Theoretical Analysis of Molecular Transport Across Membrane Channels and Nanopores - Computational Modeling of Biological Systems - page 297

Theoretical Analysis of Molecular Transport
Across Membrane Channels and Nanopores

Anatoly B. Kolomeisky

1 Introduction

A successful functioning of cellular systems requires that some molecules and
ions be transferred out of the cell while other particles should be taken in.
The bidirectional flux is accomplished with the help of a complex system of
membrane protein channels and pores [ 1 , 2 ]. It is known that molecular transport
across cellular membranes is fast, efficient, selective, and that the functioning
of channels is robust with respect to strong nonequilibrium fluctuations in the
cellular environment [ 2 ]. These observations are especially surprising because in
many cases molecular translocation does not involve the use of metabolic energy
or significant conformational changes [ 4 ]. Although in recent years significant
advances in studying molecular transport in biological systems have been achieved,
the mechanisms of translocation phenomena are still not well understood.
To develop a comprehensive picture of molecular transport across the membrane,
one has to recall that when a molecule enters into the pore its motion is slowed
mostly due to entropic barriers and possibly due to other biochemical interactions.
Additional forces are needed to overcome these barriers. In biological systems, elec-
tric fields, concentration gradients, and chemical interactions are used to speed up
the transport. There is increasing experimental evidence that the high efficiency, ro-
bustness, speed, and selectivity of many biological and artificial channels are a result
of complex processes that involve molecule/pore and intermolecular interactions
[ 3 - 17 ]. Recent high-resolution experiments on polypeptide translocations through
protein nanopores [ 14 , 15 ] have opened new possibilities in probing the effect of
molecule/pore interactions at the single-molecule level. In these experiments, it was
found that changing the location of the binding site in the pore significantly modified