29] Reverse siroheme sulfite reductase from Thiobacillus denitrificans

Abstract

This chapter describes reverse siroheme sulfite reductase from Thiobacillus denitrificans (T. denitrificans). Adenylylsulfate reductase has been found to occur only in the anaerobe T. denitrificans and in the aerobes Thiobacillus thioparus and “Thiobacillus” thiooxidans. A siroheme-containing sulfite reductase in high intracellular concentration is found in T. denitrificans. In contrast to the sulfite reductases in sulfate-reducing bacteria (SRB), in T. denitrificans the enzyme must function in the oxidative—or “reverse”—direction, oxidizing sulfane sulfur to sulfite. Sulfite reductase is measured in a manometric assay in the direction of sulfite reduction with enzymatically reduced methyl viologen as electron donor. The reduction of methyl viologen by hydrogen gas is catalyzed by purified hydrogenase from Desulfovibrio gigas. The consumption of hydrogen is recorded manometrically. The enzyme contains 24 mol of iron and 20 mol of (acid-labile) sulfur per mole of enzyme and can reduce sulfite, but not thiosulfate, dithionate, trithionate, or tetrathionate. Unlike assimilatory sulfite reductases the enzyme does not contain flavin groups.