What is collagen?

Structure and Functions

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.

Collagen is a flexible but not stretchable protein that is an important component of most connective tissues. It is the primary component of tendons and ligaments, giving them the requisite strength to connect muscles to bones and bones to other bones or organs. Cartilage found at joints and in many other structures is mostly collagen. The connective tissues found in the dermal layer of the skin, the capsules surrounding internal organs, and blood vessels are also primarily made of collagen. Bones are initially formed from collagen, which then serves as a matrix for calcium phosphate deposition. (Collagen fragments have even been extracted from fossilized dinosaur bones.) During healing, excess collagen production can lead to scar tissue formation. Collagen can be heat-treated to produce gelatin or animal-based glues, and injected collagen is often used in cosmetic procedures to plump lips or smooth out wrinkles.

Disorders and Diseases

Collagen is associated with many disorders. Osteogenesis imperfecta (brittle bone disease) is caused by mutations in the gene for the alpha-1 protein in type I collagen. An inherited form of osteoporosis is caused by a defect in the same gene. Ehlers-Danlos syndrome, which results in hyperextensible joints and fragile, stretchable skin, is caused by defects in types III and V collagen. A form of early-onset osteoarthritis is caused by a lack of functional type VI collagen, and in all forms of osteoarthritis
cartilage is lost from the ends of bones at joints. In rheumatoid arthritis, modification of type II collagen forms new antigens that are attacked by the immune system. Vitamin C deficiency decreases activity of the enzymes that add hydroxyl groups to proline, thus leading to lowered amounts of functional type I collagen, which causes
scurvy.

Bibliography

Abreu-Velez, Ana Maria, and Michael S. Howard. "Collagen IV in Normal Skin and in Pathological Processes." North American Journal of Medical Sciences 4, no. 1 (2012): 1–8.

"Collagen Vascular Disease." Medline Plus, February 9, 2011.

Fratzl, Peter. Collagen: Structure and Mechanics. New York: Springer, 2008.

Myllyharju, Joahanna, and Kari Kivirkko. “Collagen and Collagen-Related Diseases.” Annals of Medicine 33 (2001): 7–21.

"Questions and Answers about Heritable Disorders of Connective Tissue." National Institute of Arthritis and Musculoskeletal and Skin Diseases, October 2011.

"Types of OI." Osteogenesis Imperfecta Foundation, 2012.

Whitford, David. Proteins: Structure and Function. Hoboken, N.J.: John Wiley & Sons, 2005.