transfer of electrons and protons to N₂, facilitating its conversion to NH₃ under ambient temperature and pressure—processes that are energetically demanding in synthetic chemistry. FeMoco was first resolved by X‑ray crystallography in the early 1990s, and subsequent spectroscopic studies have elucidated its composition and electronic structure.
Etymology / Origin
The name “FeMoco” is a contraction of “Fe” (iron) and “Mo” (molybdenum) plus “co” for cofactor, reflecting its composition of iron and molybdenum atoms within a complex metal‑sulfur cluster. The term appears in the primary literature on nitrogenase biochemistry and is widely used in both biochemistry and bioinorganic chemistry.
Characteristics
- Composition: Fe₇MoS₉C. The cluster can be regarded as two subunits—a [Fe₄S₃] fragment and a [MoFe₃S₃] fragment—linked by three sulfide bridges and a central interstitial carbide (C⁴⁻).
- Structure: The six peripheral Fe atoms form a trigonal prismatic arrangement around the carbide, while the unique “home” Fe is ligated to a cysteine residue. The molybdenum atom is coordinated by three sulfides, a homocitrate ligand, and the imidazole side chain of a histidine residue, giving it octahedral geometry.
- Electronic properties: In the resting state the cofactor exhibits an S = 3/2 spin state; reduction to the EPR‑silent state occurs upon one‑electron addition. Formal oxidation states are commonly approximated as Mo^IV‑2Fe^II‑5Fe^III‑C⁴⁻, although precise assignments remain a topic of ongoing research.
- Biosynthesis: Assembly of FeMoco is a multistep pathway involving several nif gene products (e.g., NifS, NifU, NifB, NifE, NifN, NifV). Initial Fe‑S fragments are generated by NifS/U, merged on the NifB scaffold into an L‑cluster, transferred to NifEN for further remodeling, and finally inserted into the MoFe protein. The interstitial carbide originates from a methyl group donated by S‑adenosyl‑L‑methionine (SAM) during NifB‑mediated radical chemistry.
- Function: The exact site of N₂ binding is not definitively established, but structural and spectroscopic data suggest that Fe atoms adjacent to the carbide, and possibly the Mo center, participate in substrate activation. Binding of CO, an N₂ analogue, has been observed at Fe‑Fe edges of the cluster, providing insight into potential reaction pathways.
Related Topics
- Nitrogenase – the enzyme complex that catalyzes biological nitrogen fixation.
- Molybdenum‑iron protein (MoFe protein) – the catalytic component housing FeMoco.
- Fe protein – the electron‑donor partner of nitrogenase.
- Homocitrate – a ligand bound to the Mo atom within FeMoco.
- Nif genes – the suite of genes encoding proteins required for FeMoco biosynthesis and nitrogenase assembly.
- Metalloclusters – other biologically important metal‑sulfur clusters such as the Fe‑S clusters in ferredoxins and the H‑cluster of [FeFe] hydrogenases.
All information is derived from peer‑reviewed literature and the Wikipedia entry on FeMoco, which cites primary structural, spectroscopic, and biochemical studies.