Natural-Abundance 13C Nuclear-Magnetic-Resonance Study of Toxin II from Anemonia sulcata

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Abstract
Natural-abundance 13C NMR spectra (at 15.04 MHz) of the polypeptide toxin II from the sea anemone A. sulcata were analyzed and compared with corresponding spectra reported recently for a closely related polypeptide anthopleurin A. The spectra contained many resolved 1-C and 2-C resonances from carbonyl, aromatic and methyl C, many of which were assigned to individual C in the molecule on the basis of their chemical shifts, including their pH dependence and by comparison with 13C NMR spectrum of anthopleurin A. Analysis of effects of pH on spectrum yields estimated for pKa values of a number of functional groups in the molecule as follows: side-chain carboxylates of the 2 aspartic acid residues 2 and 3.1; COOH-terminal carboxylic acid, 3.5; imidazolium moieties of the 2 histidine residues, 6.7 and 7.6; NH2-terminal ammonium, 8. Similarity between pKa values of these functional groups in toxin II and those of corresponding groups in anthopleurin A together with close agreement between chemical shifts of conserved C indicated that many local interactions were nearly identical in the 2 molecules and supported the thesis that their overall conformations in solution were similar. Local interactions involving an aspartic acid residue were altered in toxin II. This led to a proposal for the site in these 2 molecules which was responsible for their cardiac stimulatory activity.

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