Bacterial polysulfide reductase is an integral membrane protein complex responsible for quinone-coupled reduction of polysulfide, a process important in extreme environments such as deep-sea vents and hot springs. Polysulfides are a class of compounds composed of chains of sulfur atoms, which in their simplest form are present as an anion with general formula Sn(2-). In nature, polysulfides are found in particularly high concentrations in extreme volcanic or geothermically active environments. Here, the reduction and oxidation of polysulfides are vital processes for many bacteria and are essential steps in the global sulfur cycle. In particular, the reduction of polysulfide to hydrogen sulfide in these environments is usually linked to energy-generating respiratory processes, supporting growth of many microorganisms, particularly hyperthermophiles.

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