Collagen.

Collagen is a complex protein made up of three separate polypeptide chains that form a triple helix. These polypeptides are unusual because every third amino acid is a glycine and because prolines make up an additional 17 percent of the chains. There are at least twenty-eight types of collagen made up of forty-three distinct polypeptide chains, each coded for by a different gene. For example, type I collagen, the most common type, has two chains classified as alpha-1 and alpha-2. These peptides are initially produced on the rough endoplasmic reticulum (ER) and then processed in the ER lumen, where sequences at the ends are removed and hydroxyl groups are added to many of the chains’ prolines and lysines. The triple helix then formed is called procollagen. Further processing, including preparation for secretion, takes place in the Golgi bodies. Once secreted, more end sequences are cleaved off to form collagen (also called tropocollagen). In the extracellular region, collagen molecules associate into collagen fibrils and eventually collagen fibers.