Selenocysteine glutathione peroxidase

Glutathione-peroxidase (GSH-Pxase) is an enzyme found in erythroqrtes and other tissues that has an essential selenocysteine residue involved in the catalytic decomposition of reactive oxygen species. In the erythrocyte, hydrogen peroxide is the principle reactive oxygen species available. 

All selenium-containing proteins and enzymes in animals, microorganisms and plants incorporate selenocysteine non-specifically105 or as part of Se-dependent antioxidant enzymes such as glutathione peroxidase, (EC 1.11.1.9) which has a Se-cysteine residue in its active site.116 120 An active form of Se, selenophosphate, is produced by selenophosphate synthetase in several bacteria. This active form is required for the production of Secys-tRNA, a precursor for Se-cysteine.121 In a similar vein, a Se-containing modified-tRNA nucleoside, 5-methylaminomethyl-2-selenouridine, encodes a selenouridine synthase which replaces sulfur in tRNA with selenium.122... 

Selenium is essential for many species including man, on account of its presence in a number of enzymes, notably glutathione peroxidase, an important antioxidant enzyme. It is incorporated into selenoenzymes in the form of selenocysteine. 

The second E. coli example also involves contextual signals that alter coding patterns. A few proteins in all cells (such as formate dehydrogenase in bacteria and glutathione peroxidase in mammals) require the element selenium for their activity, generally in the form of the modified amino acid selenocysteine. Although modified amino acids are generally produced in posttranslational reactions (described in Section... 

I. Chambers, J. Frampton, P. Goldfarb, N. Affara, W. McBain, and P. R. Harrison, The structure of the mouse glutathione peroxidase gene the selenocysteine in the active site is encoded by the termination codon, TGA, EMBOJ. 1986, 5, 1221-1227. 

The element Se—not really a metal—is known to play a key role in enzymes such as the well-known glutathione peroxidase, formate dehydrogenase, glycine reductase, and the previously mentioned hydrogenases (Chapter 9). The unusual amino acid selenocysteine has a unique codon on the DNA (TGA/UGA), but selenation of serine also appears to be possible [16]. A brief review on Selenium can be found in the literature [17],... 

Deagen, J.T., Butler, J.A., Beilstein, M.A., and Whanger, P.D. 1987. Effects of dietary selenite, selenocysteine and selenomethionine on selenocysteine lyase and glutathione peroxidase activities in rat tissues. J. Nutr. 117, 91—98. 

Cytoplasmic glutathione peroxidase is by far the most thoroughly studied species and has been extensively reviewed. It was also the first selenoenzyme to be purified and demonstrated to contain covalently bound selenium. The enzyme is present in the tissues of all mammalia and birds studied. Depending on the species, the molecular mass of the enzyme ranges between 76 and 94kDa. It is composed of four identical constituent subunits that contain a single selenocysteine residne each. 

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