Cytochrome ''c'' nitrite reductase (ccNiR) () is a bacterial

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. A ...

that

catalyzes Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...

the six electron reduction of

nitrite The nitrite polyatomic ion, ion has the chemical formula . Nitrite (mostly sodium nitrite) is widely used throughout chemical and pharmaceutical industries. The nitrite anion is a pervasive intermediate in the nitrogen cycle in nature. The name ...

ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous was ...

; an important step in the biological

nitrogen cycle The nitrogen cycle is the biogeochemical cycle by which nitrogen is converted into multiple chemical forms as it circulates among atmospheric, terrestrial, and marine ecosystems. The conversion of nitrogen can be carried out through both biologi ...

. The enzyme catalyses the second step in the two step conversion of nitrate to ammonia, which allows certain bacteria to use nitrite as a terminal electron acceptor, rather than oxygen, during anaerobic conditions. During this process, ccNiR draws electrons from the quinol pool, which are ultimately provided by a dehydrogenase such as

formate dehydrogenase Formate dehydrogenases are a set of enzymes that catalyse the oxidation of formate to carbon dioxide, donating the electrons to a second substrate, such as NAD+ in formate:NAD+ oxidoreductase () or to a cytochrome in formate:ferricytochrome-b1 o ...

hydrogenase A hydrogenase is an enzyme that catalyses the reversible oxidation of molecular hydrogen (H2), as shown below: Hydrogen uptake () is coupled to the reduction of electron acceptors such as oxygen, nitrate, sulfate, carbon dioxide (), and fumara ...

. These dehydrogenases are responsible for generating a proton motive force. Cytochrome ''c'' Nitrite Reductase is a homodimer which contains five c-type heme cofactors per monomer. Four of the heme centers are bis-histidine ligated and presumably serve to shuttle electrons to the active site. The active site heme, however, is uniquely ligated by a single lysine residue. This enzyme belongs to the family of

oxidoreductase In biochemistry, an oxidoreductase is an enzyme that catalyzes the transfer of electrons from one molecule, the reductant, also called the electron donor, to another, the oxidant, also called the electron acceptor. This group of enzymes usually ut ...

s, specifically those acting on other nitrogenous compounds as donors with a cytochrome as acceptor. The systematic name of this enzyme class is ammonia:ferricytochrome-c oxidoreductase.

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Further reading