title: Reaction cycle of the dissimilatory sulfite reductase from Archaeoglobus fulgidus.
abstract: A vital process in the biogeochemical sulfur cycle is the dissimilatory sulfate reduction pathway in which sulfate (SO₄⁻²) is converted to hydrogen sulfide (H₂S). Dissimilatory sulfite reductase (dSir), its key enzyme, hosts a unique siroheme-[4Fe-4S] cofactor and catalyzes the six-electron reduction of sulfite (SO₃²⁻) to H₂S. To explore this reaction, we determined the X-ray structures of dSir from the archaeon Archaeoglobus fulgidus in complex with sulfite, sulfide (S²⁻) carbon monoxide (CO), cyanide (CN⁻), nitrite (NO₂⁻), nitrate (NO₃⁻), and phosphate (PO₄³⁻). Activity measurements indicated that dSir of A. fulgidus reduces, besides sulfite and nitrite, thiosulfate (S₂O₃²⁻) and trithionate (S₃O₆²⁻) 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-SO₃²⁻complexes reveal different conformations of Argα170 and Lysα211 both interacting with the sulfite oxygens (its sulfur atom coordinates the siroheme iron), a sulfite rotation of ~60° 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.
authors.affiliation: Max-Planck-Institut für Biophysik, Max-von-Laue-Strasse 3, Frankfurt, Germany.
authors.forename: Kristian
authors.initials: K
authors.lastname: Parey
authors.forename: Eberhard
authors.initials: E
authors.lastname: Warkentin
authors.forename: Peter M H
authors.initials: PM
authors.lastname: Kroneck
authors.forename: Ulrich
authors.initials: U
authors.lastname: Ermler
mesh_terms: D019843:Archaeal Proteins / Q000737:chemistry*; D019611:Archaeoglobus fulgidus / Q000201:enzymology*; D020134:Catalytic Domain; D018360:Crystallography, X-Ray; D004583:Electrons; D050885:Hydrogensulfite Reductase / Q000737:chemistry*; D013329:Structure-Activity Relationship; D013379:Substrate Specificity; D013447:Sulfites / Q000737:chemistry*
chemical_list: D019843:Archaeal Proteins; D013447:Sulfites; D050885:Hydrogensulfite Reductase