Nitrite reductase (NiR)

Nitrite reductase refers to a family of enzymes central to the global nitrogen cycle, catalyzing the reduction of nitrite (NO₂⁻) to either ammonia (NH₃) or nitric oxide (NO), depending on the specific enzyme and organism. There are several types, most notably copper-containing nitrite reductases (CuNIR, common in bacteria and fungi) and cytochrome c nitrite reductases (ccNiR, multiheme, found in various bacteria). These enzymes are essential in both dissimilatory denitrification (energy production in bacteria under anaerobic conditions, producing gaseous nitrogen species) and assimilatory processes (production of ammonia for biosynthesis in plants/microbes). Nitrite reductases are complex, highly conserved proteins, often existing as multimers (e.g., hexamers for CuNIR) with active sites that utilize copper ions or heme groups for electron transfer and catalysis. While not a direct therapeutic target in human medicine, there is growing interest in inhibiting bacterial nitrite reductases as an anti-infective strategy, due to their role in bacterial survival under oxygen-limited or immune-evasive environments. Their environmental impact is substantial, as they are key regulators of nitrogen loss and greenhouse gas formation, affecting both agriculture and climate.

Other names

Cytochrome c nitrite reductaseCopper-based nitrite reductaseCuNIR (for copper-containing)ccNiR (for cytochrome c)Ferredoxin—nitrite reductase (NiR; plant version)Nitrite oxidoreductase (related, but distinct function)

Catalyzes the reduction of nitrite (NO₂⁻) to either nitric oxide (NO) or ammonia (NH₃) depending on type and organism. Electron transfer from donor proteins (e.g., cytochrome c, pseudoazurin, ferredoxin) to the active site metal center (copper or heme)—enabling substrate reduction. Central to the denitrification step in bacterial and environmental nitrogen cycling