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Nitrogenase (EC 18.104.22.168)  is the enzyme system responsible for biological
nitrogen fixation. Nitrogenase is an oligomeric complex which consists of two
components: component 2 is an homodimer of an iron-sulfur protein, while
component 1 which contains the active site for the reduction of nitrogen to
ammonia exists in three different forms:
A molybdenum-iron containing protein (MoFe). The MoFe protein is a hetero-
tetramer consisting of two pairs of α (nifD) and β (nifK) subunits.
A vanadium-iron containing protein (VFe). The VFe protein is a hexamer of
two pairs each of α (vnfD), β (vnfK), and delta (vnfG) subunits.
The third form of component 1 seems to only contain iron. Like the vanadium
form it is a hexamer composed of α (anfD), β (anfK), and delta
The α and β chains of the three types of component 1 are evolutionary
related and they are also related to proteins nifE and nifN, which are most
probably involved in the iron-molybdenum cofactor biosynthesis .
We selected as signature patterns for this family of proteins two stretches of
residues which are located in the N-terminal section and which each contain a
conserved cysteine thought to be one of the ligands for the metal-sulfur
May 2004 / Text revised.
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