In enzymology, a nitrous oxide reductase also known as nitrogen:acceptor oxidoreductase (N₂O-forming) is an

enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different molecule ...

that catalyzes the final step in bacterial

denitrification Denitrification is a microbially facilitated process where nitrate (NO3−) is reduced and ultimately produces molecular nitrogen (N2) through a series of intermediate gaseous nitrogen oxide products. Facultative anaerobic bacteria perform denitr ...

, the reduction of

nitrous oxide Nitrous oxide (dinitrogen oxide or dinitrogen monoxide), commonly known as laughing gas, nitrous, or nos, is a chemical compound, an oxide of nitrogen with the formula . At room temperature, it is a colourless non-flammable gas, and has ...

to dinitrogen. : N₂O + 2 reduced cytochome ''c'' N₂ + H₂O + 2 cytochrome ''c'' It plays a critical role in preventing release of a potent

greenhouse gas A greenhouse gas (GHG or GhG) is a gas that absorbs and emits radiant energy within the thermal infrared range, causing the greenhouse effect. The primary greenhouse gases in Earth's atmosphere are water vapor (), carbon dioxide (), met ...

into the atmosphere.

Function

N₂O is an inorganic metabolite of the

prokaryotic A prokaryote () is a single-celled organism that lacks a nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Greek πρό (, 'before') and κάρυον (, 'nut' or 'kernel').Campbell, N. "Biology:Concepts & Connec ...

cell during denitrification. Thus, denitrifiers comprise the principal group of N₂O producers, with roles played also by nitrifiers, methanotrophic bacteria, and

fungi A fungus (plural, : fungi or funguses) is any member of the group of Eukaryote, eukaryotic organisms that includes microorganisms such as yeasts and Mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified ...

. Among them, only denitrifying prokaryotes have the ability to convert N₂O to N₂. Conversion of N₂O into N₂ is the last step of a complete nitrate denitrification process and is an autonomous form of respiration. N₂O is generated in the denitrifying cell by the activity of respiratory NO reductase. Some microbial communities have only capability of N₂O reduction to N₂ and does not have the other denitrification pathways such communities are known as nitrous oxide reducers. Some denitrifiers do not have complete denitrification with end product N₂O

Structure

Nitrous-oxide reductase is a

homodimer In biochemistry, a protein dimer is a macromolecular complex formed by two protein monomers, or single proteins, which are usually non-covalently bound. Many macromolecules, such as proteins or nucleic acids, form dimers. The word ''dimer'' has ...

that is located in the bacterial periplasm. X-ray structures of the enzymes from '' Pseudomonas nautica'' and ''

Paracoccus denitrificans ''Paracoccus denitrificans'', is a coccoid Coccoid means shaped like or resembling a coccus, that is, spherical.The noun coccoid or coccoids may refer to: * a level of organization, characterized by unicellular, non-flagellated, non-amoeboid ...

'' 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_A. The second domain comprises a 7-bladed propeller of β-sheets that contains the catalytic site called Cu_Z, which is a tetranuclear copper-sulfide

cluster may refer to: Science and technology Astronomy * Cluster (spacecraft), constellation of four European Space Agency spacecraft * Asteroid cluster, a small asteroid family * Cluster II (spacecraft), a European Space Agency mission to study th ...

.Pomowski, A., Zumft, W. G., Kroneck, P. M. H., Einsle, O., "N2O binding at a sqbCu:2S copper-sulphur cluster in nitrous oxide reductase", Nature 2011, 477, 234. The distance between the Cu_A and Cu_Z centers within a single subunit is greater than 30Å, a distance that precludes physiologically relevant rates of intra-subunit

electron transfer Electron transfer (ET) occurs when an electron relocates from an atom or molecule to another such chemical entity. ET is a mechanistic description of certain kinds of redox reactions involving transfer of electrons. Electrochemical processes ar ...

. However, the two subunits are orientated "head to tail" such that the Cu_A center in one subunit lies only 10 Å from the Cu_Z center in the second ensuring that pairs of redox centers in opposite subunits form the catalytically competent unit. The Cu_A center can undergo a one-electron

redox Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate (chemistry), substrate change. Oxidation is the loss of Electron, electrons or an increase in the oxidation state, while reduction ...

change and hence has a function similar to that in the well-known aa₃-type cytochrome ''c'' oxidases () where it serves to receive an electron from soluble cytochromes ''c''.

Inhibitors

Acetylene Acetylene ( systematic name: ethyne) is the chemical compound with the formula and structure . It is a hydrocarbon and the simplest alkyne. This colorless gas is widely used as a fuel and a chemical building block. It is unstable in its pur ...

is the most specific inhibitor of nitrous-oxide reductase. Other inhibitors include

azide In chemistry, azide is a linear, polyatomic anion with the formula and structure . It is the conjugate base of hydrazoic acid . Organic azides are organic compounds with the formula , containing the azide functional group. The dominant ...

anion, thiocyanate,

carbon monoxide Carbon monoxide ( chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the si ...

iodide An iodide ion is the ion I−. Compounds with iodine in formal oxidation state −1 are called iodides. In everyday life, iodide is most commonly encountered as a component of iodized salt, which many governments mandate. Worldwide, iodine def ...

, and

cyanide Cyanide is a naturally occurring, rapidly acting, toxic chemical that can exist in many different forms. In chemistry, a cyanide () is a chemical compound that contains a functional group. This group, known as the cyano group, consists of ...

References

{{Portal bar, Biology, border=no EC 1.7.2 Copper enzymes Enzymes of known structure