Elastin hydrophobic interactions

Elastin is not a true rubber as it is not self-lubricating. It has elastic properties only in the presence of water. At rest, elastin is tightly folded, stabilized by hydrophobic interactions between nonpolar residues this has been termed an oiled coil. On stretching, these hydrophobic interactions are broken, and the nonpolar residues are exposed to water. This conformation is thermodynamically unstable, and once the stretching force is removed, the elastin recoils to its resting state. 

Another type of inportant structural ECM proteins, the elastin, is rich in elastic tissues and organs, such as the cardiovascular and pulmonary system and skin, and of extreme importance for their proper function. The precursors of native elastin, the tropoelastins, are composed of alternating hydrophobic domains, enabling hydrophobic interaction, and lysine-rich domains, allowing for covalent cross-linking through the mediation of the enzyme lysyl oxidase. 

As described above for elastin and resilin, the ability of elastomeric proteins to exhibit elasticity relies on the molecular movement, stmctural folding, and conformational freedom of individual components so that they can instantaneously respond to the applied force within a cross-linked network to distribute the stress throughout the system. Stretching initially will interrupt interactions between the loops such as hydrophobic interactions, hydrogen bonding, and electrostatic interactions, while at higher extensions a decrease in conformational entropy will be prevalent. To date, different models are proposed to explain the mechanisms of elasticity for resilin, based on the knowledge from elasticity models that have been proposed for elastin. 

Elastin is a macromolecule synthesized as a 70,000 single peptide chain, termed tropoelastin and secreted into the extracellular matrix where it is rapidly crosslinked to form mature elastin. The carboxy-terminal end of elastin is highly conserved with the sequence Gly-Gly-Ala-Cys-Leu-Gly-Leu-Ala-Cys-Gly-Arg-Lys-Arg-Lys. The two Cys residues that form disulfide crosslinks are found in this region as well as a positively charged pocket of residues that is believed to be the site of interaction with microfibrillar protein residues. Hydrophobic alanine-rich sequences are known to form a helices in elastin these sequences are found near lysine residues that form crosslinks between two or more chains. Alanine residues not adjacent to lysine residues found near proline and other bulky hydrophobic amino acids inhibit a helix formation. Additional evidence exists for (3 structures and 3 turns within elastin thereby giving an overall model of the molecule that contains helical stiff segments connected by flexible segments. 

Volumetric data can be essential in the thermodynamic treatment of the "polymer-solvent" interaction process. The lack of them for many important fibrous proteins is due to the difficulty of measuring the density, at controlled temperature and hydration degree, for these systems. As far as elastin is concerned, it has been reported that when completely hydrated this protein has a negative and very large coefficient of thermal expansion (15), a result which has been interpreted as evidence of a hydrophobic character of the protein (16). 

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