DETERMINATION OF THE SOLUTION STRUCTURES OF CONANTOKIN-G AND CONANTOKIN-T BY CD AND NMR-SPECTROSCOPY

Conantokin-G and conantokin-T are two paralytic polypeptide toxins ori ginally isolated from the venom of the fish-hunting cone snails of the genus Conus. Conantokin-G and conantokin-T are the only naturally occ urring peptidic compounds which possess N-methyl-D-aspartate receptor antagonist activity, produced by a selective non-competitive antagonis m of polyamine responses, They are also structurally unusual in that t hey contain a disproportionately large number of acid labile post-tran slational gamma-carboxyglutamic acid (Gla) residues, Although no preci se structural information has previously been published for these pept ides, early spectroscopic measurements have indicated that both conant okin-G and conantokin-T form alpha-helical structures, although there is some debate whether the presence of calcium ions is required for th ese peptides to adopt this fold, We now report a detailed structural s tudy of synthetic conantokin-G and conantokin-T in a range of solution conditions using CD and H-1 NMR spec troscopy. The three-dimensional structures of conantokin-T and conantokin-G were calculated from H-1 N MR-derived distance and dihedral restraints. Both conantokins were fou nd to contain a mixture of alpha- and 3(10) helix, that give rise to c urved and straight helical conformers. Conantokin-G requires the prese nce of divalent cations (Zn2+, Ca2+, Cu2+, Or Mg2+) to form a stable i v-helix, while conantokin-T adopts a stable alpha-helical structure in aqueous conditions, in the presence or absence of divalent cations (Z n2+, Ca2+, Cu2+, Or Mg2+).