Heryakusuma, ChristianJohnson, Eric F.Purwantini, EndangMukhopadhyay, Biswarup2024-01-172024-01-172023-04-202516-8290PMC10202398000482.v3 (PII)https://hdl.handle.net/10919/117388<i>Methanocaldococcus jannaschii</i> (<i>Mj</i>), 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 <i>Mj</i> 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 <i>Mj</i> 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 <i>Mj</i>Fsr reduced nitrite to ammonia with F₄₂₀H₂ with physiologically relevant <i>K</i> _m values (nitrite, 8.9 µM; F₄₂₀H₂, 9.7 µM). The enzyme also reduced hydroxylamine with a <i>K</i> _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 <i>Mj</i> could use nitrite as a nitrogen source if it is provided at a low concentration of the type that occurs in its habitat.application/pdfenCreative Commons Attribution 4.0 InternationalF420-dependent nitrite reductase (FNiR)F420-dependent sulphite reductase (Fsr)F420H2coenzyme F420deazaflavinmethanogenArticle - Refereedhttps://doi.org/10.1099/acmi.0.000482.v354Purwantini, Endang [0000-0002-4113-9995]Mukhopadhyay, Biswarup [0000-0003-0736-0298]372230552516-8290