Nitrite reductase activity in F₄₂₀-dependent sulphite 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 sulphite reductase (Fsr) in response to exposure to sulphite. This enzyme allows Mj to detoxify sulphite, a potent inhibitor of methyl coenzyme-M reductase (Mcr), by reducing it to sulphide with reduced coenzyme F₄₂₀ (F₄₂₀H₂) as an electron donor; Mcr is essential for energy production for a methanogen. Fsr allows Mj to utilize sulphite as a sulphur source. Nitrite is another potent inhibitor of Mcr and is toxic to methanogens. It is reduced by most sulphite reductases. In this study, we report that MjFsr reduced nitrite to ammonia with F₄₂₀H₂ with physiologically relevant K _m values (nitrite, 8.9 µM; F₄₂₀H₂, 9.7 µM). The enzyme also reduced hydroxylamine with a K _m 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.