Thiosulfate, oxidation

As indicated above, some forms of green sulfur bacteria are able to oxidize thiosulfate. While several pathways for thiosulfate oxidation appear to exist [6], the best-characterized [105,111] utilizes a thiosulfateicytochrome c oxidoreductase to catalyse electron transfer from thiosulfate to soluble Cyt c-551 M = 45000, 

The pathway(s) for thiosulfate oxidation in purple sulfur bacteria are poorly understood, with both a membrane-bound c cytochrome [113] and a soluble iron-sulfur protein [114] having been proposed as possible acceptors of electrons arising from thiosulfate oxidation. However, it is known that in C. vinosum electrons from thiosulfate can be used to reduce NAD(P) in an uncoupler-sensitive reaction (vide 

The authors would like to thank Dr. Davide Zannoni, Robert Blankenship and R.C. Fuller for kindly providing access to manuscripts prior to publication. Work in the authors laboratory was supported, in part, by grants (to D.B.K.) from the Robert A. Welch Foundation (D-710) and the U.S. National Science Foundation (PCM-8109635 and PCM-8408564). 

4 Kondratieva. E.N. (1979) in International Review of Microbial Biochemistry (Ouayle, J.R., ed.) Vol. 21. pp. 118-175, University Park Press. Baltimore. 

6 Truper, H.G. and Fischer, U. (1982) Phil. Trans. R. Soc. London B298, 529-542. 

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Electrocatalysis in oxidation has apparently first been shown for ascorbic acid oxidation by Prussian blue [60] and later by nickel hexacyanoferrate [61]. More valuable for analytical applications was the discovery in the early 1990s of the oxidation of sulfite [62] and thiosulfate [18, 63] at nickel [62, 63] and also ferric, indium, and cobalt [18] hexacyanoferrates. More recently electrocatalytic activity in thiosulfate oxidation was shown also for zinc [23] hexacyanoferrate. Prussian blue-modified electrodes allowed sulfite determination in wine products [64], which is important for the wine industry. 

Although these equations held true for some strains of bacteria under some growth conditions, they did not help explain the commonly observed quantitative conversion of both sulfur atoms of thiosulfate to sulfate, rather than the liberation of the sulfane-sulfur mainly as elemental sulfur. During the 1960s, cyclic reactions of polythionates and other polysulfur compounds continued to be postulated as mechanisms for thiosulfate and polythionate metabolism (Trudinger 1967), but none of these was supported at the time by strong biochemical evidence. The time was opportune for a new approach to the problem of thiosulfate oxidation in thiobacilli. 

Figure 15.1. Thiosulfate oxidation by the APS pathway in T. thioparus. Data from Peck (1960).

The scheme (Fig. 15.1) thus explained the production of both sulfate and sulfur in equimolar amounts from thiosulfate oxidation. In showing adenylylsulfate as an intermediate, it also provided a feasible route for the conservation of energy from sulfite oxidation by a substrate-level phosphorylation mechanism, in which ADP sulfurylase and adenylate kinase give rise to ATP ... 

Further Evidence for the APS Pathway of Thiosulfate Oxidation in Thiobacillus thioparus... 

Santer M. 1959. The role of phosphate in thiosulfate oxidation by Thiobacillus thioparus. Biochim Biophys Res Commun 1 9-12. 

For thiosulfate oxidation it is hypothesized that an enzyme system on the cell surface initiates the formation of polythiosulfonic acid and consequently splits the terminal S03" as sulfate. This process may evolve the intermediate of mixed anhydride -S-O-PO4" from phosphorylation (Figure 4). 

VIII. OTHER BIOLOGICAL MANGANESE SITES VIII.A. Thiosulfate Oxidizing Enzyme... 

In general, sulfide is oxidized via sulfite to sulfate, while thiosulfate can either be oxidized only to tetrathionate or may be split into sulfide and sulfite. Both compounds are then further oxidized to sulfate. During anaerobic sulfide or thiosulfate oxidation, elemental sulfur appears as sulfur globules inside or outside the cells. Sulfur metabolism in Anoxyphotobacteria has two main functions ... 

X>2 + H2S + 2H20 -- light —> 2CH20 + H2S04 and for anaerobic thiosulfate oxidation... 

Van Niel s equations clearly show that elemental sulfur is an intermediate during anaerobic sulfide or thiosulfate oxidation by whole cells. But it has turned out that elemental sulfur is not the only intermediate sulfur compound, as generally presumed. Other sulfur compounds formed during such a process are sulfite, thiosulfate, polysulfides or tetrathionate. These intermediates are also further oxidized to sulfate, if die organisms are able to do so (14-17). 

Figure 5. Anaerobic sulfide and thiosulfate oxidation by a growing culture of Chlorobium vibrioforme f. thiosulfatophilum (Reproduced with permission from Ref. 17. Copyright 1982 Springer-Verlag New York Inc.).

In Chlorobium limicola . thiosulfatophilum cytochrome c-SSl serves as electron acceptor of thiosulfate oxidation, before the electrons flow to oxidized bacteriochlorophyll via a soluble small cytochrome c-SSS and a membrane-bound cytochrome (2). The absence of cytochrome c-SSl in the non-thiosulfate-utilizing Chlorobiaceae might be one explanation why these organisms cannot use thiosulfate (4). 

Lead tetraacetate consumption is measured conveniently by iodometry.4 The reaction mixture is added to excess potassium iodide solution, usually in the presence of sodium acetate,6 and the iodine liberated is then titrated with standard thiosulfate. Oxidation may also be measured potentiometri-cally,78 210 211 a procedure especially useful for fast glycol groups,78 or with redox indicators.211... 

It is well established that green sulfur bacteria carry out linear electron flow from external donors such as S , S , 8203 and 803 to ferredoxin and NAD (see Ref. 36 and Chapter 9). The use of 02-uptake as a valid assay of linear flow in membrane preparations has also been demonstrated [61]. 8ulfide oxidation is mediated by a soluble flavocytochrome c-553 ( , -1-0.09 V) [36,62], and thiosulfate oxidation seems to require the soluble Cyt c-551 == 0.14 V) [36]. 8oluble Cyt... 

London J. and Rittenberg S. C. (1967) Path of sulfur in sulfide and thiosulfate oxidation by thiobaciUus. Arch. Microbiol. 59, 218-225. 

The genus Thiomicrospira contains three known species, all of which are chemolithoauto-trophs. These organisms thrive under microaer-ophilic conditions, and one strain can couple sulfide and thiosulfate oxidation to NO ... 

Kappler U., Friedrich C. G., Truper H. G., and Dahl C. (2001) Evidence for two pathways of thiosulfate oxidation in Starkeya novella (formerly Thiobacillus novellas). Arch. Microbiol. 175, 102-111. 

Tuttle, J.H. and Jannasch, H.W., 1973. Sulfide and thiosulfate-oxidizing bacteria in anoxic marine basins. Mar. Biol., 20 64—70. 

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