Shampoo advertisements have made us aware of collagen as a structural protein found in our hair. However, collagen is found in many more tissues. In reality, it makes up approximately 30 percent of the protein mass of our bodies. The essential role of collagen as a structural component of tissues is made even more apparent by the occurrence of diseases associated with collagen damage.
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Collagen is found in all the connective and supportive structures of the body. It is found in particularly high amounts in tissues that need to be strong, flexible or both, including cartilage, bones and tendons. Collagen can allow a tissue to withstand immense pulling pressure without stretching -- for example in a tendon transmitting the pull of a muscle to a bone. It can also be elastic, as in skin, and flexible, as in a tendon bending around a joint.
Each fiber of collagen contains thousands of individual collagen molecules that are bound together by cross-linking and staggered covalent bonds. Covalent bonds are the strongest bonds that can exist among protein molecules. The collagen molecules themselves are made from 3 individual polypeptides or strings of amino acids. The strands wind around one another in an alpha-helix. The helix forms because of the regular amino acid sequence of the strands. The sequence is a repeating pattern of glycine-proline-X, where X can be any amino acid.
Twenty eight different types of collagen have been identified in vertebrates. Collagen types I to IV are the most prevalent. The unique properties of each type are due to segments in the collagen molecules that disrupt the helical structure. These are caused by the amino acids in the X positions of the polypeptide sequence. Different tissues of the body contain different amounts of each type of collagen; for example, cartilage contains a lot of type II, whereas type IV is mostly found in basement membranes.
A number of diseases are associated with collagen abnormalities or damage. These can be acquired or caused by a genetic mutation. Ehlers-Danlos syndrome is a genetic disease that results in abnormal collagen production. Symptoms include bruising and hyperelasticity of the skin. Scurvy is an acquired collagen-associated disease caused by vitamin C deficiency. Vitamin C is needed to produce essential amino acids used to make collagen. Without it, collagen fibers cannot be repaired, and blood vessels, tendons and bones become fragile.