Figure 1.

Nitrogen cycle: Enzymatic NOS-independent synthesis. The simplified sequence of the nitrogen cycle reactions is shown. The reactions of nitrification and denitrification are associated with electron transport systems and coupled to cellular energetics and ATP production. NH₃ is widely used in the synthesis of various amine-containing biomolecules and, therefore, the NOS catalyzed oxidation of L-arginine can be considered as a heterotrophic nitrification route, in addition to bacterial autotrophic nitrification. Nitrification and NO synthesis by NOS are aerobic pathways, where molecular oxygen is required for the oxidation of L-arginine and NH₃. In contrast, denitrification usually occurs under either anaerobic or hypoxic conditions. Additionally, nitrates can be reversibly reduced to ammonia in a nitrate/nitrite type of anaerobic respiration (see (150, 151, 464, 465)) for a review of these and associated pathways).