Collagen

The fibrous, extracellular matrix protein in skin, muscles, tendons, ligaments, and bone. Collagen is composed of repeating units of glycine-proline-hydroxyproline that form linear polypeptide chains; three chains are then twisted and interlocked with hydrogen bonding to form a rigid helix structure. Collagen-containing materials, such as hides, bones and fish, have been used to make animal glues since ancient times. Collagen will irreversibly hydrolyze, or denature, in hot water, forming a water-soluble gelatin.