Quantum molecular modeling of the elastinic tetrapeptide Val-Pro-Gly-Gly

Coacervation of tropoelastin

The coacervation of tropoelastin represents the first major stage of elastic fiber assembly. The process has been modeled in vitro by numerous studies, initially with mixtures of solubilized elastin, and subsequently with synthetic elastin peptides that represent hydrophobic repeat units, isolated hydrophobic domains, segments of alternating hydrophobic and cross-linking domains, or the full-length monomer. Tropoelastin coacervation in vitro is characterized by two stages: an initial phase separation, which involves a reversible inverse temperature transition of monomer to n-mer; and maturation, which is defined by the irreversible coalescence of coacervates into large species with fibrillar structures. Coacervation is an intrinsic ability of tropoelastin. It is primarily influenced by the number, sequence, and contextual arrangement of hydrophobic domains, although hydrophilic sequences can also affect the behavior of the hydrophobic domains and thus affect coacervation. External conditions including ionic strength, pH, and temperature also directly influence the propensity of tropoelastin to self-associate. Coacervation is an endothermic, entropically-driven process driven by the cooperative interactions of hydrophobic domains following destabilization of the clathrate-like water shielding these regions. The formation of such assemblies is believed to follow a helical nucleation model of polymerization. Coacervation is closely associated with conformational transitions of the monomer, such as increased β-structures in hydrophobic domains and α-helices in cross-linking domains. Tropoelastin coacervation in vivo is thought to mainly involve the central hydrophobic domains. In addition, cell-surface glycosaminoglycans and microfibrillar proteins may regulate the process. Coacervation is essential for progression to downstream elastogenic stages, and impairment of the process can result in elastin haploinsufficiency disorders such as supravalvular aortic stenosis.

Flexibility in the solution structure of human tropoelastin

We investigated the flexibility of full-length tropoelastin in solution by using far- and near-ultraviolet circular dichroism (UV CD) and fluorescence spectroscopy to probe for structural flexibility and residue mobility within secondary and tertiary features of the monomer. Fluorescence spectroscopy revealed the presence of exposed hydrophobicity through the binding of the hydrophobic probe 4,4'-dianilino-1,1'-binaphthyl-5,5'-disulfonate (bis-ANS), which demonstrates that hydrophobic regions form clusters and are not confined to a molecular core. Near-UV CD indicated substantial mobility of aromatic residues. Structural prediction programs (PONDR, DisEMBL, and Globplot version 2.0) estimated 75 +/- 2% disorder in the tertiary structure of tropoelastin on the basis of primary sequence information. A single-site substitution of Trp for Gln (Q513W) at the tropoelastin domain 25-26 interface facilitated fluorescence spectroscopy for revealing that this region is exposed to solvent. Polarization anisotropy demonstrated substantial flexibility of W513 and little change upon denaturation of the monomer with guanidine hydrochloride. Comparable movement was found for native sequence aromatic residues in the presence of glycosaminoglycans and trifluoroethanol. These data prove the intrinsic flexibility of specific residues and adjacent sequences in any native conformation(s) they may take. This study is the first characterization of the level of mobility in defined regions of the full-length tropoelastin monomer and provides direct evidence for regions of flexible structure in tropoelastin.

The effect of a recombinant elastin-mimetic coating of an ePTFE prosthesis on acute thrombogenicity in a baboon arteriovenous shunt

A recombinant elastin-mimetic triblock protein polymer with an inverse transition temperature (approximately 20 degrees C) was used to impregnate small-diameter (4 mm i.d.) expanded polytetrafluoroethylene (ePTFE) vascular grafts. Scanning electron microscopy confirmed that initial elastin impregnation of the graft followed by further multilayer coating with elastin films filled in the fibril and node structure of the luminal surface of the ePTFE graft and was macroscopically smooth. Elastin protein polymer impregnation reduced the advancing contact angle of the luminal surface to 43 degrees, which was comparable to the advancing contact angle of 47 degrees for a cast elastin film. Attenuated total reflection infrared spectroscopy and Coomassie blue staining revealed little discernable change in the protein surface film after 24 h of shear at 500 s(-1) and 37 degrees C. Excellent short-term blood-contacting properties as determined by minimal fibrin and platelet deposition were demonstrated using a baboon extracorporeal femoral arteriovenous shunt model. The results of this study demonstrate the applicability of an elastin-mimetic triblock protein polymer as a non-thrombogenic coating or as a component of a tissue-engineered composite.

Structural changes and facilitated association of tropoelastin

Circular dichroism studies of tropoelastin secondary structure show 4+/-1% alpha-helix in aqueous solutions. This is in contrast to the substantially higher amounts (up to 23+/-7%) of alpha-helix predicted by computer algorithms, which propose that regions of alpha-helix are limited to the alanine-rich cross-linking domains. Through the addition of trifluoroethanol, the amount of alpha-helix increased to 17+/-1%, equivalent to that expected on the basis of primary structure. The physiological ability of the protein to coacervate and the critical concentration of monomer required for coacervation were unaffected by levels of alpha-helix. However, the temperature required for coacervation decreased linearly with increasing alpha-helical structure, which correlates with the participation of alpha-helices in association. We propose that the alanine-rich cross-linking domains exist as nascent helices in tropoelastin in aqueous solution. We further suggest a novel mechanism for coacervation whereby formation of alpha-helices and subsequent helical side chain interactions limit the conformational flexibility of the polypeptide, to facilitate associations between hydrophobic domains during elastogenesis.

Domain 26 of tropoelastin plays a dominant role in association by coacervation

The temperature-dependent association of tropoelastin molecules through coacervation is an essential step in their assembly leading to elastogenesis. The relative contributions of C-terminal hydrophobic domains in coacervation were assessed. Truncated tropoelastins were constructed with N termini positioned variably downstream of domain 25. The purified proteins were assessed for their ability to coacervate. Disruption to domain 26 had a substantial effect and abolished coacervation. Circular dichroism spectroscopy of an isolated peptide comprising domain 26 showed that it undergoes a structural transition to a state of increased order with increasing temperature. Protease mapping demonstrated that domain 26 is flanked by surface sites and is likely to be in an exposed position on the surface of the tropoelastin molecule. These results suggest that the hydrophobic domain 26 is positioned to play a dominant role in the intermolecular interactions that occur during coacervation.