Pro and Gly for elasticity

Disorder prevents elastic fibers from turning into amyloids, according to Sarah Rauscher, Régis Pomès (Hospital for Sick Children, Toronto, Canada), and colleagues. Groups attempting to synthesize artificial skin for medical use should thus be prepared to toss plenty of glycine and proline into the mix.

Skin gets its elasticity from elastin, which aggregates into fibrous assemblies. But at high local concentrations, almost any protein can aggregate into an amyloid, according to some proposals. So Pomès wondered, “If nature designs a protein that is supposed to aggregate, how does it avoid an amyloid state?” The key, he finds, is disorder-inducing amino acids.

Computer-simulated folding of peptides with elastin-like motifs in water revealed that constructing an elastomere requires a high degree of disorder and hydration. Such disorder only occurred when there was a threshold level of glycine and proline. Natural proteins that form amyloids fell at or below this calculated level, whereas the most elastic proteins, such as one type of spider silk, were far above the proline/glycine threshold.

Glycine, lacking any side chain, is so flexible that order is entropically unfavorable. Proline's cyclic side chain, by contrast, is “too stiff to make a regular secondary structure,” says Pomès. “Both [residues] contribute to disorder, but for opposite reasons.”

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