Secretion and aggregation

When  the  collagen  polypeptides  are  synthesized  they  have  further  amino acid  residues  (100–300)  on both  their  C and N  termini  which  are  absent  in the mature  collagen  ?ber in figure. These extension   peptides frequently contain Cys residues, that are commonly absent from the remainder of the polypeptide chain. The  addition  peptides  help  to align  properly  the  three  polypeptides  as they come together in the triple helix a procedure which may be aided through the formation of disul?de bonds among extension peptides on neighboring  polypeptide  chains. The addition peptides also prevent the premature aggregation of the procollagen triple helices within the cell. On secretion out of the ?broblast the addition peptides are erased through the action of extracellular peptidases that are shown in Figure. The resulting tropocollagen molecules then aggregate together in a staggered head- to-tail arrangement in the collagen ?ber.

Figure: 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 ?ber.