TRANSPORT-PROPERTIES OF SINGLE-FILE PORES WITH 2 CONFORMATIONAL STATES

Complex facilitative membrane transporters of specific ligands may ope rate via inner channels subject to conformational transitions. To desc ribe some properties of these systems, we introduce here a kinetic mod el of coupled transport of two species, L and w, through a two-conform ational pore. The basic assumptions of the model are: a) single-file o f, at most, n molecules inside the channel; b) each pore state is open to one of the compartments only; c) there is at most only one vacancy per pore; d) inside the channel, a molecule of L occupies the same po sitions as a molecule of w; and e) there is at most only one molecule of L per pore. We develop a general representation of the kinetic diag ram of the model that is formally similar to the one used to describe one-vacancy transport through a one-conformational single-file pore. I n many cases of biological importance, L could be a hydrophilic (ionic or nonionic) ligand and w could be water. The model also finds applic ation to describe solute (w) transport under saturation conditions. In this latter case, L would be another solute, or a tracer of w. We der ive steady-state expressions for the fluxes of L and w, and for the pe rmeability coefficients. The main results obtained from the analysis o f the model are the following. 1) Under the condition of equilibrium o f w, the expression derived for the flux of L is formally indistinguis hable from the one obtainable from a standard four-state model of liga nd transport mediated by a two-conformational transporter. 2) When L i s a tracer of w, we can derive an expression for the ratio between the main isotope and tracer permeability coefficients (P-w/P-d). We find that the near-equilibrium permeability ratio satisfies (n - 1) less th an or equal to (P-w/P-d)(eq) less than or equal to n, a result previou sly derived for the one-conformational, single-file pore for the case that n greater than or equal to 2. 3) The kinetic model studied here r epresents a generalization of the carrier concept. In fact, for the ca se that n = 1 (corresponding to the classical single-occupancy carrier ), the near-equilibrium permeability ratio satisfies 0 less than or eq ual to (P-w/P-d)(eq) less than or equal to 1, which is characteristic of a carrier performing exchange-diffusion.