Oxidative folding of omega-conotoxin MVIIC: effects of temperature and salt

Oxidative folding of omega-conotoxin MVIIC, a highly basic 26-amino acid peptide with three disulfide bonds, predominantly gave two products with mismatched disulfide bonds in 0.1M NH4OAc buffer (pH 7.7) at 21 degrees C both in the presence and absence of redox reagents such as reduced and oxidized glutathione. A low reaction temperature (5 degrees C) and a high salt concentration in buffer such as 2M (NH4)2SO4 were necessary to obtain the correctly folded biologically active product. The folding reaction was found to proceed via a two-stage pathway of (I) the formation and (II) the rearrangement of the mismatched disulfide bonds. Both the reaction temperature and the salt strongly affected the equilibrium between mismatched and correctly formed disulfide bonds in the second stage. Such an effect of salts on the rearrangement reaction could be explained by anion binding at a low concentration and the salting out effect at a high concentration by analyzing the rank order of their effectiveness. The anion-binding effect was also confirmed by examining the folding of the tetra-acetylated peptide at the Lys side chains. CD study suggested that the yield of the biologically active product was correlated with its conformational change as functions of temperature and salt concentration.