Quantitative Observation of Backbone Disorder in Native Elastin

Abstract

Elastin is a key protein in soft tissue function and pathology. Establishing a structural basis for understanding its reversible elasticity has proven to be difficult. Complementary to structure is the important aspect of flexibility and disorder in elastin. We have used solid-state NMR methods to examine polypeptide and hydrate ordering in both elastic (hydrated) and brittle (dry) elastin fibers and conclude (i) that tightly bound waters are absent in both dry and hydrated elastin and (ii) that the backbone in the hydrated protein is highly disordered with large amplitude motions. The hydrate was studied by 2H and 17O NMR, and the polypeptide by 13C and 2H NMR. Using a two-dimensional 13C MAS method, an upper limit of S < 0.1 was determined for the backbone carbonyl group order parameter in hydrated elastin. For comparison, S ∼ 0.9 in most proteins. The former result is substantiated by two additional observations: the absence of the characteristic 2H spectrum for stationary amides and "solution-like" 13C magic angle spinning spectra at 75 °C, at which the material retains elasticity. Comparison of the observed shifts with accepted values for α-helices, β-sheets, or random coils indicates a random coil structure at all carbons. These conclusions are discussed in the context of known thermodynamic properties of elastin and, more generally, protein folding. Because coacervation is an entropy-driven process, it is enhanced by the observed backbone disorder, which, we suggest, is the result of high proline content. This view is supported by recent studies of recombinant elastin polypeptides with systematic proline substitutions.