Figure 4. Model of sulfur oxidation in A. caldus.

The sulfur oxidation system involves varied sulfur oxidation pathways and the electron transfer system in different cellular compartments. Starting from the extracellular elemental sulfur (S₈), it is activated and transported into the periplasmic space as persulfide sulfur (R-SH), and then oxidized by the sulfur dioxygenase (SDO) to produce SO₃ ²⁻; SO₃ ²⁻ can enter into Sox pathway or combine with sulfur atoms to form S₂O₃ ²⁻ via a nonenzymatic reaction; S₂O₃ ²⁻ has two destinies, one is to be oxidized by the Sox pathway, the other is to form S₄O₆ ²⁻ catalyzed by thiosuirate quinine oxidoreductase (TQO); S₄O₆ ²⁻ is hydrolyzed by tetrathionate hydrolase (TetH) producing S₂O₃ ²⁻, SO₄ ²⁻, and S; S produced from hydrolysis of S₄O₆ ²⁻, oxidation of H₂S by sulfide quinone reductase (SQR) or from truncated oxidation of S₂O₃ ²⁻ by the Sox pathway can be accumulated in the form of polymeric sulfur (S_n) in the periplasm and transferred into the cytoplasm; the cytoplasmic elemental sulfur (S_n) is oxidized by sulfur oxygenase reductase (SOR) producing S₂O₃ ²⁻, SO₃ ²⁻, and H₂S, which stimulate the cytoplasmic sulfur pathways including the metabolism of S₂O₃ ²⁻ by rhodanese (TST) and heterodisulfidereductase (HDR) and the oxidation of SO₃ ²⁻ via the APS pathway. Two methods of SoxYZ regeneration are proposed, with one being the sulfur atom is provided from the sulfane intermediate (SoxYZ–S–S⁻) and the other being oxidation of SoxYZ–S–S⁻ by SDO to complete the Sox sulfur oxidation pathway. Electrons from SQR, TQO, HDR and SoxAX are mediated by the quinol pool in the inner membrane, then are utilized by terminal oxidases bd or bo₃ to produce a proton gradient to generate ATP or by the NADH complex I to generate reducing power.