National Center for
3MMA: Dissimilatory sulfite reductase phosphate complex
Biochemistry (2010) 49 p.8912-8921
A vital process in the biogeochemical sulfur cycle is the dissimilatory sulfate reduction pathway in which sulfate (SO(4)(-)(2)) is converted to hydrogen sulfide (H(2)S). Dissimilatory sulfite reductase (dSir), its key enzyme, hosts a unique siroheme-[4Fe-4S] cofactor and catalyzes the six-electron reduction of sulfite (SO(3)(2)(-)) to H(2)S. 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(2)(-)), nitrate (NO(3)(-)), and phosphate (PO(4)(3)(-)). Activity measurements indicated that dSir of A. fulgidus reduces, besides sulfite and nitrite, thiosulfate (S(2)O(3)(2)(-)) and trithionate (S(3)O(6)(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(3)(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.