The sulphur oxygenase/reductase (SOR) of the thermo-acidophilic archaeon Acidianus ambivalens is an unusual enzyme consisting of 24 identical subunits arranged in a perfectly symmetrical hollow sphere and containing a mononuclear non-heme iron centre (personal communication: A. Kletzin). At 85 degrees C in vitro, elemental sulphur is oxidised to sulphite, thiosulphate and hydrogen sulphide with no external cofactors needed. The proposed equation is: 4S + O2 + 4 H2O ---> 2 HSO3- + 2 H2S + 2 H+.

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4: The sulphur oxygenase reductase from Acidianus ambivalens is a multimeric protein containing a low-potential mononuclear non-haem iron centre. Urich T, Bandeiras TM, Leal SS, Rachel R, Albrecht T, Zimmermann P, Scholz C, Teixeira M, Gomes CM, Kletzin A; Biochem J 2004; [Epub ahead of print] PMID:15030315