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Biophys J, August 1998, p. 793-809, Vol. 75, No. 2
*Protein Dynamics Unit,
Brownian dynamics simulations have been carried out to
study ionic currents flowing across a model membrane channel under various conditions. The model channel we use has a cylindrical transmembrane segment that is joined to a catenary vestibule at each
side. Two cylindrical reservoirs connected to the channel contain a
fixed number of sodium and chloride ions. Under a driving force of 100 mV, the channel is virtually impermeable to sodium ions, owing to the
repulsive dielectric force presented to ions by the vestibular wall.
When two rings of dipoles, with their negative poles facing the pore
lumen, are placed just above and below the constricted channel segment,
sodium ions cross the channel. The conductance increases with
increasing dipole strength and reaches its maximum rapidly; a further
increase in dipole strength does not increase the channel conductance
further. When only those ions that acquire a kinetic energy large
enough to surmount a barrier are allowed to enter the narrow
transmembrane segment, the channel conductance decreases monotonically
with the barrier height. This barrier represents those interactions
between an ion, water molecules, and the protein wall in the
transmembrane segment that are not treated explicitly in the
simulation. The conductance obtained from simulations closely matches
that obtained from ACh channels when a step potential barrier of 2-3
kTr is placed at the channel neck. The
current-voltage relationship obtained with symmetrical solutions is
ohmic in the absence of a barrier. The current-voltage curve becomes
nonlinear when the 3 kTr barrier is in
place. With asymmetrical solutions, the relationship approximates the
Goldman equation, with the reversal potential close to that predicted
by the Nernst equation. The conductance first increases linearly with
concentration and then begins to rise at a slower rate with higher
ionic concentration. We discuss the implications of these findings for
the transport of ions across the membrane and the structure of ion
channels.
Biophys J, August 1998, p. 793-809, Vol. 75, No. 2
© 1998 by the Biophysical Society 0006-3495/98/08/793/17 $2.00
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