Homocysteine. Computer artwork of a molecule of homocysteine, an intermediate in the synthesis of the amino acid cysteine. The atoms are depicted as spheres whilst the sticks represent the chemical bonds that join the atoms together. The atoms are colour-coded: carbon (grey), oxygen (red), nitrogen (dark blue), sulphur (light blue), and hydrogen (white). The molecule is covered in a coloured electron density map. Cysteine, is an important component in many enzymes. The inherited disease homocystinuria is caused by excess homocysteine in the blood. Sufferers are mentally retarded and have excessively long leg bones and fingers.