Figure 2

Model of sulfur oxidation in A. ferrooxidans ATCC 23270. Reduced inorganic sulfur compound (RISC) oxidation pathways are predicted to involve various enzymes, enzyme complexes and a number of electron carriers located in different cellular compartments: in the outer membrane facing the periplasm (tetrathionate reductase, TetH), in the periplasm (high potential iron-sulfur protein, HiPIP), attached to the cytoplasmic membrane on the periplasmic side (cytochrome c, CycA2), in the cytoplasmic membrane (sulfide quinone reductase (SQR), thiosulfate quinone reductase (TQR), bc₁ complex, NADH complex I, bd and bo₃ terminal oxidases) and in the cytoplasm (heterodisulfide reductase (HDR), and ATP sulfurylase (SAT)). Insoluble sulfur is first converted to sulfane sulfate (GSSH) which is then transferred to the heterodisulfide reductase (HDR) through a cascade of sulfur transferases (DsrE, TusA and Rhd). Electrons coming from sulfide (H₂S), thiosulfate (S₂O₃^2-) or sulfane sulfate (GSSH) are transferred via the quinol pool (QH₂) either (1) directly to terminal oxidases bd or bo₃, or indirectly throught a bc₁ complex and a cytochrome c(CycA2) or a high potential iron-sulfur protein (HiPIP) probably to the aa₃ oxidase where O₂ reduction takes place or (2) to NADH complex I to generate reducing power.