Mechanism of tetravalent manganese reduction with elemental sulfur by Thiobacillus ferrooxidans.

Abstract

Washed intact cells of iron-grown T. ferrooxidans AP19-3 could reduce manganese dioxide (MnO2) enzymatically with elemental sulfur (S0) and the mechanism of Mn4+ reduction was studied. The optimum pH for Mn4+ reduction was 2.5. The amount of Mn4+ reduced by the strain was proportional to the amount of S0 added to the reaction mixture. An enzyme that directly catalyzes the reduction of Mn4+ to Mn2+ with S0 was not found in the cell-free extract of this strain. T. ferrooxidans AP19-3 was found to possess sulfur: ferric ion oxidoreductase (SFORase), which catalyzes the reduction of Fe3+ with S0 to give Fe2+ and sulfite. The reduction of Mn4+ was inhibited by a specific inhibitor of SFORase or diamide, strongly suggesting that SFORase is involved in the Mn4+ reduction in this strain. The involvement of SFORase in Mn4+ reduction was further supported by the following results. Both the amounts of Fe2+ and sulfite produced by SFORase were markedly reduced by MnO2. The amount of Mn4+ reduced increased 5.4-fold with 1 mM ferric ion and 10-fold with 2.5 mM cyanide or an inhibitor of iron oxidase, respectively. The sulfite and Fe2+ were chemically oxidized by MnO2 instantly. From the results, it is concluded that the Mn4+ reduction with S0 in this strain occurs through two steps. In the first step S0 is enzymatically oxidized by SFORase to give Fe2+ and sulfite, and in the second step these two reduced compounds are chemically reduced by MnO2 to give Mn2+