The number of moles of Mg-ATP needed for the reduction of one mole of nitrogen by nitrogenase enzyme is

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Correct option is B

Nitrogenases are enzymes that are produced by certain bacteria, such as cyanobacteria (blue-green bacteria) and rhizobacteria. These enzymes are responsible for the reduction of nitrogen (N₂) to ammonia (NH₃). Nitrogenases are the only family of enzymes known to catalyze this reaction, which is a step in the process of nitrogen fixation. Nitrogen fixation is required for all forms of life, with nitrogen being essential for the biosynthesis of molecules (nucleotides, amino acids) that create plants, animals and other organisms.

A usual assembly consists of two components:

1.The homodimeric Fe-only protein, the reductase which has a high reducing power and is responsible for a supply of electrons.

2.The heterotetrameric MoFe protein, a nitrogenase which uses the electrons provided to reduce N₂ to NH₃. In some assemblies it is replaced by a homologous alternative.

Nitrogenase is an enzyme responsible for catalyzing nitrogen fixation, which is the reduction of nitrogen (N₂) to ammonia (NH₃) and a process vital to sustaining life on Earth. There are three types of nitrogenase found in various nitrogen-fixing bacteria: molybdenum (Mo) nitrogenase, vanadium (V) nitrogenase, and iron-only (Fe) nitrogenase. Molybdenum nitrogenase, which can be found in diazotrophs such as legume-associated rhizobia, is the nitrogenase that has been studied the most extensively and thus is the most well characterized. Vanadium nitrogenase and iron-only nitrogenase can both be found in select species of Azotobacter as an alternative nitrogenase. Equations 1 and 2 show the balanced reactions of nitrogen fixation in molybdenum nitrogenase and vanadium nitrogenase respectively.

The transfer of electrons requires an input of chemical energy which comes from the binding and hydrolysis of ATP. The hydrolysis of ATP also causes a conformational change within the nitrogenase complex.

Information Booster

The MoFe protein is a heterotetramer consisting of two α subunits and two β subunits, with a mass of approximately 240-250kDa. The MoFe protein also contains two iron–sulfur clusters, known as P-clusters, located at the interface between the α and β subunits and two FeMo cofactors, within the α subunits. The oxidation state of Mo in these nitrogenases was formerly thought Mo(V), but more recent evidence is for Mo(III). (Molybdenum in other enzymes is generally bound to molybdopterin as fully oxidized Mo(VI)).

The core (Fe₈S₇) of the P-cluster takes the form of two [Fe₄S₃] cubes linked by a central sulfur atom. Each P-cluster is linked to the MoFe protein by six cysteine residues.

​Each FeMo cofactor (Fe₇MoS₉C) consists of two non-identical clusters: [Fe₄S₃] and [MoFe₃S₃], which are linked by three sulfide ions. Each FeMo cofactor is covalently linked to the α subunit of the protein by one cysteine residue and one histidine residue.

Electrons from the Fe protein enter the MoFe protein at the P-clusters, which then transfer the electrons to the FeMo cofactors. Each FeMo cofactor then acts as a site for nitrogen fixation, with N₂ binding in the central cavity of the cofactor.