Enzymes of sulfur metabolism in Acidithiobacillus ferrooxidans

This study describes the distribution, characterization, and possible significance of the sulfur-oxidizing enzymes in A. ferrooxidans. A. ferrooxidans is a Gram-negative, acidophilic, chemolithotrophic bacterium able to derive energy for growth from the oxidation of ferrous to ferric iron and elemental sulfur or reduced inorganic sulfur compounds to sulfate using oxygen as electron acceptor. The first step of the sulfur compound oxidation is enzymatic oxidation of thiosulfate to tetrathionate. The enzyme, catalyzing this reaction is thiosulfate dehydrogenase, a periplasmic enzyme with pH optimum 3.0. Tetrathionate hydrolase is a periplasmatic enzyme with pH optimum 2.5. The rate of tetrathionate oxidation was strongly stimulated by the presence of sulfate ions. After formation of sulfur in the periplasm, sulfur was actively transported across the cell membrane. Further oxidation via sulfite to sulfate occurs in the cytoplasm. Sulfuroxygenase is a cytoplasmatic enzyme with pH optimum 7.4. Enzyme required GSH for its activity. The final step in the oxidation of reduced sulfur compounds; oxidation of sulfite to sulfate, is catalyzed by sulfite dehydrogenase, a cytoplasmatic enzyme with pH optimum 8.0.