Influence of protein surface charge on the bimolecular kinetics of a potassium channel peptide inhibitor

This study investigates the influence of a through-solution electrostatic interaction on the kinetics of ion channel blockade by the high-affinity peptide inhibitor Lq2. Membrane patches containing many Shaker K+ channels were removed from Xenopus oocytes and placed in a rapid perfusion chamber. Lq2 association and dissociation rate constants were determined from the relaxations to equilibrium blockade following rapid changes in toxin concentration. The association and dissociation rate constants were 8.5 x 10(7) M-1 s-1 and 0.71 M-1 s-1, respectively, in 100 mM NaCl solution, pH 7.1, at room temperature (21-23 degrees C). Charge-altering mutations introduced at position 422 on the ion channel affect toxin affinity in a manner consistent with a through-solution electrostatic interaction. The full effect of the charge mutations is expressed kinetically on the association rate; toxin dissociation remains unaltered. An electrostatic influence on the association rate alone is expected if diffusion of toxin up to (and away from) its receptor on the channel is fast compared to the rate of formation of short-range contacts that are necessary to produce the bound state.