Synthetic study of peptide toxins for the elucidation of functions of ion channels

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1.Syntheses of two chimeric analogs (δω and ωδ) of ω-conotoxin TxVII, an L-type calcium channel blocker, and δ-conotoxin TxVIA, a peptide toxin that slows the inactivation of sodium channels were examined. Air oxidation of a linear precursor of 8w afforded desired analog in good yield. However, that of ωδ gave complex mixture of disulfide bond isomers. Two-step selective disulfide bond formation strategy also failed to give correct product, suggesting that δω lack the residues essential for the 3D structure formation. 3D structure of δ-conotoxin TxVIA was analyzed by NMR and simulated annealing calculations. δ-Conotoxin TxVIA showed an unusual hydrophobic patch on one side of the molecule, which may play am important role in, the sodium channel binding.2.Two analogs of μ-conotoxin GIIIA, a selective blocker of muscle sodium channels, were synthesized by replacing C-terminal Ala22 with acidic Glu (A22E) and basic Lys (A22K) residues. A22E was 90-fold less active than native μGJIIA, however it showed high affinity against E765K mutant of sodium channel, indicating that C-terminal part of μGIIIA closely associates with domain II of sodium channels. A22K was also less active than native μGIIIA, suggesting that the large side chain of Lys residue may interfere the binding.3.Two peptide toxins, κ-hefutoxin 1 and 2, isolated from the venom of the scorpion Heterometrus fulvipes were chemically synthesized and confirmed to be identical to native toxins. The disulfide bond pairings were determined by enzymatic digestion. NMR studies indicated that κ-hefutoxin 1 adopts a unique three-dimensional fold of two parallel helices linked by two disulfide bridges without any β-sheets. κ-Hefutoxin 1 not only blocks the voltage-gated potassium channels, Kv1.3 and Kv1.2, but also slows the activation kinetics of Kv1.3 currents.