Evidence on the presence of two distinct enzymes responsible for the reduction of selenate and tellurite in Bacillus sp. STG-83

Bacillus sp. STG-83 that was isolated from Neidasht spring – north of Iran – is a gram positive faculative anaerobic rod which is capable of reducing selenate anion to elemental selenium. This bacterium is also able to catalyze the reduction of tellurite. The chromatographic behavior demonstrated that the reduction of tellurite and selenate in Bacillus sp. STG-83 was carried out by two distinct enzymes. Both enzymes were purified by the same purification procedure. The purification process involved (NH4)2SO4 precipitation, followed by phenyl-Sepharose and Sephacryl S-200 chromatography. Using gel filtration, the relative molecular mass of the native selenate reductase and tellurite reductase was determined to be 182 kDa and 197 kDa, respectively. Values of 2.7 μmol min−1 mg−1 and 3.24 mM were obtained for the Vmax and Km of selenate reductase, respectively. The Km for selenite reduction was determined to be 10.9 mM and the Vmax was 1.6 μmol min−1 mg−1 of protein. For tellurite the Km was 2.63 mM and the Vmax was 5.2 μmol min−1 mg−1 of protein. Optimal pH range for selenate reductase was 5.5–6, and the maximum activity was achieved at 38 °C. Tellurite reductase exhibited its highest activity at 35 °C and the optimum pH of the purified enzyme was 8. Cations such as Ca2+, Mg2+ and Na+ abolished the selenate reductase activity and had no effect on the tellurite reductase activity. Tellurite reductase was inactivated by EDTA. Elemental analysis revealed the presence of Zn and Mo in tellurite reductase and Cu in selenate reductase.