| NONCOLLAGENOUS PROTEINS IN THE EXTRACELLULAR MATRIX
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| Figure 27.4 Collagen crosslink formation. Allysine (and hydroxyallysine) are precursors of collagen crosslink formation by (A) aldol condensation and (B) Schiff base (imine) intermediates. |
The flexibility required for function of our blood vessels, lungs, ligaments and skin is contributed by a network of
elastic fibers in the ECM of these tissues. The predominant protein of elastic fibers is elastin. Unlike the multigene collagen family, there is only one gene for elastin - a polypeptide about 750 amino acids long. In common with collagens, it is rich in glycine and proline residues, but elastin is more hydrophobic: one in seven of its amino acids is a valine. Unlike collagens, elastin contains little hydroxyproline and no hydroxylysine or carbohydrate chains, and does not have a regular secondary structure. Its primary structure consists of alternating hydrophilic and hydrophobic, lysine, and valine-rich domains. The lysines are involved in intermolecular crosslinking, while the weak interactions between valine residues in the hydrophobic domains impart elasticity to the molecule.
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| Elastin can stretch in two dimensions
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| The soluble monomeric form of elastin initially synthesized on the RER is called tropoelastin. Except for some hydroxylation of proline, tropoelastin does not undergo post-translational modification. During the assembly process in the extracellular space, lysyl oxidase generates allysine in specific sequences: -Lys-Ala-Ala-Lys- and -Lys-Ala-Ala-Ala-Lys-. As with collagen, the reactive aldehyde of allysine condenses with other allysines or with unmodified lysines. Allysine and dehydrolysinonorleucine on different tropoelastin chains also condense to form pyridinium crosslinks - heterocyclic structures known as desmosine or isodesmosine (Fig. 27.5). Because of the way in which elastin monomers are crosslinked in polymers, elastin can stretch in two dimensions.
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