BioPHYSICAL chemistry

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)