Collagens (types I, II, III, IV and V) are synthesized as precursor molecules called procollagens. These contain propeptide sequences, at both the amino-terminal and the carboxy-terminal ends. The function of these propeptides is to facilitate the winding of procollagen molecules into a triple-helical conformation within the endoplasmic reticulum. The propeptides are cleaved off from the collagen triple helix molecule during its secretion. The triple helix collagens then polymerize into extracellular collagen fibrils.
