Figure 9.

Current model for cellular H₂S detoxification in Staphylococcus aureus highlighting the known or proposed enzymatic activities encoded by the cst operon. (this work). Extracellular H₂S freely penetrates the cell membrane and exists as HS⁻ once in the cytoplasm. SQR then carries out the initial two-electron oxidation of HS⁻ to form SQR-bound persulfide and the SQR-bound persulfide is transferred to reduced cellular LMW thiols (LMW-SH) to form LMW persulfides (LMW-SSH), which can also be generated from HS⁻ directly via reaction with oxidized LMW thiols (LMW-S-S-LMW). In the presence of molecular oxygen (O₂), CstB^PDO further oxidizes LMW persulfides to possibly form a LMW thiol-S-sulfonate or CstB-S-sulfonate (denoted R-S-SO₃⁻) intermediate, which requires further investigation. Free thiosulfate (TS) is then immediately generated either through the persulfide transferase activity of CstB^Rhod or other cellular rhodanese proteins (other Rhod), or directly generated via reaction of a LMW persulfide with LMW thiol-S-sulfonate or CstB-S-sulfonate. Finally, accumulating thiosulfate is processed by the thiosulfate sulfurtransferase activities of CstB^Rhod, CstA^Rhod or other cellular rhodanese proteins. Sulfite (SO₃²⁻) is effluxed from the cell (perhaps by TauE) and sulfane sulfur (S⁰) proposed to be transferred to as yet unknown downstream cellular sulfur acceptor(s) for assimilation.