Thiosulfate reductase

The extracts contained a reduced methylviologen-dependent thiosulfate reductase ... 

Considering only the enzyme data, a weakness of the scheme could have been in the mechanism and activity of the thiosulfate reductase. This reductase activity was demonstrable only with methylviologen as the reductant, and no activity was seen with NADPH (Peck 1960). This showed that the reductase required an extremely electronegative electron donor the Eq for methylviologen is -446 mV, whereas that for NADPH is only -320 mV. This requirement for reduced methylviologen is consistent with the Eo value for... 

The activities of thiosulfate reductase originally reported were low (-0.8nmol sulfide per minute per milligram protein), when assayed at pH... 

The thiosulfate reductase/rhodanese/APS reductase system is thus supported by evidence from direct enzyme assay, whole-cell metabolism and energetics, and S-labehng experiments and provides a robust hypothesis to explain thionate oxidation and energy conservation in at least some chemolithotrophs. 

Prieto JL, Perez Castiniera JR, Vega JM. 1997. Thiosulfate reductase from Chlamy-domonas. J Plant Physiol 151 385-9. 

Anoxyphotobacteria which do not possess a "reverse" sulfite reductase are nevertheless able to form sulfite from sulfide, if they contain a flavocytochrome and a thiosulfate reductase. Therefore Trueper and Fischer (22) assume that in these organisms the following reaction mechanism might be possible ... 

Reductive cleavage of sulfur-sulfur bonds in inorganic sulfur species is exploited in terminal electron transfer in some bacteria. The tetrathionate and thiosulfate reductases of the enteric bacterium Salmonella enterica LT2 and the polysulfide reductase of the rumen bacterium Wolinella succinogenes effect such reactions. ... 

In the sulfate-reducing bacteria there is another cytochrome which resembles cytochrome c3 in the spectral properties and redox potential but differs from this cytochrome in molecular mass this is cytochrome c3 (26 kDa) which has eight heme C molecules in the molecule. The cytochrome molecule is composed of two polypeptides of 13 kDa (Loufti et al., 1989). On the basis of the amino acid sequence, however, the 13 kDa polypeptide differs from cytochrome c3 (Guer-lesquin et al., 1982 LeGall and Peck, 1987 Loufti et al., 1989). Desulfovibrio gigas cytochrome c3 (26 kDa) molecule is composed of two 13 kDa molecules bound to each other by an S-S bond (Bruschi et al., 1996). It is claimed that cytochrome c3 (26 kDa) is very effective as the electron donor for thiosulfate reductase (Hat-chikian et al., 1972). 

The physiological role of cytochrome c3 (Mr 26,000) has not been elucidated. It can be reduced by hydrogenase in a similar way to cytochrome c3 (13,000) [49,76]. D. gigas cytochrome c3(Mr 26,000) was shown to stimulate the reduction of thiosulfate by a partially purified extract, using H2/hydrogenase as electron donor [77], but this was not observed with purified thiosulfate reductase [78]. 

At lease four intracellular enzymes may be involved for cyanide detoxification. The generalized reactions of rhodanese, mercaptopyruvate sulfurtransferase, thiosulfate reductase, and cystathionase are shown within the cell. 

Eventually the gas is converted to sulfite in the mitochondria by thiosulfate reductase, and the sulfite is further oxidized to thiosulfate and sulfate by sulfite oxidase. The sulfates are excreted in the urine. 

Fig. 3. Regulation of the bound pathway for the assimilation of sulfate into cysteine and associated processes. Carrier refers to an endogenous thiol of uncertain identity in higher plants. Enzymes associated with the sulfate assimilation pathway and the synthesis of O-acetylseiine are (1) high-ailinity sulfate uptake mechanism, (2) ATP-sulfurylase, (3) adenosine S -phosphosulfate (APS) sulfotransferase, (4) organic thiosulfate reductase, (5) cysteine synthase, and (6) serine transacetylase. Cysteine sulfhydrase (7), an enzyme of cysteine catabolism, and nitrate reductase (8), the first enzyme of the nitrate assimilation pathway, are also shown. Inhibitory control of the pathways is shown by discontinuous lines (----) and enhancement by continuous lines (------).

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