Conformational heterogeneity in polypeptide cardiac stimulants from sea anemones.

High-resolution 1H NMR spectra at 300 MHz of the polypeptide cardiac stimulants anthopleurin-A and Anemonia sulcata toxin II reveal conformational heterogeneity in both molecules. The two conformations, manifest in a number of split 1H resonances, are in slow exchange over a wide range of pH and temperature. Heterogeneity affects a region of these molecules containing the structurally and functionally important Asp residues. By comparison with a homologous polypeptide Anemonia sulcata toxin I, which does not show this type of heterogeneity, it is suggested that the heterogeneity may originate in cis-trans isomerism of the Gly-40 to Pro-41 peptide bond.