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Collagen

From Greek: kolla (κόλλα) - glue
German: Collagen

1 Definition

Collagen is a fibrous protein and the most abundant protein in human body. It represents 25-30 % of the weight of all body proteins. Moreover it is the major protein found in connective tissue and provides structure and strenght to the following body tissues: teeth, bones, tendons, cartilage, skin, cornea and blood vessels.

2 Types of collagen

Until now, 28 types of collagen are discovered. They can be subdivided into:

  • fibrous
  • non-fibrous

The most common types of collagen are:

Collagen type I is the most common type (over 90% of the whole body collagen)

3 Structure of collagen

Collagen is a protein that has a triple helix structure composed of three polypeptide chains (called α-chains/peptides). Each α-chains has an left-handed conformation while all three together form the right-handed triple helix. These triple helical arranged α-chains are interconnected via covalent bonds (called cross links) and hydrogen bonds and they are composed of about 1000 amino acids.

α-chain composition in relation to the types of collagen:

Type of collagen Chains composition
I two α1 chains and one α2 chain
II three α1 chains
III three α1 chains
IV three α1 chains or three α2 chains

Collagen is rich in gycine, proline, hydroxyproline and hydroxylysine (both are derived amino acids formed by hydroxylation of proline or lysine respectively). Glysine is expected every third amino acid in the collagen strutrure and proline makes up about 17% in collagen.

As an overall structure of collagen the following stringing togethers are assumed:

  • Glysine - Proline - X
  • Glysine - X - hydroxyproline/hydroxylysine

(X could be any amino acid present in proteins)

4 Synthesis of collagen

First an inactive procollagen is synthesised within fibroblasts, osteoblasts or chondroblasts. Important steps inside these cells:

  • synthesis of the three α-chains (polypeptide chains)
  • hydroxylation of various proline and lysine amino asids to hydroxyproline and hydroxylysine (reaction catalyzed by prolyl - and lysyl hydroxylase, enzymes which use Vitamin C as coenzyme)
  • glycosylation of hydroxylysine residues (usually addition of glucose or galactose)
  • formation of characteristic triple helix (after glycosylation)

Now the procollagen consist of a triple helical center framed by a nonhelical structure called propeptides, telopeptides or extension peptides. The telopeptides contain a lot of cystein residuse which form disulfide bonds.

After these processes the procollagen is released from the cell (fibroblasts, chondroblasts, osteoblasts) and the telopeptides are cleaved by an enzyme called procollagen peptidase. In the last stage of collagen synthesis the triple helical collagen molecules form collagen fibrils which again are stabilized by intermolecular cross links.

5 Pathophysiology

Collagen synthesis is Vitamin C dependent, better the hydroxylation reaction of proline and lysine is Vitamin C dependent. Vitamin C acts as coenzyme for prolyl hydroxylase and lysyl hydroxylase. Thats the reason why deficiency in Vitamin C leads to scurvy.

6 Diseases

7 References

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