Whereas these structures provide insight into the molecular mechanism
of transport, many questions remain. For example, how are the movements
of the substrate and ions physically coupled? A
hint is provided by the structure of a Na+/Cl−-
dependent neurotransmitter transporter. In the
structure one of the two sodium atoms bound in
the transporter comes into contact with a bound
leucine molecule (Figure 18.6). The binding site
is formed by a partially unwound transmembrane
helix devoid of water with main-chain atoms and
helix dipoles providing binding interactions.
Ion channels
Cell membranes possess ion channels that are pro-
teins designed to transport specific ions across the
cell membrane. Ion channels can be distinguished
from ion transporters by certain characteristics.
Channels can transport ions at a significantly
faster rate than transporters. Also, the rate of ion
CHAPTER 18 MEMBRANE POTENTIALS 397
192 R 195
180
75
N
N
H
Water dipole
reorientation
Electrostatic
repulsion
Size
restriction
intracellular
extracellular
(b) Cytoplasm
Figure 18.5The aquaporins. (a) The three-
dimensional structure of aquaporin. (b) A schematic
representation of the water channel of aquaporin.
Peter Agre won the Nobel Prize in Chemistry in
2003 for his work on aquaporin.
Na1
A22
(O)
G26(N) F253(O)
G258(N)
I262(N)
G260(N)
A261(N)
E62
H 2 O
T254(O)
S256(Oγ)
Leu
L25(N)
Y108(OH)
1b
6a
6b
2
1a
8
Figure 18.6The leucine-binding site of the
Na+/Cl−transporter showing the presence of
a bound Na+ion. Modefied from Yamashita
et al. (2005).
(a)