Collagen - an example for fibrous protein
Collagen is the most abundant protein found in humans. Unlike globular structures discussed above, collagen forms a long coiled fibrous structure. Each collagen molecule consists of three polypeptide chains which forms an elongated triple helical structure as described in figure 3.16
The amino acid sequence of these polypeptide chains are always repeating units of Gly-X-Y. Where X is often proline and Y is either hydroxyproline or hydroxylysine. The hydroxyl group of hydroxylysine can also be glycosylated with glucose or galactose.
There are various types of collagen which can be broadly categorized into three groups.
a. The fibril forming collagens present in skin, bone, cartilages, tendons and blood vessels etc provide tensile strength to the corresponding tissues.
b. The network forming collagens form network like structures beneath the membranes providing them mechanical strength.
c. The fibril associated collagens connect two fibril forming collagens or a fibril forming collagen with other components of extra cellular matrix.
More than 1000 disease causing mutations have been identified both directly in collagen genes or in the genes of enzymes involved in the synthesis of collagen. The diseases associated with collagen malfunction are known as collagenopathies. Elhers-Danlos syndrome (EDS) is a prominent collagenopathy which is due to inherited mutations in collagen processing enzymes. Stretchy skin, loose joints and vascular problems are associated with EDS.
Osteogenesis Imperfecta (O.I) is another prominent collagenopathy characterized by brittle bones, hunch back, twisted spine and retarded wound healing. The mutations in glycine residues of collagen and thereby an improper triple helical structure leads to this disease.