Synthesis

Collagen is an excellent example of the production of a complex protein structure from a simple amino acid chain. It is made in cells such as fibroblasts:

• individual polypeptide chains are synthesized on membrane-bound ribosomes according to mRNA templates; this is the pro-alpha chain
• a signal peptide on the polypeptide ensures that the chain is passed into the lumen of the endoplasmic reticulum
• proline and lysine residues on the chain are hydroxylated
• some hydroxylysine residues may be glycosylated
• a triple alpha helix of 3 polypeptide chains forms spontaneously; this is procollagen:
  • hydroxyproline forms interchain hydrogen bonds
  • hydroxylysine cross-links collagen molecules
• procollagen is secreted into the extracellular space where proteolytic enzymes cleave the terminal peptides
• resulting tropocollagen molecules spontaneously combine with other tropocollagen molecules to form linear array within collagen fibril
• collagen fibrils orientate themselves along the line of collagen fibre made from several collagen fibrils