The gramicidin a channel: A review of its permeability characteristics with special reference to the single-file aspect of transport

Abstract

Gramicidin A forms univalent cation-selective channels of approximately 4 A diameter in phospholipid bilayer membranes. The transport of ions and water throughout most of the channel length is by a single-file process; that is, cations and water molecules cannot pass each other within the channel. The implications of this single-file mode of transport for ion movement are considered. In particular, we show that there is no significant electrostatic barrier to ion movement between the energy wells at the two ends of the channel. The rate of ion translocation (e.g., Na+ or Cs+) through the channel between these wells is limited by the necessity for an ion to move six water molecules in single file along with it; this also limits the maximum possible value for channel conductance. At all attainable concentrations of NaCl, the gramicidin A channel never contains more than one sodium ion, whereas even at 0.1 M CsCl, some channels contain two cesium ions. There is no necessity to postulate more than two ion-binding sites in the channel or occupancy of the channel by more than two ions at any time.

Estimation of Na+ dwell time in the gramicidin A channel. Na+ ions as blockers of H+ currents

“…From the numerical fit of a simple first-order block mechanism to the data we obtained Na + dwell times of 8.4.10 -9 s and 12-10 -9 s and non-equilibrium Na + affinities of 0.17 M -~ and 0.24 M-] at membrane potentials of 100 mV and 200 mV, respectively. These values agree well with the 'weak-binding' Na + transport rate constants of Finkelstein and Andersen [1].…”

“…For the application of this theory one must start with a specific kinetic model for the ion transport mechanism. We start with the 2-site, 3-barrier model (2S3B) of Finkelstein and Andersen ( [1]; Fig, 1A) for proton conduction, and accomodate the flux of sodium by extending this model to nine states allowing sodium ions to compete with protons for the same binding sites (Fig. 1B).…”

“…1A. For the solid curve the starting values of the fit were the 'weak binding' rate constants of Finkelstein and Andersen [1], while for the dotted curve the "tight binding' values of Urry et al [3,4] were used. The fitted values (listed in Table If) are very similar to the starting values, and fit the experimental points indistinguishable well even dtough two of the rate constants differ by two orders of magnitude between the two sets.…”

“…logues having a reduced number of amino acids also indicated that the gramicidin A channel becomes doubly occupied by sodium ions [5]. Finkelstein and Andersen [1], however, concluded from analysis of the concentration dependence of conductance, current-voltage relationships, and flux ratios tha~. the first Na + binds very loosely (<< 1 M-t) and that double occupancy does not occur.…”

“…The set of rate constants as proposed by Finkelstein and Andersen [1] is now widely accepted, but it is of interest to attempt to distinguish between the 'tight binding" and "weak binding" theories by measuring the dwell time of Na ÷ in the channel. By analogy with the work of Lansman et al [6] in calcium channels, in which blockages of Na ÷ current by calcium ions was used to estimate the affinity of calcium for the channel, we wished to find experimental conditions under which the sodium ions could be considered as a blocking agent for a probe current.…”