Sequence of amino acid in collagen ?

In order to form a triple-helix, a polypeptide chain (a-chain) must contain glycine as every third residue in the sequence.This is because only the glycine is small enough to be accommodated in the limited space available down the central core of the triple helix.Each turn of polypeptide chain (a-chain) contains three amino acid residues, and glycine (Gly) is present at every third position.Thus glycine constitutes 33% of the total amino acid residues.The repeating amino acid residues, represented as (Gly-X-Y)n, is an absolute requirement for formation of triple helix.X and Y can be any amino acids, but most of the time X is proline (10% of the total amino acid residues) and most of the time Y is hydroxyproline.Other impoant amino acids found in collagen are lysine and hydroxylysine.