BioPHYSICAL chemistry

in the mammalian central nervous system is the amino acid glutamate.
Glutamate transporters transport three Na+ions and one proton across
the membrane while glutamate and one K+ion
are transferred in the opposite direction. The
glutamate-transporter family has properties that
are distinct from those of the major facilitator
superfamily, in particular only eight transmem-
brane helices. The fold of homolog of a glutamate
transporter shows the presence of two segments
that contain αhelices, forming short hairpin loops,
which could serve to gate the transport process
(Figure 18.3). In addition to the ability of short
helices to serve as a gate through a loop movement,
the transmembrane helices are often bent and the
flexing of a helix around its bend may provide
a means of changing the opening of transporters
(Figure 18.4).

CHAPTER 18 MEMBRANE POTENTIALS 395

Figure 18.2The three-dimensional structures of the transporters (a) LacY and (b) GlpT.

1

2

6

5

7a

3

4c

4b

4a

7b

8

HP2

HP1

Figure 18.3Representation of the glutamate
transporter showing the αhelices as cylinders.
The hairpin helices that are proposed to serve as
a gate are identified as HP1 and HP2. Modified
from Yernool et al. (2004).

(a) (b)