Threonine. Molecular model of the amino acid threonine. Its chemical formula is C4.H9.N.O3. Atoms are represented as rods and are colour- coded: carbon (blue), hydrogen (gold), oxygen (red) and nitrogen (dark blue). Amino acids are the building blocks of proteins. Threonine 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 cottage cheese, poultry, fish, meat, lentils, and sesame seeds. Threonine is important in liver function and in producing collagen, the supportive tissue in skin, nails, and connective tissue. It also supports the immune system by aiding in the production of antibodies.