Collagen is a fibrous protein, formed by the association of several polypeptide chains; hence it is an example of quaternary structure.
The basic unit in a collagen fiber is the tropocollagen molecule, a triple helix made of three identical polypeptide chains (A, B and C in this model), each one with approximately 1000 residues (here only 30 are displayed) and the characteristic secondary structure, the collagen helix (already studied in the secondary structure section).
Remember that the sequence consists of a Gly–X–Y repeat, where X is usually proline and Y is usually either proline or hydroxyproline.
The small sidechain of glycine (a single hydrogen atom) orients inwards, allowing a close association of the three polypeptide chains (triple helix, characteristic and exclusive for collagen). Gly residues in the three chains (A, B and C) are rendered here using a spacefilling model (spheres).
In this example, triple helix is less compact at the middle, where an alanine residue substitutes for glycine. The methyl group in alanine, bulkier than the hydrogen in glycine, prevents packing of the helices.
Contrarily to glycines, the sidechains in proline and hydroxyproline residues
are oriented outwards the triple helix, interacting with the solvent.
colour Gly, Pro, Hyp, Ala