Chapter 4: The biosynthetic pathway in the cell.

Various intracellular and extracellular events are involved in the formation of a collagen fibre. First of all a transcript of the collagen genes, located on the long arms of human chromosomes 7 (a₂(I)) and 17 (a₁(I)), has to be made. The product of transcription, the mRNA, contains the protein coding exons divided by about 50 introns. After capping, tailing and methylation of the mRNA to protect it from hydrolysis, the non-coding introns are removed in at least 50 splicing steps (Prockop and Kivirikko, 1984). The exons *), all beginning with a codon for Gly and ending with a codon for the Y-position, now form an active mRNA with less than 40% of the original length. This mRNA is going from the nucleus of the cell to the rough Endoplasmatic Reticulum (ER) in the cytoplasm for translation. Actual synthesis of the protein starts from this moment on ribosomes on the ER.

Already during the synthesis of the a-chain, selected prolines and lysines are hydroxylated at the propagating chain (Cardinale and Udenfriend, 1974). Also glycosylation takes place at this moment. These post-translational modifications require at least eight specific enzymes (Prockop and Kivirikko, 1984) to modify up to 100 amino acids in each a-chain.

Three new synthesized a-chains assemble and fold together with help of intra- and interchain disulfide bonds between cysteines in the C-terminal propeptides of the three different a-chains. Once a triple-helix is formed, post- translational modifications of the chains ceases. The triple-helix 'procollagen' molecule is now ready for secretion. Procollagen is collagen that posses 'propeptides'. These are also called 'telopeptides' or 'registration peptides'. They exist of the signal peptides for the ER and the extension peptides. The last are necessary for the time controlled, extracellular activation to prevent premature fibre formation.

With help of small secretory vesicles the procollagen molecules are transported from the ER-compartment over the cell membrane to the extracellular space. From this moment on no substances are bonded to the collagen proteins anymore.

After secretion of procollagen, the N-propeptides are cleaved by specific proteinases. The C-propeptides by separate enzymes *). For every type of collagen and for both the C- and the N-terminal end a different peptidase is needed. The collagen monomers (=tropocollagen) then self- assemble into fibrils near the cell surface.

Finally, lysyl oxidase deaminates some of the lysine and hydroxylysine residues. It converts them to aldehyde derivates. These form spontaneously covalent aldol cross- links with similar residues in adjacent molecules. Some of these bonds are modified to a variety of even more stable cross-links, but this is tissue dependent. Hydroxypyridinium cross-links are formed between head and tail of neighbouring collagens. The type of bonds in cross- links are unique to collagen. Collagen cross-links are at precise sites which have been identified as the amino acid residues 9, 103, 946 and 1047 (Tanzer, 1985). The extent and type of cross-linking varies with the physiological function and age of the tissue. Type I collagen results in mature collagen fibres.