Several decades after the discovery of
selenium as an essential
trace element in vertebrates approximately 20 eukaryotic and more than 15 prokaryotic
selenoproteins containing the 21st proteinogenic
amino acid,
selenocysteine, have been identified, partially characterized or cloned from several species. Many of these
proteins are involved in redox reactions with
selenocysteine acting as an essential component of the catalytic cycle.
Enzyme activities have been assigned to the
glutathione peroxidase family, to the
thioredoxin reductases, which were recently identified as
selenoproteins, to the iodothyronine deiodinases, which metabolize
thyroid hormones, and to the
selenophosphate synthetase 2, which is involved in
selenoprotein biosynthesis. Prokaryotic
selenoproteins catalyze redox reactions and formation of selenoethers in (stress-induced) metabolism and energy production of E. coli, of the clostridial cluster XI and of other prokaryotes. Apart from the specific and complex biosynthesis of
selenocysteine,
selenium also reversibly binds to
proteins, is incorporated into
selenomethionine in bacteria, yeast and higher plants, or posttranslationally modifies a catalytically essential
cysteine residue of
CO dehydrogenase. Expression of individual eukaryotic
selenoproteins exhibits high tissue specificity, depends on
selenium availability, in some cases is regulated by
hormones, and if impaired contributes to several pathological conditions. Disturbance of
selenoprotein expression or function is associated with deficiency syndromes (Keshan and
Kashin-Beck disease), might contribute to
tumorigenesis and
atherosclerosis, is altered in several bacterial and
viral infections, and leads to
infertility in male rodents.