Nitrite reductase activity in F420-dependent sulfite reductase (Fsr) from Methanocaldococcus jannaschii

Methanocaldococcus jannaschii (Mj), a hyperthermophilic and evolutionarily deeply rooted methanogenic archaeon from a deep-sea hydrothermal vent, produces F420-dependent sulfite reductase (Fsr) in response to exposure to sulfite. This enzyme allows Mj to detoxify sulfite, a potent inhibitor of methyl coenzyme-M reductase (Mcr), by reducing it to sulfide with reduced coenzyme F420 (F420H2) as electron donor; Mcr is essential for energy production for a methanogen. Fsr allows Mj to utilize sulfite as a sulfur source. Nitrite is another potent inhibitor of Mcr and toxic to methanogens. It is reduced by most sulfite reductases. In this study, we report that MjFsr reduced nitrite to ammonia with F420H2 with physiologically relevant Km values (nitrite, 8.9 μM; F420H2, 9.7 μM). The enzyme also reduced hydroxylamine with a Km value of 112.4 μM, indicating that it was an intermediate in the reduction of nitrite to ammonia. These results open the possibility that Mj could use nitrite as a nitrogen source if it is provided at a low concentration of the type that occurs in its habitat.