The definite structural features of all collagen molecules is the triple helix. This unique confirmation is due to three polypeptide chains (α -chains) twisted around each other into a right handed major helix. [See Fig]
Extending from the amino and carboxyl terminal ends of both helical domains of the α -chains are nonhelical components called telopeptides. In the major interstitial collagens, the helical domains are continuous, whereas in the other collagen classes the helical domains may be interrupted by 1 to 12 nonhelical segments.
The primary structure of the helical domain of the α-chain is characterized by the repeating triplet Gly-X-Y. X and Y can be any amino acid but are most frequently praline and hydroxyproline, respectively. Overall, approximately 25 percent of the residues in the triple helical domains consist of praline and hydroxyproline. Hydroxylysine is also commonly found.
In the most abundant interstitial collagens (i.e., type I, II), the triple helical region contains about 1000 amino acid residues, (Gly-X-Y)333..
Major Collagen Classes
Fibril-forming (interstitial)-types I, II, III, V, XI. The most abundant collagen class, these collagens form the extracellular fabric of the major connective tissues. They have the same tensile strength as steel wire.
Fibril –associated collagens with interrupted triple helices (FACIT)-types IX, XII, XIV, XVI, XIX. These collagens are associated with the interstitial (fibrillar) collagens and occur in the same tissues.
Collagens with specialized structure or functions:
o Basement membrane collage – type IV
o Nonfibrillar collagen – types VI, VII, XIII, XV, XVII, XVIII
o Short-chain collagen – types VIII, X
