A typical collagen molecule is long, stiff, extracellular structure in which
three polypeptides are wound around one another in a rope like triple helix.
The chains are held together by hydrogen bonds. Variations in the amino acid sequence
of the alpha chain result in collagen molecules with slightly different
properties.
Type I,II and III collagens are fibrillar, a and are found in
skin, tendon, bone cornea and blood vessels. Type IX and XII are fibril
associated and are found in cartilage, tendon and ligaments. Type IV and VII
from networks in basement membrane and beneath stratified squamous epithelia.
Collagen is rich in glycine and proline. The glycine residues
are part of a repeating sequence-Gly-X-Y-where X is frequently proline and Y is
often hydroxyproline and hydroxyllysine.
Hydroxyproline and hydroxyline result from the hydroxylation
by specific hydroxylases of proline and lysine residues after their
incorporation into alpha chains. The enzymes require ascorbic acid as a cofactor.
The hydroxyl group of the hydroxylysine residues of collagen may be enzymatically
glycosylated.
The precursors of collagen alpha chains are formed in
fibroblasts, osteoblasts and chondrobalsts, and travel via the endoplasmic
reticulam and Golgi to the extracellular matrix. There, the N terminal and C
terminal properties are removed by procollagen peptides. In some collagen, the
collagen molecules self assemble into fibrils in which the adjacent triple
helices are arranged in a staged pattern, each overlap ping its neighbor by a
length approximately three quarters of a molecules. The triple helices are then
cross linked, giving the fibrillar array great tensile strength.
