Structure of Collagen

Collagen is the oldest and most abundant extracellular matrix protein which finds many applications in the food, cosmetic, pharmaceutical, and biomedical industries. The collagen molecule is comprised of a triple helical region and two nonhelical regions at either end of the helix. The triple-helix conformation, the defining structural element of all collagens, comprises three separate polypeptide chains, each in a left-handed polyproline II-like helix, that are wound together into a tight, right-handed superhelix.

Fig. 1 Structure of collagen

A consequence of the tightly wound conformational structure is that every third residue in the chain resides inside the triple helix structure and only the smallest amino acid, Gly (glycine), can be accommodated. This results in the characteristic repeating amino acid sequence (Gly-Xaa-Yaa) observed in the sequences of all triple helix structural domains. In addition to close packing of the three chains, the high content of proline (Pro) and hydroxyproline (Hyp) promotes the polyproline- ii structure of the individual chains, thus stabilizing the triple helix structure ^[1].

During the biosynthesis of collagen chains, Pro residues added into the Yaa position of the Gly-Xaa-Yaa repeating sequence are post-translationally modified by prolyl-4-hydroxylase (P4H) to generate Hyp. The high content of Hyp, which usually accounts for more than 10% of all residues, is a distinctive feature of animal collagens. Hyp residues provide additional thermal stability to the triple helix structure over Pro residues due to stereoelectronic effects and hydrogen bonding interactions with the hydration network. In addition, Hyp is associated with the nucleation and efficiency of triple-helix folding and may also be critical for collagen fibril formation and collagen interaction with certain receptors and ECM proteins.

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