Elastin Confers Extensibility Recoil On Lung Blood Vessels Ligaments

Elastin is a connective tissue protein that is responsible for properties of extensibility and elastic recoil in tissues. Although not as widespread as collagen, elastin is present in large amounts, particularly in tissues that require these physical properties, eg, lung, large arterial blood vessels, and some elastic ligaments. Smaller quantities of elastin are also found in skin, ear cartilage, and several other tissues. In contrast to collagen, there appears to be only one genetic type of elastin, although variants arise by alternative splicing (Chapter 37) of the hnRNA for elastin. Elastin is synthesized as a soluble monomer of 70 kDa called tropoelastin. Some of the prolines of tropoelastin are hydroxylated to hydroxy-proline by prolyl hydroxylase, though hydroxylysine and glycosylated hydroxylysine are not present. Unlike collagen, tropoelastin is not synthesized in a pro- form with extension peptides. Furthermore, elastin does not contain repeat Gly-X-Y sequences, triple helical structure, or carbohydrate moieties.

After secretion from the cell, certain lysyl residues of tropoelastin are oxidatively deaminated to aldehydes by lysyl oxidase, the same enzyme involved in this process in collagen. However, the major cross-links formed in elastin are the desmosines, which result from the condensation of three of these lysine-derived aldehydes with an unmodified lysine to form a tetrafunctional crosslink unique to elastin. Once cross-linked in its mature, extracellular form, elastin is highly insoluble and extremely stable and has a very low turnover rate. Elastin exhibits a variety of random coil conformations that permit the protein to stretch and subsequently recoil during the performance of its physiologic functions.

Table 48-5 summarizes the main differences between collagen and elastin.

Deletions in the elastin gene (located at 7q11.23) have been found in approximately 90% of subjects with Williams syndrome, a developmental disorder affecting connective tissue and the central nervous system. The mutations, by affecting synthesis of elastin, probably play a causative role in the supravalvular aortic stenosis often found in this condition. A number of skin diseases (eg, scleroderma) are associated with accumulation of elastin. Fragmentation or, alternatively, a decrease of elastin is found in conditions such as pulmonary emphysema, cutis laxa, and aging of the skin.

Table 48-5. Major differences between collagen and elastin.




Many different genetic

One genetic type



Triple helix

No triple helix; random coil

conformations permitting



(Gly-X-Y)n repeating

No ( Gly-X-Y)n repeating




Presence of hydroxylysine

No hydroxylysine



No carbohydrate


Intramolecular aldol

Intramolecular desmosine




Presence of extension

No extension peptides present

peptides during bio-

during biosynthesis


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