Cysteine. Molecular model of the amino acid cysteine. Its chemical formula is C3.H7.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. Cysteine is a non-essential amino acid. It can be synthesised by the body and so does not need to come from the diet. Cysteine is an important structural and functional component of many enzymes (proteins that catalyse metabolic reactions). For example, it allows coenzyme proteins to bind to metal cofactor molecules. Without this binding ability, the coenzyme proteins are unable to function as enzymes.