Role of Disulfide Bonds in the Structure and Potassium Channel Blocking Activity of ShK Toxin

ShK toxin, a potassium channel blocker from the sea anemone Stichodactyla helianthus, is a 35 residue polypeptide cross-linked by three disulfide bridges:  Cys3−Cys35, Cys12−Cys28, and Cys17−Cys32. To investigate the role of these disulfides in the structure and channel-blocking activity of ShK toxin, a series of analogues was synthesized by selective replacement of each pair of half-cystines with two α-amino-butyrate (Abu) residues. The remaining two disulfide pairs were formed unambiguously using an orthogonal protecting group strategy of Cys(Trt) or Cys(Acm) at the appropriate position. The peptides were tested in vitro for their ability to block Kv1.1 and Kv1.3 potassium channels and their ability to displace [125I]dendrotoxin binding to rat brain synaptosomal membranes. The monocyclic peptides showed no activity in these assays. Of the dicyclic peptides, [Abu12,28]ShK3-35,17-32 (where the subscript indicates disulfide connectivities) had weak activity on Kv1.3 and Kv1.1. [Abu17,32]ShK3-35,12-28 block...