Collagen is the most abundant protein in mammals, making up about 25 percent of all protein in the human body. Although there are 16 types of collagen, all serve the same purpose: helping tissues withstand stretching. Collagen's triple-helix structure directly contributes to its function, and mutations in genes that code for collagen have dire consequences to the entire organism.
Structure
The basic structural unit of collagen is a triple helix. All collagen types have three-stranded helical segments; they differentiate themselves by the segments that interrupt the triple helix and fold into other three-dimensional shapes. Even though there are 16 types of collagen, more than 85 percent of the collagen in the body consists of Type I, Type II or Type III. The triple helix forms because of the amino acid sequence of the proteins that make up the strands of the helix. Each helix has a regular, repeating pattern of amino acids in the sequence of gly-pro-X, where gly stands for the amino acid glycine, pro stands for the amino acid proline and X stands for any other amino acid.
Strength
Short segments at the end of collagen chains contain an amino acid called hydroxylysine. These amino acids form cross-links that stabilize the side-by-side packing of collagen chains, creating fibrils. Fibrils are extremely strong, even when significantly stretched. This strength is important to their function of connecting muscles to bones, where they must withstand enormous forces. According to "Molecular Cell Biology" by Lodish et. al., "Gram for gram, Type I collagen is stronger than steel."
Importance of Vitamin C
Collagen contains a large amount of two uncommon amino acids, hydroxyproline and hydroxylysine. They are made from the more common amino acids, proline and lysine, through enzymatic processes that require vitamin C. Deficiencies of vitamin C result in a disease called scurvy. Without enough vitamin C, the collagen fibers are not properly made, resulting in unstable collagen structures that are degraded by the cell. Without the structural support provided by collagen, blood vessels, tendons, and skin become fragile, and the cells are unable to make or repair collagen worn out by everyday activities. Symptoms of scurvy include loss of teeth and easy bruising.
Brittle Bone Disease
Mutations in collagen can also lead to osteogenesis imperfecta, or brittle bone disease. Because the amino acid glycine must be at every third position for the triple helix to form properly, mutations that result in amino acids other than glycine in that position produce unstable helices. When unstable helices form in the collagen associated with tendons and bones, such as Type I collagen, severely weak bone formation results.
Properties
Different collagen types form fibers, and networks of fibers, that have different properties. For example, Type II collagen forms cartilage, which has fibrils that are smaller in diameter than those of Type I collagen and are organized into a more random pattern. The resulting structure is more suitable for absorbing shocks in joints than collagen formed by the longer, straighter Type I fibers.
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