Nitric-oxide reductase

Nitric oxide reductase was assigned Enzyme Commission number (EC) 1.7.2.5. Enzyme Commission numbers are the standard naming system used for enzymes.[5] The EC identifies the class, subclass, sub-subclass, and serial number of the enzyme.[5] Nitric oxide reductase is in Class 1, therefore it is an oxidoreductases.[5]

Figure 1. The Nitrogen Cycle. Nitric oxide (NO) and nitrous oxide (N₂O) are intermediates in the denitrification of nitrate (NO₃⁻) to nitrogen gas (N₂). Nitric oxide reductase reduces NO to N₂O.

Nitric oxide reductase belongs to the family of oxidoreductases, specifically those acting on other nitrogenous compounds as donors with other acceptors. The systematic name of this enzyme class is nitrous-oxide:acceptor oxidoreductase (NO-forming). Other names in common use include nitrogen oxide reductase, and nitrous-oxide:(acceptor) oxidoreductase (NO-forming).

Organisms reduce nitrate (NO₃⁻) to nitrogen gas (N₂) through the process of denitrification, see Figure 1.[1][2] Two important intermediates of the reduction pathway are nitric oxide (NO) and nitrous oxide (N₂O).[1][2] The reducing reaction that transforms NO into N₂O is catalyzed by nitric oxide reductase (NOR).[1][2][3][4]

NO is reduced to N₂O also to prevent cellular toxicity.[4][6] N₂O, a potent greenhouse gas, is released.[1][4]

In enzymology, a nitric oxide reductase (NOR) catalyzes the chemical reaction:

2 NO + 2 e⁻ + 2 H⁺ ${\displaystyle \rightleftharpoons }$ N₂O + H₂O[4]

The enzyme acts on 2 nitric oxide (substrate).[2] The enzyme converts NO, electrons and protons to products: nitrous oxide, and H₂O.[2]

Inputs: 2 molecules of NO, 2 electrons, 2 protons[2]

Outputs: 1 molecule of N₂O, 1 molecule of H₂O[2]

NOR catalyzes the formation of nitrogen to nitrogen (N--N)  bonding.[1][3][6] The conformation changes of the active site and attached ligands (ie. Glu211) allows NO to be positioned in the crowded binuclear center and form N--N bonds.[4]

The precise mechanism of catalysis is still unknown, although hypotheses have been proposed.[3][4]

Cordas et al. 2013 proposes three options: the trans-mechanism, the cis-FeB and the cis-heme b3 mechanisms.[3]

Based on the structure of the enzyme, Shiro 2012 proposes the following mechanism: (1) NO molecules bind at the binuclear center, (2) electrons are transferred from the ferrous irons to the NO, (3) charged NO molecules have the potential to form N to N bonds, and (4) N to O bonds are potentially broken by water, allowing for the N₂O and H₂O to be released.[4]

According to Hino et al. 2010, the changing charge of the active site causes NO to bind, form N₂O and leave the enzyme. The NOR active site is positioned near two hydrogen bound glutamic acids (Glu). The Glu groups provide an electron-negative charge to the active site.[1] The electro-negative charge reduces the reaction potential for heme b3 and allows NO to bind to the binuclear activation site.[1] Glu residues also provide protons needed for removal of N₂O and production of H₂O.[1]

Bacteria, archaea and fungi use NOR.[4][6] qNOR is found in denitrifying bacteria and archaea, as well as pathogenic bacteria not involved in denitrification.[4] Denitrifying fungi reduce NO using P-450nor soluble enzyme.[6]

Three types of NOR were identified from bacteria: cNOR, qNOR, and qCuNOR.[3] cNOR was found in denitrifying bacteria: Paracoccus denitrificans, Halomonas halodenitrificans, Pseudomonas nautica, Pseudomonas stutzeri, and Pseudomonas aeruginosa.[3] cNOR was first isolated from P. aeruginosa.[1][4] qNOR was isolated from Geobacillus stearothermophilus.[4]