Sulfide : quinone oxidoreductase (SQR) from the lugworm Arenicola marina shows cyanide‐ and thioredoxin‐dependent activity

The lugworm Arenicola marina inhabits marine sediments in which sulfide concentrations can reach up to 2 m m . Although sulfide is a potent toxin for humans and most animals, because it inhibits mitochondrial cytochrome  c oxidase at micromolar concentrations, A. marina can use electrons from sulfide for mitochondrial ATP production. In bacteria, electron transfer from sulfide to quinone is catalyzed by the membrane‐bound flavoprotein sulfide : quinone oxidoreductase (SQR). A cDNA from A. marina was isolated and expressed in Saccharomyces cerevisiae , which lacks endogenous SQR. The heterologous enzyme was active in mitochondrial membranes. After affinity purification, Arenicola SQR isolated from yeast mitochondria reduced decyl‐ubiquinone ( K m  = 6.4 μ m ) after the addition of sulfide ( K m  = 23 μ m ) only in the presence of cyanide ( K m  = 2.6 m m ). The end product of the reaction was thiocyanate. When cyanide was substituted by Escherichia coli thioredoxin and sulfite, SQR exhibited one‐tenth of the cyanide‐dependent activity. Six amino acids known to be essential for bacterial SQR were exchanged by site‐directed mutagenesis. None of the mutant enzymes was active after expression in yeast, implicating these amino acids in the catalytic mechanism of the eukaryotic enzyme.