Collagen biosynthesis and modifying enzymes

Collagen propeptides typically undergo a number of post-translational modifications. Proline and lysine residues are hydroxylated by prolyl 3-, prolyl 4- and lysyl hydroxylases. 4-hydroxyproline is essential for intramolecular hydrogen bonding and stability of the triple helical collagenous domain. Prolyl 4-hydroxylase may also have a role in alpha chain association as no association of the C-propeptides of type XII collagen was seen in the presence of prolyl 4-hydroxylase inhibitors (Mazzorana et al. 1993, 1996). In fibril forming collagens approximately 50% of prolines are 4-hydroxylated; the extent of this is species dependent, lower hydroxylation correlating with lower ambient temperature and thermal stability. Similarly the extent of 3-hydroxyproline and lysine hydroxylation varies between tissues and collagen types. Hydroxylysine molecules can form cross-links between collagen molecules in fibrils, and are sites for glycosyl- and galactosylation.

Collagen molecules fold and assemble through a series of distinct intermediates. Individual collagen polypeptide chains are translocated co-translationally across the membrane of the endoplasmic reticulum (ER). Intra-chain disulfide bonds are formed within the N-propeptide, and hydroxylation of proline and lysine residues occurs within the triple helical domain. When the peptide chain is fully translocated into the ER lumen the C-propeptide folds, the conformation being stabilized by intra-chain disulfide bonds. Pro alpha-chains associate via the C-propeptides, or NC2 domains for FACIT family collagens to form an initial trimer which can be stabilized by the formation of inter-chain disulfide bonds, though these are not a prerequisite for further folding. The triple helix then nucleates and folds in a C- to N- direction. The association of the individual chains and subsequent triple helix formation are distinct steps. The N-propeptides associate and in some cases form inter-chain disulfide bonds. Procollagen is released via carriers into the exracellular space. Fibrillar procollagens undergo removal of the C- and N-propeptides by procollagen C and N proteinases respectively, both Zn2+ dependent metalloproteinases. Propeptide processing is a required step for normal collagen I and III fibril formation, but collagens can retain some or all of their non-collagenous propeptides. Retained collagen type V and XI N-propeptides contribute to the control of fibril growth by sterically limiting lateral molecule addition. Processed fibrillar procollagen is termed tropocollagen, which is considered to be the unit of higher order fibrils and fibres. Tropocollagens of the fibril forming collagens I, II, III, V and XI sponteneously aggregate in vitro in a manner that has been compared with crystallization, commencing with a nucleation event followed by subsequent organized aggregation. Fibril formation is stabilized by lysyl oxidase catalyzed crosslinks between adjacent molecules.