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Nitrate reductase enzymes are a group of enzymes that reduce [nitrate]] (NO3) to nitrite (NO2).

Bacteria can be differentiated based on whether they have nitrate reductases through the application of the nitrate reductase test.

Catalytic mechanism

The nitrate molecule binds to the active site with the molybdenum ion in the +6 oxidation state. Electron transfer to the active site occurs only in the proton-electron transfer stage, where the molybdenum(V) species plays an important role in catalysis. The presence of the sulfur atom in the molybdenum coordination sphere creates a pseudo-dithiolene ligand that protects it from any direct attack from the solvent. Upon the nitrate binding there is a conformational rearrangement of this ring that allows the direct contact of the nitrate with molybdenum(VI) ion. This rearrangement is stabilized by the conserved methionines Met141 and Met308. The reduction of nitrate into nitrite occurs only in the second step of the mechanism where the two dimethyl-dithiolene ligands have a key role in spreading the excess of negative charge near the molybdenum atom to make it available for the chemical reaction. The reaction involves the oxidation of the sulfur atoms and not of the molybdenum as previously suggested. The mechanism involves a molybdenum and sulfur-based redox chemistry instead of the currently accepted redox chemistry based only on the molybdenum ion. The second part of the mechanism involves two protonation steps that are promoted by the presence of molybdenum(V) species. Molybdenum(VI) intermediates might also be present in this stage depending on the availability of protons and electrons. Once the water molecule is generated only the molybdenum(VI) species allow water molecule dissociation, and, the concomitant enzymatic turnover[1].


  1. ^ Cerqueira, N M F S A · Gonzalez, P J · Brondino, C D · Romão, M J · Romão, C C · Moura, I · Moura, J J G, The effect of the sixth sulfur ligand in the catalytic mechanism of periplasmic nitrate reductase., Journal of Computational Chemistry, 2009 vol. 30 (15) pp. 2466-84

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