Selenium is naturally found in fossil fuels, shales, alkaline soils and as a constituent in over 40 minerals (i.e. ferroselite, challomenite, schmeiderite). Extensive deaths and deformities in waterfowl and other wildlife are attributed to micromolar concentrations of selenium at the Kesterton reservoir. The soluble oxyanions selenate (Se(VI)) and selenite (Se(IV)) are the primary forms of selenium in oxic environments. Se(VI) does not readily undergo chemical reduction under physiological conditions of pH and temperature.
The toxicity of selenium involves the substitution of selenium for sulfur in thiol-containing proteins. In animals, selenium toxicity is conferred primarily by ingestion of food that is rich in seleniferous compounds (e.g. selenomethionine). Selenium poisoning can result in conditions known as alkali disease and the blind staggers. Plants and microbes can incorporate inorganic selenium oxyanions into their tissues by assimilatory reduction to the level of selenide, followed by their assembly into proteins.
At present, only the selenate reductase of Thauera selenatis has been well-characterized. This enzyme is a periplasmic, trimeric enzyme that is very substrate specific and contains molybdenum. It reduces four molecules of selenate to four molecules of selinite (Stolz and Oremland, 1999).
Selenate respiring bacteria have been shown to occur in a wide range of environments and not confined to any specific genus. The full impact of their activities on the biogeochemical cycling of selenium is only now being realized. Furthermore, these organisms, and perhaps the enzymes themselves, may be useful in the bioremediation of selenium-contaminated environments.
The following is a text-format selenate pathway map. Organisms which can initiate the pathway are given, but other organisms may also carry out later steps. Follow the links for more information on compounds or reactions. This map is also available in graphic (22k) format.
Selenate Selenate Thauera selenatis Sulfurospirillum barnesii SES-3 \ Bacillus arseniciselenatis E-1H \ / selenate \ / membrane-bound reductase \ / selenate reductase \ / \ / \ / Selenite | | | spontaneous | | v Selenodiglutathione | | | spontaneous | | v Glutathionyl selenide anion | | | spontaneous | | v Hydrogen Selenide | | | selenophosphate synthetase | | v Selenophosphate | | | | | v Intermediary Metabolism (KEGG)
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