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Estimating the p K a values of basic and acidic side chains in ion channels using electrophysiological recordings: A robust approach to an elusive problem
Author(s) -
Cymes Gisela D.,
Grosman Claudio
Publication year - 2011
Publication title -
proteins: structure, function, and bioinformatics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.699
H-Index - 191
eISSN - 1097-0134
pISSN - 0887-3585
DOI - 10.1002/prot.23087
Subject(s) - protonation , deprotonation , chemistry , conductance , side chain , ion channel , yield (engineering) , torpedo , lipid bilayer , ion , biophysics , chemical physics , stereochemistry , acetylcholine receptor , membrane , thermodynamics , receptor , biochemistry , physics , organic chemistry , condensed matter physics , biology , polymer
As a step toward gaining a better understanding of the physicochemical bases of p K a ‐value shifts in ion channels, we have previously proposed a method for estimating the proton affinities of systematically engineered ionizable side chains from the kinetic analysis of single‐channel current recordings. We reported that the open‐channel current flowing through mutants of the (cation‐selective) muscle nicotinic acetylcholine receptor (AChR) engineered to bear single basic residues in the transmembrane portion of the pore domain fluctuates between two levels of conductance. Our observations were consistent with the idea that these fluctuations track directly the alternate protonation–deprotonation of basic side chains: protonation of the introduced basic group would attenuate the single‐channel conductance, whereas its deprotonation would restore the wild‐type‐like level. Thus, analysis of the kinetics of these transitions was interpreted to yield the p K a values of the substituted side chains. However, other mechanisms can be postulated that would also be consistent with some of our findings but according to which the kinetic analysis of the fluctuations would not yield true p K a s. Such mechanisms include the pH‐dependent interconversion between two conformations of the channel that, while both ion permeable, would support different cation‐conduction rates. In this article, we present experimental evidence for the notion that the fluctuations of the open‐channel current observed for the muscle AChR result from the electrostatic interaction between fixed charges and the passing cations rather than from a change in conformation. Hence, we conclude that bona fide p K a values can be obtained from single‐channel recordings. Proteins 2011; © 2011 Wiley‐Liss, Inc.