Procollagen 1 extension peptides

Amino- and carboxyl-terminal Procollagen 1 extension peptides By-product of collagen biosynthesis... 

Fig. 4. Role of the extension peptides in the folding and secretion of procollagen. Once secreted out of the cell, the extension peptides are removed and the resulting tropocollagen molecules aggregate and are cross-linked to form a microfibril.

The individual chains are synthesized as procollagen a chains, which, with a mass of 150,000 Da, have additional extension peptides at both the amino and carboxyl termini. The amino- and carboxyl-terminal regions from the three a chains each fold to form globular structures, which then interact to guide the formation of the triple helix. Interchain disulfide bonds stabilize the carboxyl-terminal domain and the triple helix coils up from this domain (Fig. 5-15). 

The procollagen molecule is secreted from the cell, and the extension propeptides are excised by two specific procollagen peptidases to form the tropocollagen molecule. The removal of the peptides (Mr = 20,000 and 35,000) allows the tropocollagen molecules to self-assemble to form fibrils. This assembly is regulated to some extent by the cells and by other extracellular components to produce the wide variety of structures found in collagen fibers. 

---