Elastin elasticity, models

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. 

This coacervation process forms the basis for the self-assembly, which takes place prior to the crosslinking. The assembly of tropoelastin is based on an ordering process, in which the polypeptides are converted from a state with little order to a more structured conformation [8]. The insoluble elastic fiber is formed via the enzymatic crosslinking of tropoelastin (described in Sect. 2.1). Various models have been proposed to explain the mechanism of elasticity of the elastin fibers. 

The fundamental driving force behind the remarkable elastic properties of the elastin polymer is believed to be entropic, where stretching decreases the entropy of the system and elastic recoil is driven by a spontaneous return to maximum entropy. The precise molecular basis for elasticity has not been fully elucidated and a number of models exist. Two main categories of structure-function models have been proposed those in which elastin is considered to be isotropic and devoid of structure, and those which consider elastin to be anisotropic with regions of order (Vrhovski and Weiss, 1998). 

Weis-Fogh, T., and Anderson, S. O. (1970). New molecular model for the long-range elasticity of elastin. Nature 227, 718-721. 

Several models of the macromolecular structure of elastin have been suggested to account for its elasticity cross-linked globular elastin subunits, cross-linked... 

In one study, a model for elastin, the main protein that confers elasticity on solid structures in mammals, had its mobility investigated by examining 1H-13C and 1H dipolar couplings extracted from isotropic-anisotropic correlation experiments.29 The elastic properties of elastin are almost certainly conferred by molecular degrees of freedom, so such studies are important in understanding how this material works in Nature. The motional amplitudes determined from these experiments were found to depend upon the degree of hydration, with the mean square fluctuation angles found to be 11-18° in the dry protein and 16-21° in the 20% hydrated protein. 

Uny et also reported the chemical synthesis of protein polymers based on the (Val-Pro- Ala-Val-Gly) repeat sequence in which glycine is replaced by the D-alanine residue. The hetero-chiral Pro- Ala diad would be erqrected on the basis of stereochemical considerations to adopt a type-II p-tum conformation. Stmctural analyses of small-molecule "Pro- Ala turn models support the formation of the type-II p-mm conformation in solution and the solid state. Polymers based on the (Val-Pro- Ala-Val-Gly) repeat sequence display a thermo-reversible phase transition similar to the corresponding polypeptides derived from the parent (Val-Pro-Gly-Val-Gly) sequence, albeit with a shift of the Tt to approximately 5-10 ° G below the latter due to a slight inaease in hydrophobic character due to the presence of the alanine residue. NMR spectroscopic analyses of the (Val-Pro- Ala-Val-Gly) polymer suggest that the repeat unit retains the p-tum stmcture on the basis of comparison to the corresponding behavior of the (Val-Pro-Gly-Val-Gly) polymer. Stress-strain measurements on cross-linked matrices of the (Val-Pro- Ala-Val-Gly) polymer indicate an elastomeric mechanical response in which the elastic modulus does value in comparison to the (Val-Pro-Gly-Val-Gly) polymer. These smdies of glycine suhstitution support the hypothesis that type-II p-tum formation can he associated with the development of elastomeric behavior with native elastins and elastin-derived polypeptide sequences. Several investigators have proposed that the (Val-Pro-Gly-Val-Gly) pentapeptide represents the minimal viscoelastic unit... 

D.W. Urry,T. Hugel, M. Seitz, H. Gaub, L. Sheiba, J. Dea, J. Xu, L. Hayes, F. Prochazka, and T. Parker, Ideal Protein Elasticity The Elastin Model. In Elastomeric Proteins Structures, Biomechanical Properties and Biological Roles P.R. Shewry, A.S. Tatham, and A.J. Bailey, Eds. Cambridge University Press, The Royal Society Chapter Four, pages 54-93,2003. 

Early reviews are available on the development of the molecular conformation of the tetrapep-tide, pentapeptide, and hexapeptide repeating sequences of elastin and on the mechanism of elasticity of the basic elastic-contractile model protein. These may be sought for historical background. Recent reviews of the mechanism of elasticity and its relation to contractility may be examined for a more... 

T. Weis-Fogh and S.O. Andersen, New Molecular Model for the Long-range Elasticity of Elastin. Nature, 111, 718-721,1970. 

Description of the mechanics of elastin requires the understanding of two interlinked but distinct physical processes the development of entropic elastic force and the occurrence of hydrophobic association. Elementary statistical-mechanical analysis of AFM single-chain force-extension data of elastin model molecules identifies damping of internal chain dynamics on extension as a fundamental source of entropic elastic force and eliminates the requirement of random chain networks. For elastin and its models, this simple analysis is substantiated experimentally by the observation of mechanical resonances in the dielectric relaxation and acoustic absorption spectra, and theoretically by the dependence of entropy on frequency of torsion-angle oscillations, and by classical molecular-mechanics and dynamics calculations of relaxed and extended states of the P-spiral description of the elastin repeat, (GVGVP) . The role of hydrophobic hydration in the mechanics of elastin becomes apparent under conditions of isometric contraction. 

Figure 2. Schematic representation of phase diagrams for several model elastic proteins based on the elastin repeat, (GVGVP) (see text for discussion). Solid curves adapted with permission from Sciortino et al (1990) and (1993).

In an ideal or perfect elastomer the energy repeatedly invested in extension is repeatedly and completely recovered during relaxation. Ideality increases as the elastic force results from a decrease in entropy upon extension, because this occurs without stressing bonds to the breaking point. Elastin models and elastin itself in water provide examples of such entropic elastomers with about 90% of the elastic force being entropic, that is, the /e//ratio of Equation (4) is about 0.1. This is essential to human life expectancy, because the half-life of elastin in the mammalian elastic fiber is on the order of 70 years. This means that the elastic fibers of the aortic arch and thoracic aorta, where there is twice as much elastin as collagen, will have survived some billion demanding stretch-relaxation cycles by the start of the seventh decade of life. This represents an ultimate in ideal elasticity. 

Figure 5. AFM single-chain force-extension curves of the model elastic proteins based on elastin, Cys-(GVGVP) ,a5i-Cys (A) and Cys-(GVGIP) ,<26o-Cys (B), showing the ideal elasticity due to perfect reversibility of traces 2 and 5 from bottom of A and marked hysteresis in the lower trace of B. See text for discussion. Reproduced with permission from Urry et al. (2002a).

ELRs are a promising model of biocompatible protein-based polymers. The basic structure of ELRs involves a repeating sequence based on the recurring sequences found in the mammalian elastic protein elastin [4]. As far as their properties are concerned, some of their main characteristics are derived from those of the natural protein. Elastin is an extracellular matrix protein that is present in aU vertebrate connective tissue. Its functions include the provision of elasticity and resilience to tissues, such as large elastic blood vessels (aorta), elastic ligaments, lung and skin, which are subjected to repetitive and reversible deformation [5, 6]. 

---