28] Enzymes of dissimilatory sulfide oxidation in phototrophic sulfur bacteria

Abstract

This chapter reviews enzymes of dissimilatory sulfide oxidation in phototrophic sulfur bacteria. The pathway of reduced sulfur oxidation in phototrophic bacteria includes three essential steps: (1) formation of elemental sulfur that appears as globules, inside or outside the bacteria, during oxidation of sulfide or thiosulfate, (2) oxidation of sulfide or elemental sulfur to sulfite, and (3) formation of sulfate as the final product. The splitting of thiosulfate to elemental sulfur and sulfite is catalyzed by an enzyme with rhodanese (thiosulfate sulfurtransferase) activity. Sulfite is oxidized and reacts with AMP to form adenylylsulfate catalyzed by the enzyme adenylylsulfate (APS) reductase. The intermediary formation of APS demands an additional enzyme to split off the sulfate moiety. This step allows conservation of the phosphate-bond energy contained in APS. The enzyme ADP-sulfurylase replaces the sulfate moiety by inorganic phosphate, thus producing adenosine diphosphate. Sulfite reductase catalyzes sulfite reduction with enzymatically reduced methyl viologen as electron donor. The reaction mixture contains hydrogenase as an auxiliary enzyme, catalyzing methyl viologen reduction with hydrogen. Sulfite reductase activity can be measured as H 2 consumption in a Warburg apparatus.