Selenium is a nonmetal element on the periodic table (between sulfur and tellurium), while Cysteine is a semi-essential amino acid. But when Selenium performs its biological functions, it does so through selenoproteins. Selenoproteins have selenium in them as the form of the 21st amino acid, selenocysteine (also called Sec), which is a cysteine analog.
Selenocysteine is encoded by the UGA codon (one of three) in mRNA translation for non-selenoprotein genes. Selenocysteine is a proteinogenic amino acid. Unlike the 20 regular amino acids (both essential and non-essential) selenocysteine is biosynthesized on its tRNA from the amino acid serine.
Interestingly, there are 25 selenoproteins, like selenocysteine and selenomethionine (the latter of which replaces methionine amino acid residues, and is sometimes randomly substituted for methionine), which are encoded in our human genome.
Study on selenium and selenocysteine on health
Selenocysteine, according to S Kurokawa and MJ Berry, in their publication titled Selenium. Role of the essential metalloid in health discuss selenocysteine and its role in health. They say that selenocysteine (Sec) is 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.”
Even though selenium can be considered toxic if the dose is too high, it is still required for health purposes in the bigger picture, selenocysteine notwithstanding. According to the researchers the selenium field (which includes the selenoproteins, and selenomethionine, etc.) has “grown dramatically in the last few decades, and research on selenium biology is providing extensive new information regarding its importance for human health.”
Selenocysteine, itself, is a building block of selenoproteins, contains selenium, and is present in several enzymes such as glutathione peroxidases, glycine reductases, methionine-R-sulfoxide reductase B1 (SEPX1), and so on). Glutathione and glycine are standard amino acids. The biochemist, Theresa Stadtman (married to Earl R. Stadtman) at the National Institutes of Health, discovered selenocysteine.