Collagen is a fibrous protein that is found in tendons, bones, muscles, skin and blood. It comprises three-quarters of the skin and a third of all the protein in the human body.The primary structure is the sequence of amino acids that gives rise to an unbranched polypeptide chain. Every third amino-acid is glycine. The other amino-acids are mostly proline, hydroxyproline and lysine.The secondary structure consists of a tightly wound molecule.The molecules has a tertiary structure; it is twisted into a second helix.The quaternary structure consists of three helices tightly wound round each other and cross-linked by lysine residues.Every third amino-acid is glycine to ensure the individual chains can get close to each other to be tightly wound. The ends are staggered to ensure that a line of weakness does not run through the molecule.