Characterization of the Proline Residues in Elastin

Abstract

Elastin is responsible for the stretch and recoil cycles of extensible mammalian tissues. The abundance of proline in elastomeric proteins hints that this residue plays an important role in elasticity. The putative function of proline in elastin is to prevent the formation of compact structures stabilized by regular hydrogen bonding patterns. However, this hypothesis is not confirmed experimentally, because the high- resolution structure of this protein is still unknown, as solution nuclear magnetic resonance spectroscopy and X-ray diffraction are incompatible with elastin’s insolubility and non-crystallinity. Solid-state NMR spectroscopy has emerged as the method of choice in the analysis of this biopolymer. The isotopic enrichment and the subsequent application of state-of-the-art NMR techniques allow detailed structural and dynamic information to be obtained for each amino acid. Elastin samples, incorporated with 13C or 15N at the prolines, were prepared from neonatal rat smooth muscle cells using a feedback-by-inhibition strategy. The percent enrichment is about 80%. 13C, 1H and 15N isotropic chemical shifts of proline from one- and two-dimensional NMR studies of this enriched biopolymer show that the prolines adopt both trans and cis conformations. Furthermore, a labile type II β- turn is identified in elastin. The structural information is consistent with Tamburro’s model for elasticity, describing equilibrium of states. The plausible changes that might occur at prolyl residues upon stretching are cis-trans isomerization and conversion of turn to extended conformations. In terms of dynamics, the prolines in elastin were confirmed to have liquid-like mobility in the hydrated state. By using model peptides of varying proline content, relaxation measurements show that increasing amounts of this amino acid enhance segmental motions in the peptide backbone. A majority of the prolines in elastin oscillate in a motional regime that is believed to be critical for elasticity. This study on the prolines in elastin highlighted the role of this unique amino acid in promoting elastomeric properties. The insights gleaned from this residue allowed the evaluation of the various models for elasticity. Expanding this approach to the rest of the key amino acids in elastin will eventually lead to determination of the structure-function relationships of this macromolecule.