Tropoelastin coacervation

Coacervation occurs in tropoelastin solutions and is a precursor event in the assembly of elastin nanofibrils [42]. This phenomenon is thought to be mainly due to the interaction between hydro-phobic domains of tropoelastin. In scanning electron microscopy (SEM) picmres, nanofibril stmc-tures are visible in coacervate solutions of elastin-based peptides [37,43]. Indeed, Wright et al. [44] describe the self-association characteristics of multidomain proteins containing near-identical peptide repeat motifs. They suggest that this form of self-assembly occurs via specific intermolecular association, based on the repetition of identical or near-identical amino acid sequences. This specificity is consistent with the principle that ordered molecular assembhes are usually more stable than disordered ones, and with the idea that native-like interactions may be generally more favorable than nonnative ones in protein aggregates. 

The coacervation of tropoelastin plays a crucial role in the assembly into elastic fibers. This coacervation is based on the LCST behavior of tropoelastin, which causes tropoelastins structure to become ordered upon raising the temperature. The loss of entropy of the biopolymer is compensated by the release of water from its chain [2, 18, 19]. This release of water results in dehydration of the hydrophobic side chains, and this is the onset of the self-assembly leading to the alignment of tropoelastin molecules. 

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. 

Fig. 5 Coacervation curve of 40 mg/mL tropoelastin in 10 mM sodium phosphate pH 7.4, containing 150 mM NaCl. Maximal coacervation was obtained at 37 °C. Reproduced from [20] with permission from John Wiley and Sons, copyright 1997...

The process of coacervation is finely tuned to the physiological conditions of the extracellular matrix. Optimal coacervation of human tropo-elastin occurs at 37 °G, 150 mM NaCl, and pH 7-8 (Vrhovski et al, 1997). The arrangement of sequences in tropoelastin is critical to this process of coacervation, where association through hydrophobic domains depends on their contextual location in the molecule (Toonkool et al., 2001b). Tropoelastin association rapidly proceeds through a monomer to tuner transition, with little evidence of intermediate forms (Toonkool et al, 2001a). 

Jensen, S. A., Vrhovski, B., and Weiss, A. S. (2000). Domain 26 of tropoelastin plays a dominant role in association by coacervation./. Biol. Chem. 275, 28449-28454. Kagan, H. M., and Sullivan, K. A. (1982). Lysyl oxidase Preparation and role in elastin biosynthesis. Methods Enzymol. 82, 637-650. 

Recombinant tropoelastin and elastin-derived peptides and proteins have also been employed to investigate the mechanism of coacervation above the transition temperamre in vitro. For the recombinant human derivative SHELA26A, the coacervation process has been demonstrated to proceed in phases that depend... 

Despite the foregoing evidence, the type-II p-tum has not been unequivocally established as the determinative element for the development of elastomeric behavior within the elastomeric domains of elastin and other native polypeptide materials. Spectroscopic analyses of native elastin and elastin-mimetic polypeptides have provided strong evidence for the presence of alternative conformations, particularly above the inverse transition temperature, which may contribute to the elastomeric behavior of the material in its coacervate state. In addition, the elastomeric domains of native tropoelastin display quite substantial sequence diversity, although it appears that Pro-Gly sequences are conserved... 

D. Volpin, D.W. Urry, I. Pasquali-Ronchetti, and L. Gotte, Studies by Electron Microscopy on the Structure of Coacervates of Synthetic Polypeptides of Tropoelastin. Micron, 7,193-198,1976. 

Bressan, G. M., Castellani, I., Giro, G. M., Volpin, D., Fomieri, C., and Pasquali-Ronchetti, I., 1983, Banded fibers in tropoelastin coacervates at physiological temperatures, J. Ultrastruct. Res. 82 335-340. 

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