Fig. 1.

Overview of the biogeochemical cycle of nitrogen. The steps common to the human nitrate-nitrite-NO pathway are highlighted with thicker lines and bold characters. The catalytic centers (d₁ heme and copper T2 center) of the enzymes responsible for the nitrite reduction to NO in those steps are also presented. Denitrification (a "respiratory" pathway), blue arrows; anaerobic ammonium oxidation (AnAmmOx; "respiratory" pathway), grey arrows; "denitrification/intra-aerobic methane oxidation" ("respiratory" pathway that links the nitrogen and carbon cycles), violet arrows; dinitrogen fixation (nitrogen assimilatory pathway), yellow arrow; assimilatory ammonification (nitrogen assimilatory pathway), orange arrows; "organic nitrogen pool", pink arrows; dissimilatory nitrate reduction to ammonium ("respiratory" pathway), green arrows; nitrification and complete ammonium oxidation (ComAmmOx) ("respiratory" pathways), black arrows. Adapted from Ref. [8] with permission.