Pro-α chain formation: Collagen is one of many proteins that normally function outside of cells. Like most proteins produced for export, the newly synthesized polypeptide precursors of α chains (prepro-α chains) contain a special amino acid sequence at their amino (N)-terminal ends. This sequence acts as a signal that, in the absence of additional signals, targets the polypeptide being synthesized for secretion from the cell. The signal sequence facilitates the binding of ribosomes to the rough endoplasmic reticulum (RER) and directs the passage of the prepro-α chain into the lumen of the RER. The signal sequence is rapidly cleaved in the lumen to yield a precursor of collagen called a pro-α chain (see Fig. 4.7).
2.
Hydroxylation: The pro-α chains are processed by a number of enzymic steps within the lumen of the RER while the polypeptides are still being synthesized (see Fig. 4.7). Proline and lysine residues found in the Y-position of the –Gly–X–Y– sequence can be hydroxylated to form hydroxyproline and hydroxylysine residues. These hydroxylation reactions require molecular oxygen, ferrous iron (Fe2+), and the reducing agent vitamin C (ascorbic acid, see p. 381), without which the hydroxylating enzymes, prolyl hydroxylase and lysyl hydroxylase, are unable to function (see Fig. 4.6). In the case of ascorbic acid deficiency (and, therefore, a lack of proline and lysine hydroxylation), interchain H-bond formation is impaired, as is formation of a stable triple helix. Additionally, collagen fibrils cannot be cross-linked (see 7. below), greatly decreasing the tensile strength of the assembled fiber. The resulting deficiency disease is known as scurvy. Patients with scurvy often show ecchymoses (bruise-like discolorations) on the limbs as a result of subcutaneous extravasation (leakage) of blood due to capillary fragility (Fig. 4.8).
3.
Glycosylation: Some hydroxylysine residues are modified by glycosylation with glucose or glucosyl-galactose (see Fig. 4.7).
4.
Assembly and secretion: After hydroxylation and glycosylation, three pro-α chains form procollagen, a precursor of collagen that has a central region of triple helix flanked by the nonhelical N-and carboxyl (C)terminal extensions called propeptides (see Fig. 4.7). The formation of procollagen begins with formation of interchain disulfide bonds between the C-terminal extensions of the pro-α chains. This brings the three α chains into an alignment favorable for triple helix formation. The procollagen molecules move through the Golgi apparatus, where they are packaged in secretory vesicles. The vesicles fuse with the cell membrane, causing the release of procollagen molecules into the extracellular space.
5.
Extracellular cleavage of procollagen molecules: After their release, the triple-helical procollagen molecules are cleaved by N-and Cprocollagen peptidases, which remove the terminal propeptides, producing tropocollagen molecules.
6.
Collagen fibril formation: Tropocollagen molecules spontaneously associate to form collagen fibrils. They form an ordered, parallel array, with adjacent collagen molecules arranged in a staggered pattern, each overlapping its neighbor by a length approximately three quarters of a molecule (see Fig. 4.7).
7.