Methionine. Molecular model of the amino acid methionine. Its chemical formula is C5.H11.N.O2.S. Atoms are represented as spheres and are colour- coded: carbon (blue), hydrogen (gold), nitrogen (dark blue), oxygen (red) and sulphur (yellow). Amino acids are the building blocks of proteins. Methionine is an essential amino acid. It cannot be synthesised by the body and so needs to be obtained from food. It is most concentrated in seeds, fish, lentils, nuts, meats and cheeses. Methionine is a lipotrope, a chemical that helps the liver to process fats and ensures normal liver function. It is also essential for producing collagen, which is used to make skin, nails, and connective tissue.