Nitric oxide reductase, an enzyme, catalyzes the reduction of nitric oxide (NO) to nitrous oxide (N2O). The enzyme participates in nitrogen metabolism and in the microbial defense against nitric oxide toxicity. The catalyzed reaction may be dependent on different participating small molecules: Cytochrome c (EC: 1.7.2.5, Nitric oxide reductase (cytochrome c)), NADPH (EC:1.7.1.14), or Menaquinone (EC:1.7.5.2). Nomenclature Nitric oxide reductase was assigned Enzyme Commission number (EC) 1.7.2.5. Enzyme Commission numbers are the standard naming system used for enzymes. The EC identifies the class, subclass, sub-subclass, and serial number of the enzyme. Nitric oxide reductase is in Class 1, therefore it is an oxidoreductases. 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). Function Organisms reduce nitrate (NO3−) to nitrogen gas (N2) through the process of denitrification, see Figure 1. Two important intermediates of the reduction pathway are nitric oxide (NO) and nitrous oxide (N2O). The reducing reaction that transforms NO into N2O is catalyzed by nitric oxide reductase (NOR). NO is reduced to N2O also to prevent cellular toxicity. N2O, a potent greenhouse gas, is released. Reaction In enzymology, a nitric oxide reductase (NOR) catalyzes the chemical reaction: 2 NO + 2 e− + 2 H+ N2O + H2O The enzyme acts on 2 nitric oxide (substrate). The enzyme converts NO, electrons and protons to products: nitrous oxide, and H2O. Inputs: 2 molecules of NO, 2 electrons, 2 protons Outputs: 1 molecule of N2O, 1 molecule of H2O Mechanism NOR catalyzes the formation of nitrogen to nitrogen (N--N) bonding. 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. The precise mechanism of catalysis is still unknown, although hypotheses have been proposed. Cordas et al. 2013 proposes three options: the trans-mechanism, the cis-FeB and the cis-heme b3 mechanisms. 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