Structure-function study on a de novo synthetic hydrophobic ion channel

Abstract

Ion conduction properties of a de novo synthesized channel, formed from cyclic octa-peptides consisting of four alternate L-alanine (Ala) and N'- acylated 3-aminobenzoic acid (Aba) moieties, were studied in bilayer membranes. The single-channel conductance was 9 pS in symmetrical 500 mM KCl. The channel favored permeation of cations over anions with a permeability ratio (P(Cl-)/P(K+)) of 0.15. The selectivity sequence among monovalent cations based on permeability ratio (P(x+)/P(K+)) fell into an order: NH4+(1.4) > Cs+(1.1) ≥ K+(1.0) > Na+(0.4) >> Li+(0). The conductance- activity relationship of the channel in K+ solutions followed simple Michaelis-Menten kinetics with a half-maximal saturating activity of 8 mM and a maximal conductance of 9 pS. The permeability ratio P(Na+)/P(K+) remained constant (~0.40) under biionic concentrations from 10 to 500 mM. These results suggests that the channel is a one-ion channel. The pore diameter probed by a set of organic cations was ~6 Å. The single-channel current was blocked by Ca2+ in a dose-dependent manner that followed a single-site titration curve with a voltage-dependent dissociation constant of 0.6 mM at 100 mV. The electric distance of the binding site for Ca2+ was 0.07 from both entrances of the channel, indicating the presence of two symmetrical binding sites in each vicinity of the channel entrance. Correlations between conduction properties and structural aspects of the channel are discussed in terms of a three-barrier and two-binding-site (3B2S) model of Eyring rate theory. All available structural information supported an idea that the channel was formed from a tail-to-tail associated dimer of the molecule, the pore of which was lined with hydrophobic acyl chains. This is the first report to have made a systematic analysis of ion permeation through a hydrophobic pore.