<prompt> Sequence of amino acid in collagen ?Ala-X-YLys-X-YGly-X-YHis-X-Y  <response>: 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-Yn, 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. 