Molecular Biology: Protein Structure and Function - Stepanov V.M. 2005

Fibrous proteins
Elastin

Elastin is an insoluble fibrillar protein that plays a vital role in The formation of elastic Tissues, particularly the walls of Arteries, Lungs, and ligaments. The polypeptide chain of its precursor, tropoelastin—synthesized by fibroblasts just like Collagen—consists of approximately 760 amino acid residues. More than half of these are Glycine and Alanine, a quarter are typically hydrophobic Amino Acids (with valine being especially abundant), while the number of hydrophilic amino acids is very small. The Proline content in elastin (11%) is significantly lower than in collagen. Some of the proline residues are hydroxylated, but hydroxyproline apparently does not play a significant role in elastin and is likely formed incidentally as a consequence of the high activity of prolyl hydroxylase in fibroblasts, where collagen is also synthesized.

The Introduction/19.html">Primary Structure of elastin is characterized by an Abundance of repeating Sequence Motifs. For instance, the sequence Pro—Gly—Val—Gly—Val appears at least 11 times in succession within elastin. Elastin is constructed from A large number of alternating hydrophobic and hydrophilic segments of the polypeptide chain, which correspond to a multitude of relatively short exons in the structural Gene of this protein. The hydrophilic regions are enriched in Lysine.

Apparently, the hydrophobic regions are responsible for aggregation, which is extremely characteristic of elastin. The hydrophilic regions, in turn, determine the formation of intermolecular cross-links that render aggregation irreversible. Approximately in the middle of each hydrophilic segment, there are lysine residues separated by two to three residues of Other Amino Acids—often alanine—such as Lys—Ala—Ala—Lys, Lys—Ser—Ala—Ala—Lys—Val—Ala—Ala—Lys, and Lys—Ala—Pro—Lys.

During tropoelastin maturation, almost all lysine residues are converted into allysine through the action of lysyl oxidase, the same copper-containing enzyme that modifies collagen. The resulting aldehyde groups undergo Condensation reactions with other allysine residues, as well as with the ε-amino group of an as-yet-unoxidized lysine residue. As a result, four closely spaced lysine residues within the aggregate yield desmosine—an amino acid featuring a pyridine ring that links four Regions of the elastin peptide chain into a single junction:

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Cross-links of this type make elastin insoluble in Water, whereas the intervening segments, adopting a random coil conformation, retain The ability to undergo reversible stretching, which accounts for the extensibility of ligaments and arterial walls. Inactivation of lysyl oxidase—which can be triggered by copper ion deficiency or the blocking of pyridoxal in the enzyme's Active Site (due to a reaction with the amino group of β-aminopropionitrile, or lathyrine)—inhibits elastin maturation and leads to a loss of elasticity in ligaments and arteries, occasionally resulting in aortic rupture.