Computer artwork of Methionine, an essential amino acid. The human body is not able to manufacture it by itself. That means that a constant blood serum level has to be maintained through nutrition and diet. The main functions of methionine include the building of various protein molecules and the synthesis of the equally sulphurous amino acid cysteine. Atoms are represented as spheres and are colour-coded: carbon (black), hydrogen (white), nitrogen (blue) and oxygen (red). Illustration.