Disulfide-rich peptides from the venoms of cone snails have excellent potency and selectivity profiles for a range of neuronal receptors, and are promising leads for drug design (1). However, several factors still limit the full-scale exploitation of these peptides in a clinical setting. One such limitation is the instability, in reducing environments, of disulfide bonds important for structure and activity.
Replacement of disulfides with non-reducible dicarba bonds represents one approach to improving their utility but this can have unpredictable effects on structure and activity (2). The α-conotoxin Vc1.1 is a potent analgesic that is reportedly both an α9α10 nAChR antagonist and a GABA_BR agonist (3), although the precise mechanism by which it induces analgesia remains unclear. Vc1.1 is a 16-residue peptide with a globular fold stabilised by two disulfide bonds. The aim of this study was to investigate the effects of replacing individual disulfides with dicarba bridges on both the structure and activity of this peptide using NMR spectroscopy and electrophysiology, respectively.
Four peptides with dicarba bonds in place of each disulfide were synthesized. Substitution of the Cys2-8 disulfide results in loss of activity at nAChRs, while substitution of the Cys3-16 disulfide results in loss of activity at the GABA_BR – most importantly, without any dramatic changes in the three-dimensional structure of the peptides. Molecular dynamics simulations indicated a loss of contacts between the toxin dicarba alkenes and key receptor binding site residues, relative to wt Vc1.1, that may partly explain the reduced potency of the cis-[2,8]-dicarba analogue.
Our results highlight the fact that the disulfide bonds in conopeptides are not only critical for the structure of the peptide but may also contribute directly to binding. Inclusion of dicarba bridges is a viable option for improving the stability of α-conotoxin Vc1.1 and enhancing its therapeutic potential.