The structure of the F420-dependent sulfite-detoxifying enzyme from Methanococcales reveals a prototypical sulfite-reductase with assimilatory traits

The coenzyme F 420 -dependent sulfite reductase (Fsr group I) protects hydrogenotrophic methanogens, one of the main contributors in worldwide methane emission, from toxic sulfite. Fsr is a single peptide composed of a F 420 H 2 -oxidase and a novel class of sulfite reductase. Both catalytic domains have been proposed to be the ancestors of modern F 420 -oxido/reductases and dissimilatory/assimilatory sulfite reductases. Here, we describe the X-ray crystal structures of Fsr natively isolated from Methanocaldococcus jannaschii ( Mj Fsr) and Methanothermococcus thermolithotrophicus ( Mt Fsr), respectively refined to 2.30 Å and 1.55 Å resolution. In both organisms, Fsr oligomerizes as a 280-kDa homotetramer, where each siroheme–[4Fe–4S] is catalytically active, in contrast to dissimilatory homologues. The siroheme–[4Fe–4S], embedded in the sulfite reductase domain, is electronically connected to the flavin in the F 420 H 2 -oxidase domain by five [4Fe–4S]-clusters. EPR spectroscopy determined the redox potentials of these [4Fe–4S] 2+/1+ clusters (−435 to -275 mV), through which electrons flow from FAD to the siroheme–[4Fe–4S] 2+/1+ (siroheme, -114 mV; [4Fe–4S] -445 mV). The electron relay is mainly organized by two inserted ferredoxin modules, which stabilize the higher degree of oligomerization. While the F 420 H 2 -oxidase part is similar to the β-subunit of F 420 -reducing hydrogenases, the sulfite reductase domain is structurally analogous to dissimilatory sulfite reductases, whereas its siroheme–[4Fe–4S] cofactor is bound in the same way as in assimilatory ones. Accordingly, the reaction of Mt Fsr is unidirectional, reducing sulfite or nitrite with F 420 H 2 . Our results provide the first structural insights into this unique fusion, a snapshot of a primitive sulfite reductase that turns a poison into an elementary block of Life.