Collagen

Collagen is the most abundant protein in mammals. It is a fibrous protein that gives tensile strength to tendons, ligaments, skin, cartilage, bone and the walls of arteries.Structure of one molecule. A collagen molecule is made of three polypeptide chains, each about 1000 amino acids long. Each chain is a tight helix, and the three chains are wound around each other to form a triple helix, like a rope.The primary structure is highly repetitive. Glycine occurs at every third position, in the pattern Gly–X–Y, where X and Y are often proline and hydroxyproline. Glycine has the smallest R group (a single H atom), so it can fit in the centre of the triple helix, allowing the three chains to pack very tightly.The three chains are held together by many hydrogen bonds between them.Fibrils and fibres. Many collagen molecules lie side by side in parallel rows, with their ends staggered so that there are no weak points in line. Neighbouring molecules are joined by covalent cross-links, forming fibrils. Fibrils bundle together into collagen fibres.How the structure suits its function:
• triple helix, hydrogen bonds and covalent cross-links give very high tensile strength
• staggered ends avoid weak points along the fibre
• it is insoluble and stable, so it lasts in the body
• it is flexible but does not stretch much, ideal for tendons, which transmit the pull of muscle to boneWorked example: a collagen chain of 1050 amino acids has glycine every third position, so it contains 1050 ÷ 3 = 350 glycines.