Selenium-containing enzymes in mammals: Chemical perspectives |
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Authors: | Gouriprasanna Roy Bani Kanta Sarma Prasad P Phadnis and G Mugesh |
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Institution: | (1) Department of Inorganic and Physical Chemistry, Indian Institute of Science, 560 012 Bangalore, India |
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Abstract: | The chemical and biochemical route to the synthesis of the 21st amino acid in living systems, selenocysteine, is described.
The incorporation of this rare amino acid residue into proteins is described with emphasis on the role of monoselenophosphate
as selenium source. The role of selenocysteine moiety in natural mammalian enzymes such as glutathione peroxidase (GPx), iodothyronine
deiodinase (ID) and thioredoxin reductase (TrxR) is highlighted and the effect of other amino acid residues located in close
proximity to selenocysteine is described. It is evident from various studies that two amino acid residues, tryptophan and
glutamine, appear in identical positions in all known members of the GPx family. According to the three-dimensional structure
established for bovine GPx, these residues could constitute a catalytic triad in which the selenol group of the selenocysteine
is both stabilized and activated by hydrogen bonding with the imino group of the tryptophan (Trp) residue and with the amido
group of the glutamine (Gln) residue. The ID enzymes, on the other hand, do not possess any Trp or Gln residues in close proximity
to selenium, but contain several histidine residues, which may play important roles in the catalysis. The TrxR enzymes also
possess some basic histidines, but the most important amino acid residues are the cysteines which constitute the internal
cofactor systems along with the catalytically active selenocysteine. The catalytic activity and substrate specificity of all
three selenoenzymes are described. The reactivity of selenocysteine residues in selenoenzymes towards metal-based drugs such
as goldthioglucose is also described. |
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Keywords: | Antioxidants cysteine deiodination monoselenophosphate selenium selenocysteine selenoenzymes thyroxine |
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