3.D.4.10.1
Nitric oxide reductase (EC #1.7.99.7) (NorBC) (component of the anerobic, respiratory chain that converts NO₃^- to N₂; denitrification) [reaction catalyzed by Nor: 2 nitric oxide (NO) + 2e^- + 2H⁺ → nitrous oxide (N₂0) + H₂O].  This enzyme does not pump protons across the bacterial membrane (Reimann et al. 2007), but the protons needed for the reaction are taken from the periplasmic side of the membrane (from which side the electrons are donated). P. denitrificans NOR uses a single defined proton pathway with residues Glu-58 and Lys-54 from the NorC subunit at the entrance (ter Beek et al. 2016).  norC and norB encode the cytochrome-c-containing subunit II and cytochrome b-containing subunit I of nitric-oxide reductase (NO reductase), respectively. norQ encodes a protein with an ATP-binding motif and is similar to NirQ from Pseudomonas stutzeri and Pseudomonas aeruginosa and CbbQ from Pseudomonas hydrogenothermophila. norE encodes a protein with five putative transmembrane alpha-helices and has similarity to CoxIII, the third subunit of the aa3-type cytochrome-c oxidases. norF encodes a small protein with two putative transmembrane alpha-helices. Mutagenesis of norC, norB, norQ or norD resulted in cells unable to grow anaerobically. Nitrite reductase and NO reductase (with succinate or ascorbate as substrates) and nitrous oxide reductase (with succinate as substrate) activities were not detected in these mutant strains. Nitrite extrusion was detected in the medium, indicating that nitrate reductase was active. The norQ and norD mutant strains retained about 16% and 23% of the wild-type level of NorC, respectively. The norE and norF mutant strains had specific growth rates and NorC contents similar to those of the wild-type strain, but had reduced NOR and NIR activities, indicating that their gene products are involved in regulation of enzyme activity (de Boer et al. 1996).