Structure

The  major  structure  of  every  polypeptide  in  collagen  is  characterized  through  a repeating tripeptide sequence of Gly–X–Y where X is frequent but not exclusively, Y and Pro is often Hyp. Every of the three polypeptide chains in collagen is some

                 Figure: Formation of hydroxyproline and hydroxylysine.

1000 residues long and they each fold up into a helix which has only 3.3 residues per turn, certain than the 3.6 residues per turn of an α-helix. This secondary structure is distinctive to collagen and is frequently called the collagen helix. The three  polypeptide  chains  lie wind  round  and parallel  one another  with  a slight right-handed  rope-like twist to form a triple-helical  cable. Each third residue  of every polypeptide  passes by  the center of the triple helix, that  is  so  crowded  which is  only  the  small  side-chain  of  Gly  can  ?t  in.  This describes the absolute needs for Gly at every third residue. This residues in the X and Y positions are situated on the outside of the triple-helical  cable where there  is room  for  the  bulky  side-chains  of  Pro  and  other  residues.  These  three polypeptide  chains  are also  staggered  so in which  the  Gly  residue  in one  chain  is aligned  with  the X residue  in the second  and  the Y residue  in the third.  The triple helix is held together through an extensive network of hydrogen bonds, in particular among the primary amino collection of Gly in one helix and the major carboxyl set of Pro in position X of one of the another helices. Additionally, the hydroxyl collection of Hyp residues participates in stabilizing the structure.  The relatively in?exible Hyp and Pro also confer rigidity on the collagen structure.

The value of Gly at every third residue is shown when a mutation in the DNA encoding Type I collagen leads to the incorporation of a various amino acid at just one position in the 1000 residue polypeptide chain. For instance, if a mutation  leads  to  the  incorporation  of  Cys  in the place of  Gly  the  triple  helix  is disrupted as the -CH₂-SH side-chain of Cys is too huge to ?t in the interior of the triple helix. This process leads to a partly unfolded structure which is susceptible to excessive glycosylation   and hydroxylation and is not ef?ciently   secreted through the ?broblast cells.  In this, turn results in a defective collagen structure which can provide rise to skeletal deformities and brittle bones.  A overall spectrum  of like mutations  are  known  that  cause  the  production  of  defective  collagen  and give the result in osteogenesis imperfecta or brittle bones.

                Figure: Arrangement of the three polypeptide chains in collagen.