Selenocysteine is an amino acid that's present in several enzymes (for example glutathione peroxidases, tetraiodothyronine 5' deiodinases, thioredoxin reductases, formate dehydrogenases, glycine reductases and some hydrogenases). Nomenclature The joint nomenclature committee of the IUPAC/IUBMB has officially recommended the three-letter symbol Sec and the one-letter symbol U for selenocysteine. Structure Selenocysteine has a structure similar to cysteine, but with an atom of selenium taking the… (More on Selenocysteine)