enzymology An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...

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

enzyme An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...

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 factitious air, among others, is a chemical compound, an Nitrogen oxide, oxide of nitrogen with the Chemical formula, formula . At room te ...

dinitrogen Nitrogen is a chemical element; it has symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at seventh ...

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

greenhouse gas Greenhouse gases (GHGs) are the gases in the atmosphere that raise the surface temperature of planets such as the Earth. Unlike other gases, greenhouse gases absorb the radiations that a planet emits, resulting in the greenhouse effect. T ...

into the atmosphere.

Function

N₂O is an inorganic metabolite of the

prokaryotic A prokaryote (; less commonly spelled procaryote) is a single-celled organism whose cell lacks a nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Ancient Greek (), meaning 'before', and (), meaning 'nut' ...

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

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

. 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 only have the capability of N₂O reduction to N₂ 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 N₂O

Structure

Nitrous-oxide reductase is a

homodimer In biochemistry, a protein dimer is a macromolecular complex or protein multimer, multimer formed by two protein monomers, or single proteins, which are usually Non-covalent interaction, non-covalently bound. Many macromolecules, such as proteins ...

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_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.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, ion, or molecule, to another such chemical entity. ET describes the mechanism by which electrons are transferred in redox reactions. Electrochemical processes are ET reactio ...

. 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 or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is t ...

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 (Chemical nomenclature, systematic name: ethyne) is a 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 u ...

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 Thiocyanates are salts containing the thiocyanate anion (also known as rhodanide or rhodanate). is the conjugate base of thiocyanic acid. Common salts include the colourless salts potassium thiocyanate and sodium thiocyanate. Mercury(II) t ...

carbon monoxide Carbon monoxide (chemical formula CO) is a poisonous, flammable gas that is colorless, odorless, tasteless, and 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 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 deficiency ...

, and

cyanide In chemistry, cyanide () is an inorganic chemical compound that contains a functional group. This group, known as the cyano group, consists of a carbon atom triple-bonded to a nitrogen atom. Ionic cyanides contain the cyanide anion . This a ...

References

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