in molecular biology a selenoprotein is any protein that includes a selenocysteine (se-cys) amino acid residue. among functionally characterized selenoproteins are five glutathione peroxidases (gpx) and three thioredoxin reductases, (trxr/txnrd) which both contain only one se-cys. selenoprotein p is the most common selenoprotein found in the plasma. it is unusual because in humans it contains 10 se-cys residues, which are split into two domains, a longer n-terminal domain that contains 1 se-cys, and a shorter c-terminal domain that contains 9 se-cys. the longer n-terminal domain is likely an enzymatic domain, and the shorter c-terminal domain is likely a means of safely transporting the very reactive selenium atom throughout the body. == species distribution == selenoproteins exist in all major forms of life, eukaryotes, bacteria and archaea. among eukaryotes, selenoproteins appear to be common in animals, but rare or absent in other phyla (one has been identified in the green alga chlamydomonas, but none in other plants or in fungi). among bacteria and archaea, selenoproteins are only present in some lineages, while they are completely absent in many other phylogenetic groups. these observations have recently been confirmed by whole genome analysis, which shows the presence or absence of selenoprotein genes and accessory genes for the synthesis of selenoproteins in the respective organism. == types == besides the selenocysteine-containing selenoproteins, there are also some selenoproteins known from bacterial species, which have selenium bound noncovalently. most of these proteins are thought to contain a selenide-ligand to a molybdopterin cofactor at their active sites (e.g. nicotinate dehydrogenase of eubacterium barkeri, or xanthine dehydrogenases). selenium is also specifically incorporated into modified bases of some trnas (as 2-seleno-5-methylaminomethyl-uridine). in addition, selenium occurs in proteins as unspecifically incorporated selenomethionine, which r
Selenoproteins