Principles of Biochemistry Volume 1 - A. Lehninger 1985

Biomolecules
Fibrous Proteins
The structure of elastin imparts unique properties to elastic tissue

The major types of Connective Tissue rich in Elastin, yet containing a small amount of Collagen, include the yellow elastic tissue of ligaments and the elastic connective tissue layer found in the walls of large Arteries. These resilient arterial walls help distribute The Heart-pumped Blood throughout The Vascular System and smooth out the pulse pressure fluctuations generated by cardiac contractions. Elastin-containing connective tissue comprises a fibrillar protein that shares several properties with collagen while differing markedly in others. The primary subunit of elastin fibrils—tropoelastin—has a Molecular Weight of approximately 72,000 and consists of about 800 amino acid residues. Like collagen, elastin is rich in Glycine and Alanine. However, tropoelastin differs from tropocollagen by containing a high proportion of Lysine residues and relatively few Proline residues. Tropoelastin forms a unique type of helix that differs from both the α-Helix and the Collagen helix. The tropoelastin molecule consists of glycine-rich helical segments separated by shorter regions containing lysine and alanine residues.

Class="center">

Fig. 7-15. One type of cross-link between parallel collagen chains. Such bonds are formed enzymatically by linking two lysine residues belonging to adjacent chains.

Fig. 7-16. Desmosine residue, a unique amino acid found exclusively in elastin. Its central cyclic Structure is formed through the interaction of the R groups (highlighted in red) of four lysine residues, resulting in cross-links between The polypeptide chains of elastin (see Fig. 7-17).

Fig. 7-17. Tropoelastin molecules that make up the interconnected network of elastin polypeptide chains.

A. Segment of a tropoelastin molecule. B. The exact structure of elastin has not been fully elucidated. It is known, however, that tropoelastin molecules are cross-linked to form a continuous two- or three-dimensional network possessing a high degree of elasticity. In addition to desmosine residues (highlighted in color), which link two, three, or four tropoelastin molecules together as shown in the figure, elastin contains Other types of cross-links (also highlighted in red).

The helical regions stretch under tension and snap back to their original length when the load is removed. The regions containing lysine residues participate in The formation of covalent cross-links. Four lysine R groups come into close proximity and are enzymatically converted into desmosine (Section 5.10), The structure of which is shown in Fig. 7-16, or into the structurally similar isodesmosine. In this manner, tropoelastin polypeptide chains can assemble into systems capable of undergoing reversible stretching in all directions (Fig. 7-17).