Translations:Collagen/23/en

1. Inside the cell
   1. Two types of alpha chains – alpha-1 and alpha 2, are formed during translation on ribosomes along the rough endoplasmic reticulum (RER). These peptide chains known as preprocollagen, have registration peptides on each end and a signal peptide.
   2. Polypeptide chains are released into the lumen of the RER.
   3. Signal peptides are cleaved inside the RER and the chains are now known as pro-alpha chains.
   4. Hydroxylation of lysine and proline amino acids occurs inside the lumen. This process is dependent on and consumes ascorbic acid (vitamin C) as a cofactor.
   5. Glycosylation of specific hydroxylysine residues occurs.
   6. Triple alpha helical structure is formed inside the endoplasmic reticulum from two alpha-1 chains and one alpha-2 chain.
   7. Procollagen is shipped to the Golgi apparatus, where it is packaged and secreted into extracellular space by exocytosis.
2. Outside the cell
   1. Registration peptides are cleaved and tropocollagen is formed by procollagen peptidase.
   2. Multiple tropocollagen molecules form collagen fibrils, via covalent cross-linking (aldol reaction) by lysyl oxidase which links hydroxylysine and lysine residues. Multiple collagen fibrils form into collagen fibers.
   3. Collagen may be attached to cell membranes via several types of protein, including fibronectin, laminin, fibulin and integrin.