Figure 6.

Electron transfer pathways to S° and from menaquinol in P. carbinolicus. Abiotic reaction with H₂S may reduce sulfur (S₈) to polysulfides. Electron transfer (orange arrows) may take various pathways (numbered) to reduce polysulfides to H₂S. (1) Periplasmic thioredoxins that derive electrons from NADPH may reduce polysulfides to smaller, more soluble S° species. (2) A cytoplasmic S° reductase may extract individual sulfur atoms from a chain and reduce them to H₂S with NADH. (3) Alternatively, S° may be reduced by PpcA, a periplasmic triheme c₇-type cytochrome that receives electrons from menaquinol (MQH₂) through the Act complex. Reduction of S° by MQH₂ requires reverse electron transport, but the source of energy is unknown. The ActC subunit of the Act complex oxidizes MQH₂, releasing two protons to the periplasm. In a Q loop, both electrons pass to iron-sulfur cluster-binding protein ActB, pentaheme c-type cytochrome ActA, monoheme c-type cytochrome ActE, and other c-type cytochromes such as PpcA. (4) In a Q cycle, only one electron takes this route while the other passes to menaquinone (MQ) with proton uptake from the cytoplasm, perhaps involving the ActF subunit. (5) ActE could form a channel lined with eight cysteine residues for reverse electron transport to a disulfide-based cytoplasmic electron carrier such as lipoyl carrier protein LarG. Dihydrolipoamide dehydrogenase reoxidizes dihydrolipoyl-LarG and reduces NAD. The bottom part of the figure depicts in one-letter amino acid code the transmembrane segments and one cytoplasmic loop of the unique ActE of P. carbinolicus.