In enzymology, a nitrous oxide reductase also known as nitrogen:acceptor oxidoreductase (N<sub>2</sub>O-forming) is an enzyme that catalyzes the final step in bacterial denitrification, the reduction of nitrous oxide to dinitrogen.
It plays a critical role in preventing release of a potent greenhouse gas into the atmosphere.
N<sub>2</sub>O is an inorganic metabolite of the prokaryotic cell during denitrification. Thus, denitrifiers comprise the principal group of N<sub>2</sub>O producers, with roles played also by nitrifiers, methanotrophic bacteria, and fungi. Among them, only denitrifying prokaryotes have the ability to convert N<sub>2</sub>O to N<sub>2</sub>. Conversion of N<sub>2</sub>O into N<sub>2</sub> is the last step of a complete nitrate denitrification process and is an autonomous form of respiration. N<sub>2</sub>O is generated in the denitrifying cell by the activity of respiratory NO reductase. Some microbial communities only have the capability of N<sub>2</sub>O reduction to N<sub>2</sub> and do not possess the other denitrification pathways. Such communities are known as nitrous oxide reducers. Some denitrifiers do not have complete denitrification with end product N<sub>2</sub>O
Nitrous-oxide reductase is a homodimer that is located in the bacterial periplasm. X-ray structures of the enzymes from Pseudomonas nautica and Paracoccus denitrificans have revealed that each subunit (MW=65 kDa) is organized into two domains. One cupredoxin-like domain contains a binuclear copper protein known as Cu<sub>A</sub>.
The second domain comprises a 7-bladed propeller of ò-sheets that contains the catalytic site called Cu<sub>Z</sub>, which is a tetranuclear copper-sulfide cluster. The distance between the Cu<sub>A</sub> and Cu<sub>Z</sub> centers within a single subunit is greater than 30à, a distance that precludes physiologically relevant rates of intra-subunit electron transfer. However, the two subunits are orientated "head to tail" such that the Cu<sub>A</sub> center in one subunit lies only 10 àfrom the Cu<sub>Z</sub> center in the second ensuring that pairs of redox centers in opposite subunits form the catalytically competent unit. The Cu<sub>A</sub> center can undergo a one-electron redox change and hence has a function similar to that in the well-known aa<sub>3</sub>-type cytochrome c oxidases () where it serves to receive an electron from soluble cytochromes c.
Acetylene is the most specific inhibitor of nitrous-oxide reductase. Other inhibitors include azide anion, thiocyanate, carbon monoxide, iodide, and cyanide.