Structures of the sulfite detoxifying F 420 -dependent enzyme from Methanococcales.
Jespersen, M., Pierik, A.J., Wagner, T.(2023) Nat Chem Biol 
- PubMed: 36658338 
- DOI: https://doi.org/10.1038/s41589-022-01232-y
- Primary Citation of Related Structures:  
7NP8, 7NPA - PubMed Abstract: 
Methanogenic archaea are main actors in the carbon cycle but are sensitive to reactive sulfite. Some methanogens use a sulfite detoxification system that combines an F 420 H 2 -oxidase with a sulfite reductase, both of which are proposed precursors of modern enzymes. Here, we present snapshots of this coupled system, named coenzyme F 420 -dependent sulfite reductase (Group I Fsr), obtained from two marine methanogens. Fsr organizes as a homotetramer, harboring an intertwined six-[4Fe-4S] cluster relay characterized by spectroscopy. The wire, spanning 5.4 nm, electronically connects the flavin to the siroheme center. Despite a structural architecture similar to dissimilatory sulfite reductases, Fsr shows a siroheme coordination and a reaction mechanism identical to assimilatory sulfite reductases. Accordingly, the reaction of Fsr is unidirectional, reducing sulfite or nitrite with F 420 H 2 . Our results provide structural insights into this unique fusion, in which a primitive sulfite reductase turns a poison into an elementary block of life.
Organizational Affiliation: 
Max Planck Institute for Marine Microbiology, Bremen, Germany.