In molecular biology a selenoprotein is any protein that includes a selenocysteine (Sec, U, Se-Cys) amino acid residue.
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 almost none in other plants or in fungi.
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.