Selenium is an essential micronutrient in mammals, but is also recognized
as toxic in excess. It is a non-metal with properties that are intermediate
between the chalcogen elements sulfur and tellurium. Selenium exerts its
biological functions through selenoproteins. Selenoproteins contain selenium in
the form of the 21st amino acid, selenocysteine (Sec), which is an analog of
cysteine with the sulfur-containing side chain replaced by a Se-containing side
chain. Sec is encoded by the codon UGA, which is one of three termination codons
for mRNA translation in non-selenoprotein genes. Recognition of the UGA codon as
a Sec insertion site instead of stop requires a Sec insertion sequence (SECIS)
element in selenoprotein mRNAs and a unique selenocysteyl-tRNA, both of which
are recognized by specialized protein factors. Unlike the 20 standard amino
acids, Sec is biosynthesized from serine on its tRNA. Twenty-five selenoproteins
are encoded in the human genome. Most of the selenoprotein genes were discovered
by bioinformatics approaches, searching for SECIS elements downstream of
in-frame UGA codons. Sec has been described as having stronger nucleophilic and
electrophilic properties than cysteine, and Sec is present in the catalytic site
of all selenoenzymes. Most selenoproteins, whose functions are known, are
involved in redox systems and signaling pathways. However, several
selenoproteins are not well characterized in terms of their function. The
selenium field has grown dramatically in the last few decades, and research on
selenium biology is providing extensive new information regarding its importance
for human health.