Selenocysteine. Molecular model of the amino acid selenocysteine. Its chemical formula is C3.H7.N.O2.Se. Atoms are represented as balls and are colour-coded: carbon (blue), hydrogen (yellow), nitrogen (dark blue), oxygen (red) and selenium (orange). Amino acids are the building blocks of proteins. Selenocysteine is an essential amino acid. Selenium, its core component, needs to be obtained from food in order for the body to make the amino acid. Brazil nuts have the highest concentrations of selenium, but it is also found in seafood, fish, meats, eggs and cereals. Selenocysteine is an important component of many enzymes, such as glutathione peroxidases, a group of antioxidant enzymes.