Inorganic Sulfur Compounds as Electron Donors in Purple Sulfur Bacteria

Abstract

Most anoxygenic phototrophic bacteria can use inorganic sulfur compounds (e.g. sulfide, elemental sulfur, polysulfides, thiosulfate, or sulfide) as electron donors for reductive carbon dioxide fixation during photolithoautotrophic growth. In these organisms, light energy is used to transfer electrons from sulfur compounds to the level of the more highly reducing electron carriers NAD(P)+ and ferredoxin. In this chapter the sulfur oxidizing capabilities of the different groups of anoxygenic phototrophic bacteria are briefly summarized. This chapter then focuses on the pathways of sulfur compound oxidation in purple sulfur bacteria of the families Chromatiaceae and Ectothiorhodospiraceae. A variety of enzymes catalyzing sulfur oxidation reactions have been isolated from members of this group and Allochromatium vinosum, a representative of the Chromatiaceae, has been especially well characterized also on a molecular genetic level. In this organism intracellular sulfur globules are an obligate intermediate during the oxidation of thiosulfate and sulfide to sulfate. Thiosulfate oxidation is strictly dependent on the presence of three periplasmic Sox proteins encoded by the soxBXA and soxYZ genes. Sulfide oxidation does not appear to require the presence of Sox proteins. Flavocytochrome c is also not essential leaving sulfide:quinone oxidoreductase as the probably most important sulfide-oxidizing enzyme. Polysulfides are intermediates en route of sulfide to stored sulfur. Sulfur is deposited in the periplasm and present as long chains probably terminated by organic residues at one or both ends. The oxidation of stored sulfur is completely dependent on the proteins encoded in the dsr operon. These include siroamide-containing sulfite reductase (DsrAB), a transmembrane electrontransporting complex (DsrMKJOP) and a iron–sulfur flavoprotein with NADH:acceptor oxidoreductase activity (DsrL). The last step of reduced sulfur compound oxidation in purple sulfur bacteria is the oxidation of sulfite. This can occur either via the enzymes adenosine 5'-phosphosulfate (APS) reductase and ATP sulfurylase which are non-essential in Alc. vinsoum or via direct oxidation to sulfate. The nature of the enzyme catalyzing the latter step is still unresolved in purple sulfur bacteria.