Elastin

Some tissues, such as ligaments and arterial blood vessels, need highly elastic fibers. Such tissues contain large amounts of the fibrous protein elastin.

The polypeptide chain of elastin is rich in glycine, alanine, and valine and is very flexible and easily extended. In fact, its conformation probably approximates that of a random coil, with little secondary structure at all. However, the sequence also contains frequent lysine side chains, which can be involved in cross-links. These cross-links prevent the elastin fibers from being extended indefinitely, causing the fibers to snap back when tension is removed. The cross-links in elastin are rather different from those in collagen, for they are designed to hold several chains together. Four lysine side chains can be combined to yield a desmosine cross-link: (see here)

Because the carbons of four separate chains are connected, only a small amount of such cross-linking is needed to convert elastin fibers into a highly interconnected, rubbery network.

See also: Fibrous Proteins