 Collagen triple helix

A collagen triple helix is a protein structure in which three polypeptide chains wind together into a right-handed superhelix; each individual chain has a left-handed, polyproline-II-like conformation. It is characteristic of collagen and differs from a single alpha helix. The close-packed structure connects protein primary structure to a three-chain assembly: glycine commonly occurs at every third residue in the triple-helical region, conventionally written Gly–X–Y.^[ref][ref]

Glycine's small side group permits close approach near the central axis. Proline and hydroxyproline are frequent at the other positions, while interchain hydrogen bonds and interactions with water help organize the assembly. A model segment with ten Gly–X–Y repeats has 30 residues per chain and 90 across three chains; the three-chain count is distinct from the three-residue sequence period.

The structural picture developed from fiber-diffraction constraints in work by G. N. Ramachandran, G. Kartha and other investigators, rather than from directly seeing every atom in an intact tissue. Later, a synthetic collagen-like peptide provided a sharper test. Its 1994 X-ray crystallography structure, reported at $1.9$ angstrom resolution, confirmed triple-helical packing and interchain bonding. A deliberately introduced glycine-to-alanine substitution caused local untwisting and replaced direct interchain contacts near the substitution with water-mediated bridges.^[ref]

The experiment illustrates why sequence changes can affect a material: even a small side-group change can disturb local packing. The triple-helical molecule is one level of organization; the larger collagen fiber includes additional assembly beyond these three chains.