Coenzyme F 420 -dependent sulfite reductase (Fsr) protects methanogenic archaea inhabiting deep-sea hydrothermal vents from the inactivation of methyl coenzyme M reductase (Mcr), one of their essential energy production enzymes. Anaerobic methanotrophic archaea (ANME) that oxidize methane and rely on Mcr, carry Fsr homologs that form a distinct clade.

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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.

No abstract

Methanocaldococcus jannaschii ( Mj ), a hyperthermophilic and evolutionarily deeply rooted methanogenic archaeon from a deep-sea hydrothermal vent, produces F 420 -dependent sulfite reductase (Fsr). This enzyme allows Mj to detoxify sulfite, a potent inhibitor of methyl coenzyme-M reductase (Mcr), by reducing it to sulfide with reduced coenzyme F 420 (F 420 H 2 ) as electron donor, and to utilize this oxyanion as a sulfur source; Mcr is essential for energy production for a methanogen. Nitrite is another potent inhibitor of Mcr and toxic to methanogens. It is reduced by most sulfite reductases. In this study, we report that Mj Fsr reduced nitrite to ammonia with F 420 H 2 and exhibiting physiologically relevant K m values (nitrite, 8.9 μM; F 420 H 2 , 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 occur in its habitat.