Fig. 2

Enzymatic pathways for oxidation of electrochemically reduced hydrogen sulfide. In the Sox (Sulfide oxidation) pathway (a), located in the periplasm of the microorganism, sulfide is bound to the SoxY enzyme through a cysteine-sulfur atom (SoxY-S^-) and is sequentially oxidized to sulfate. SoxCD is believed to catalyze the oxidation through to sulfite (SO₃^-), with the final oxidation to sulfate (SO₄^2-) catalyzed by SoxB. The sulfide quinone oxidoreductase (SQR) pathway (b), includes the formation of the free intermediates elemental sulfur (S⁰), sulfite (SO₃^2-) and APS (adenosine-5′-phosphosulfate). In this pathway, hydrogen sulfide is first oxidized to sulfur in a 2-electron reaction by a sulfide:quinone reductase (SQR). In Beggiatoa this sulfur precipitates and is stored in intracellular granules. When the supply of sulfide has been depleted, elemental sulfur can be converted back to soluble sulfide and sent to the cytoplasm by the Dissimilatory sulfite reductase (Dsr), a membrane spanning siroheme. Sulfide is further oxidized to sulfite by reverse Dsr (rDsr), then to sulfate by either APS reductase and ATP sulfurylase, or Adenosine 5’-monophosphate (AMP)-independent sulfite dehydrogenase (Sdh). This cycle is completed when sulfate is electrochemically reduced back to sulfide at the cathode. This figure was compiled with information from references [103, 104, 137, 138]