Nitrogenase Proteins

Fe protein
(electron transfer to MoFe)

MoFe protein
(reduces N2)

Nitrogenase is an enzyme found in certain bacteria that can transform atmospheric dinitrogen into ammonia. It consists of two separate multi-chain proteins. The Fe contains a [4Fe4S] ferredoxin unit that acts as an electron-transfer cofactor to the MoFe protein. The MoFe protein is the site of N2 reduction, and consists of four subunits, containing two sets of metalloclusters. The clusters are:

P Cluster
(electron transfer)

FeMo Cluster
(N2 activation)

The P cluster is bound to the protein by 6 cysteine residues (4 terminal and 2 bridging), so that each Fe atom is tetrahedrally coordinated by sulfur atoms. In effect, this cluster is two [4Fe4S] ferredoxin units that share in common one vertex as a μ6sulfur atom.

The FeMo cluster is bound to the protein through one Fe by a cysteine sulfur and through the Mo by a histidine. There is also a bidentate homocitrate ligand completing octahedral coordination at Mo. The cluster consists of two [4M3S] units bridged by three sulfur atoms. High-end spectroscopy and careful reanalysis of crystallographic diffraction data has revealed a μ6-carbon atom in the centre of this cluster. Click the box to view the carbon:

The mechanistic details of the function of this enzyme -- such as the location of N2 binding, the oxidation states of the metals, the mechanistic steps of N2 reduction -- remain points of intense debate and investigation. In other words, we don't have a clue, really.

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