Composition and post-translational modi?cations

Composition of the amino acid of collagen is quite unique.  Nearly one-third of its residues are Gly although another one-quarter are Pro signi?cantly higher proportions than are found in another proteins. The hydroxylated amino acids 4- hydroxyproline Hyp and 5-hydroxylysine Hyl are found only in collagen. These hydroxylated amino acids are formed from the parent amino acid through the action of lysine hydroxylase and proline hydroxylase, respectively. These enzymes have a Fe²⁺ ion at their active site and needs ascorbic acid like vitamin C for activity.  The ascorbic acid behave as an antioxidant keeping

                      Figure: Overview of the biosynthesis of collagen.

the Fe²⁺ ion in its decrease state. The lysine hydroxylase and Proline hydroxylase are dioxygenases by using a molecule of O₂. The citric acid cycle α-Ketoglutarate intermediate is an obligatory substrate and is converted to succinate in during of the reaction. Both  enzymes  will hydroxylate  only Lys and Pro residues  which are incorporated  in a polypeptide  chain and then only when the residue  is on  the  N-terminal  side  of Gly.  The Hyp is most important in stabilizing the structure of collagen by hydrogen bond formation. In vitamin C de?ciency, Hyp and Hyl are not synthesized resulting in the weakening of the collagen ?bers. This leads to the fragile blood vessels, skin lesions and poor wound healing which are characteristic of the disease scurvy.

The other post-translational modi?cation which occurs to collagen is glycosylation. In this case the sugar  residues,  commonly  only galactose, glucose  and their disaccharides   are  attached  to  the  hydroxyl  group  in  the  newly  formed  Hyl residues,  rather  than to Asn or Ser or Thr residues  as occurs  in the more  broad- spread  N- and O-linked  glycosylation  (see Topic H5). The quantity of attached carbohydrate in collagen varies from 0.4 to 12 percent through weight depending on the tissue in that it is synthesized.