Characterization of a novel thiosulfate-forming enzyme isolated from Desulfovibrio vulgaris

Abstract

An enzyme that formed thiosulfate from bisulfite and trithionate was purified from extracts ofDesulfovibrio vulgaris. This enzyme, designated as "thiosulfateforming" enzyme, required the presence of both bisulfite and trithionate. Various 35S-labeling studies showed that thiosulfate was formed from bisulfite and the inner sulfur atom of trithionate. This involved a nucleophilic attack by the bisulfite ion, resulting in the displacement of the two outer sulfonate groups of trithionate that recycled to participate as free bisulfite in subsequent reactions.This reaction required a reduction, presumably by a concerted mechanism with thiosulfate formation. The natural electron carrier cytochrome C3 participated in this reductive formation ofthiosulfate. This reaction was coupled to the bisulfite reductase-catalyzed reaction, which resulted in the reconstruction of a thiosulfate-forming pathway from bisulfite.

Revisiting the dissimilatory sulfate reduction pathway

“…In detail, the trithionate pathway seemingly requires two reactions that are not required by Rees: the reductions of trithionate and thiosulfate. These reactions require enzymes (trithionate reductase and thiosulfate reductase), which have been partially characterized in some sulfate-reducing bacteria (Ishimoto et al, 1955;Haschke & Campbell, 1971;Hatchikian, 1975;Drake & Akagi, 1977;Akagi et al, 1994). An important disclaimer, and as noted by Akagi et al (1994) is that presence of these enzymes does not confer their participation or importance in DSR.…”

“…They also point out that the relevant enzymes are not universal in sulfate reducers. For example, Drake & Akagi (1977) describe an enzyme from Desulfovibrio vulgaris that forms thiosulfate from sulfite and trithionate. Peck and Legall state that while present in D. vulgaris, this enzyme is absent in D. gigas.…”

“…For trithionate, at least two reduction enzymes have been reported from Desulfovibrio (Akagi et al, 1994). The first enzyme, referred to as thiosulfate-forming enzyme (TF) (Drake & Akagi, 1977;Akagi et al, 1994), catalyzes the reaction of trithionate with a sulfite molecule, forming thiosulfate and two molecules of sulfite (Fig. 4, Table 1).…”

“…(A) The putative reaction catalyzed by TF (thiosulfate-forming enzyme). This reaction mechanism has been determined via isotope labeling and involves the reaction of trithionate with sulfite, producing thiosulfate and two sulfites (Drake & Akagi, 1977). (B) The putative reaction catalyzed by trithionate-reducing enzyme (TR).…”

“…Similarly, during the formation of thiosulfate in Dsr, the more reduced sulfur atom is likely sourced from zerovalent S derived from DsrAB, while the more oxidized S is from the sulfite pool. Thiosulfate may also be produced from trithionate via one of the enzymes described by Drake & Akagi, 1977; in which case thiosulfate inherits the intramolecular isotopic composition of trithionate plus any fractionation associated with this reaction. Under neutral to mildly alkaline conditions, the formation of thiosulfate may also proceed abiotically, by the reaction of sulfite with sulfide (Siu & Jia, 1999;Shirodkar et al, 2011).…”