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Publication Abstract from PubMed

A vital process in the biogeochemical sulfur cycle is the dissimilatory sulfate reduction pathway in which sulfate (SO(2)) is converted to hydrogen sulfide (HS). Dissimilatory sulfite reductase (dSir), its key enzyme, hosts a unique siroheme-[4Fe-4S] cofactor and catalyzes the six-electron reduction of sulfite (SO(2)) to HS. To explore this reaction, we determined the X-ray structures of dSir from the archaeon Archaeoglobus fulgidus in complex with sulfite, sulfide (S(2)) carbon monoxide (CO), cyanide (CN), nitrite (NO), nitrate (NO), and phosphate (PO(3)). Activity measurements indicated that dSir of A. fulgidus reduces, besides sulfite and nitrite, thiosulfate (SO(2)) and trithionate (SO(2)) and produces the latter two compounds besides sulfide. On this basis, a three-step mechanism was proposed, each step consisting of a two-electron transfer, a two-proton uptake, and a dehydration event. In comparison, the related active site structures of the assimilatory sulfite reductase (aSir)- and dSir-SO(2)complexes reveal different conformations of Argalpha170 and Lysalpha211 both interacting with the sulfite oxygens (its sulfur atom coordinates the siroheme iron), a sulfite rotation of ~60 degrees relative to each other, and different access of solvent molecules to the sulfite oxygens from the active site cleft. Therefore, solely in dSir a further sulfite molecule can be placed in van der Waals contact with the siroheme-ligated sulfite or sulfur-oxygen intermediates necessary for forming thiosulfate and trithionate. Although reported for dSir from several sulfate-reducing bacteria, the in vivo relevance of their formation is questionable.

Reaction cycle of the dissimilatory sulfite reductase from Archaeoglobus fulgidus.,Parey K, Warkentin E, Kroneck PM, Ermler U Biochemistry. 2010 Oct 19;49(41):8912-21. PMID:20822098^[1]

From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.