Stability and structure-forming properties of the two disulfide bonds of alpha-conotoxin GI.

alpha-Conotoxin GI is a 13 residue snail toxin peptide cross-linked by Cys2-Cys7 and Cys3-Cys13 disulfide bridges. The formation of the two disulfide bonds by thiol/disulfide exchange with oxidized glutathione (GSSG) has been characterized. To characterize formation of the first disulfide bond in each of the two pathways by which the two disulfide bonds can form, two model peptides were synthesized in which Cys3 and Cys13 (Cono-1) or Cys2 and Cys7 (Cono-2) were replaced by alanines. Equilibrium constants were determined for formation of the single disulfide bonds of Cono-1 and Cono-2, and an overall equilibrium constant was measured for formation of the two disulfide bonds of alpha-conotoxin GI in pH 7.00 buffer and in pH 7. 00 buffer plus 8 M urea using concentrations obtained by HPLC analysis of equilibrium thiol/disulfide exchange reaction mixtures. The results indicate a modest amount of cooperativity in the formation of the second disulfide bond in both of the two-step pathways by which alpha-conotoxin GI folds into its native structure at pH 7.00. However, when considered in terms of the reactive thiolate species, the results indicate substantial cooperativity in formation of the second disulfide bond. The solution conformational and structural properties of Cono-1, Cono-2, and alpha-conotoxin GI were studied by 1H NMR to identify structural features which might facilitate formation of the disulfide bonds or are induced by formation of the disulfide bonds. The NMR data indicate that both Cono-1 and Cono-2 have some secondary structure in solution, including some of the same secondary structure as alpha-conotoxin GI, which facilitates formation of the second disulfide bond by thiol/disulfide exchange. However, both Cono-1 and Cono-2 are considerably less structured than alpha-conotoxin GI, which indicates that formation of the second disulfide bond to give the Cys2-Cys7, Cys3-Cys13 pairing induces considerable structure into the backbone of the peptide.