and H2O to be released. According to Hino et al. 2010, the changing charge of the active site causes NO to bind, form N2O and leave the enzyme. The NOR active site is positioned near two hydrogen bound glutamic acids (Glu). The Glu groups provide an electron-negative charge to the active site. The electro-negative charge reduces the reaction potential for heme b3 and allows NO to bind to the binuclear activation site. Glu residues also provide protons needed for removal of N2O and production of H2O. Structure Subunits NOR is made up of two subunits, NorC (small) and NorB (large), with a binuclear iron centre. The binuclear iron center is the active site. It is composed of two b-type hemes and a non-heme iron (FeB). The ligands are connected through a μ-oxo bridge. Histidine (His) residues are attached to the heme b3 in the small subunit. The hydrophilic region of the larger subunit has His and methionine (Met) ligands. Structure is similar to cytochrome oxidases. The active site is conserved between cNOR and qNOR, although differences (ie. heme type) occur between cNOR and qNOR. Folding Enzymatic folding produced 13 alpha-helices (12 from NorB, 1 from NorC) located within and through the membrane. The folded metalloenzyme transverses the membrane. Species distribution Bacteria, archaea and fungi use NOR. qNOR is found in denitrifying bacteria and archaea, as well as pathogenic bacteria not involved in denitrification. Denitrifying fungi reduce NO using P-450nor soluble enzyme. Types Three types of NOR were identified from bacteria: cNOR, qNOR, and qCuNOR. cNOR was found in denitrifying bacteria: Paracoccus denitrificans, Halomonas halodenitrificans, Pseudomonas nautica, Pseudomonas stutzeri, and Pseudomonas aeruginosa. cNOR was first isolated from P. aeruginosa. qNOR was isolated from Geobacillus stearothermophilus. References Further reading EC 1.7.99 Enzymes of known structure