Compositions comprising an engineered or non-naturally occurring polypeptides comprising two or more domains selected from Sulfide quinone oxidoreductase, Thiosulfate Sulfurtransferase and persulfide dioxygenase, and a phosphate binding motif are provided along with methods of detoxifying bacterial endotoxins and H.sub.2S with such compositions.

A method of producing sulfur-containing amino acids or derivatives of the sulfur-containing amino acids.

Method of using sulfide: quinone oxidoreductase (SQR) for persulfide-mediated generation of mitochondrial membrane potential and energy production, and method of using SQR for catalyzing proton and electron transfer from hydropersulfides to a mitochondrial electron transport chain.

A method for improving mitochondrial functions includes a step for supplementing mitochondria with an efficient amount of a reactive persulfide, and a step for increasing a membrane potential of mitochondria by said reactive persulfide.