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Translational, Cotranslational, and Early Post-translational Events

After the gene is transcribed, it is spliced to remove introns and to yield a functional mRNA that contains about 3,000 bases. Specific mRNAs for each chain and collagen type are translocated to the cytoplasm and translated into proteins in the rough endoplasmic reticulum (RER) on membrane-bound polysomes.

As the collagen polypeptide is synthesized in the RER, it is modified in important ways. Two major constituents of collagen are the modified amino acids hydroxyproline and hydroxylysine, but neither of these can be directly incorporated into proteins. Instead, proline and lysine are incorporated and then modified by two hydroxylating enzymes, prolyl and lysyl hydroxylases.

These enzymes require ferrous iron, ascorbate, and α-ketoglutarate for their activity. The degree of hydroxylation differs from tissue to tissue and depends on availability of substrate, rate of synthesis, turnover, and the time during which the molecule remains in the presence of the hydroxylating enzymes. The time required for the synthesis of a complete pro α chain is about 6.7 min.

As lysyl residues in the newly synthesized pro α chains are hydroxylated, sugar residues are added to the resulting hydroxylysyl groups. Glycosylation is catalyzed by two specific enzymes, a galactosyltransferase and glucosyltransferase. Once the translation, modifications, and additions are completed, the individual pro α chains become properly aligned for the triple helix to form.

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