Procollagen molecule structure

Question Once the procollagen molecule is hydroxylated, glycosylated, and coiled into a triple helix, how is it processed to form higher structures ... 

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. 

To understand how structural defects in collagen could cause OI, we need to follow the collagen fiber synthesis pathway (Fig. 3-3). It starts from synthesis of precursor (procollagen) chains. Association of two pro-al(I)- and one pro-a2(I)-chains at their C-propeptides initiates procollagen folding, which proceeds in a zipperlike manner from C- to N-terminal end of the molecule (Engel and Prockop, 1991).These processes occur inside the endoplasmic reticulum (ER) and are assisted by a variety of ER chaperones and enzymes (Lamande and Bateman, 1999). 

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