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 replaces methionine residues. Proteins containing such unspecifically incorporated selenomethionine residues are not regarded as selenoproteins. However, replacement of all methionines by selenomethionines is a widely used, recent technique in solving the phase problem during X-ray crystallographic structure determination of many proteins (MAD-phasing). While the exchange of methionines by selenomethionines appears to be tolerated (at least in bacterial cells), unspecific incorporation of selenocysteine in lieu of cysteine seems to be highly toxic. This may be one reason for the existence of a rather complicated pathway of selenocysteine biosynthesis and specific incorporation into selenoproteins, which avoids the occurrence of the free amino acid as intermediate. Thus, even if a selenocysteine-containing selenoprotein is taken up in the diet and used as selenium source, the amino acid must be degraded prior to synthesising a new selenocysteine for incorporation into a selenoprotein.
Read more about this topic: Selenoprotein
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