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enzymology Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. ...
, 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 substrates, and the enzyme converts the substrates into different molecules known as products ...
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 denit ...
, 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 ha ...
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 (), methane ...
into the atmosphere.


Function

N2O is an inorganic metabolite of the prokaryotic 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 funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately fr ...
. 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 that is located in the bacterial periplasm. X-ray structures of the enzymes from ''
Pseudomonas nautica ''Marinobacter hydrocarbonoclasticus'' is a species of bacteria found in sea water which are able to degrade hydrocarbons. The cells are rod-shaped and motile by means of a single polar flagellum.Garrity, George M. (2005). Bergey's manual of s ...
'' and ''
Paracoccus denitrificans ''Paracoccus denitrificans'', is a coccoid bacterium known for its nitrate reducing properties, its ability to replicate under conditions of hypergravity and for being a relative of the eukaryotic mitochondrion (endosymbiotic theory). Descripti ...
'' have revealed that each subunit (MW=65 kDa) is organized into two domains. One cupredoxin-like domain contains a binuclear
copper protein Copper proteins are proteins that contain one or more copper ions as prosthetic groups. Copper proteins are found in all forms of air-breathing life. These proteins are usually associated with electron-transfer with or without the involvement ...
known as CuA. The second domain comprises a 7-bladed propeller of β-sheets that contains the catalytic site called CuZ, 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 CuA and CuZ centers within a single subunit is greater than 30Å, a distance that precludes physiologically relevant rates of intra-subunit electron transfer. However, the two subunits are orientated "head to tail" such that the CuA center in one subunit lies only 10 Å from the CuZ center in the second ensuring that pairs of redox centers in opposite subunits form the catalytically competent unit. The CuA center can undergo a one-electron
redox Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or ...
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 pure ...
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 appli ...
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 simpl ...
, iodide, 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