A Statistical Thermodynamic Model for Ligands Interacting With Ion Channels: Theoretical Model and Experimental Validation of the KCNQ2 Channel.

Fang Bai,Huaiyu Yang,Hualiang Jiang,Xicheng Wang,Ping Li,Pingzheng Zhou,Xiaoping Pi,Min Li,Zhaobing Gao

doi:10.3389/fphar.2018.00150

Abstract

Ion channels are important therapeutic targets, and their pharmacology is becoming increasingly important. However, knowledge of the mechanism of interaction of the activators and ion channels is still limited due to the complexity of the mechanisms. A statistical thermodynamic model has been developed in this study to characterize the cooperative binding of activators to ion channels. By fitting experimental concentration-response data, the model gives eight parameters for revealing the mechanism of an activator potentiating an ion channel, i.e., the binding affinity (KA), the binding cooperative coefficients for two to four activator molecules interacting with one channel (γ, μ, and ν), and the channel conductance coefficients for four activator binding configurations of the channel (a, b, c, and d). Values for the model parameters and the mechanism underlying the interaction of ztz240, a proven KCNQ2 activator, with the wild-type channel have been obtained and revealed by fitting the concentration-response data of this activator potentiating the outward current amplitudes of KCNQ2. With these parameters, our model predicted an unexpected bi-sigmoid concentration-response curve of ztz240 activation of the WT-F137A mutant heteromeric channel that was in good agreement with the experimental data determined in parallel in this study, lending credence to the assumptions on which the model is based and to the model itself. Our model can provide a better fit to the measured data than the Hill equation and estimates the binding affinity, as well as the cooperative coefficients for the binding of activators and conductance coefficients for binding states, which validates its use in studying ligand-channel interaction mechanisms.

Highlights

The KCNQ family of potassium channels, a type of ion channel, is widely expressed in different tissues (Brown, 2008)
By fitting the concentrationresponse data of an activator potentiating a channel, we obtain these parameters. These parameters provide insight into the binding affinity of the activator with the channel; the cooperative effects of two, three, and four activator molecules binding to the channel together; and the channel conductance properties of each configuration (CF1-CF4)
Based on the statistical thermodynamic principles, we have deduced a quantitative model for the interactions between activators and K+ channels

Summary

Introduction

The KCNQ family of potassium channels, a type of ion channel, is widely expressed in different tissues (Brown, 2008). There are five isoforms with more than 40% amino acid identity within the six-transmembrane segment core regions. These channels are activated at sub-threshold membrane potentials. In an effort to examine the chemical repertoire of KCNQ activators, Li’s lab screened a collection of 20,000 compounds against KCNQ2 homomeric channels and identified multiple series of structures with distinct chemotypes. Through mutagenesis and electrophysiological determination in conjunction with molecular modeling and simulation, we have identified the binding sites of ztz240 at the voltage-sensing domain (VSD) of KCNQ2. The mutation of F137, one of the identified binding sites, to alanine (A) largely abolished ztz240 activity (Li et al, 2013)

Methods

Results

Conclusion