Sodium and Potassium e Channels and Pumps - Biological Inorganic Chemistry - page 188

transported out of the cells into the circulation by a passive glucose uniport situated on the capillary side of the cell
( Figure 9.12 ) . For this system to continue functioning, ATP hydrolysis, which maintains the intra-cellular Na þ
concentration through the (Na þ -K þ )-ATPase, is absolutely required.

I L C

Na + -glucose symport

Glucose
uniport

Glucose

Glucose Glucose

Na + Na +

Na +

ATP

K + K +

ADP + P
i

(Na +_ K + )- ATPase

Brush border cell

FIGURE 9.12 Epithelial brush border cells of the small intestine concentrate glucose from the intestinal lumen in symport with Na þ : this is
driven by the Na þ -K þ -ATPase located on the capillary side of the cell. The glucose is then exported out of the brush border cell by a passive
uniport system. (Adapted from Voet & Voet, 2004 . )

Glutamate transporters, also referred to as excitatory amino acid transporters (EAATs), clear synaptically
released glutamate from the extracellular space, thus ensuring the precise control of excitatory synaptic trans-
mission. In addition, excessive extracellular concentrations of glutamate can be neurotoxic and the efficient
removal of glutamate limits pathological conditions associated with excitotoxic cell death. The transport cycle and
stoichiometry of EAATs, of which five mammalian isoforms have been characterised, is presented in Figure 9.13 .

1 Glu - , 3 Na + , 1 H +

1 K +

T T T

1 Glu - , 3 Na + , 1 H + 1 K +

(1)

(6)

Glutamate
translocation

Transporter
reorientation

(2)

(5)

(3)

(4)

T T T

1 K +

1 Glu - , 3 Na + , 1 H +

1 K +

1 Glu - , 3 Na + , 1 H +

IN

FIGURE 9.13 Transport cycle and stoichiometry of excitatory amino acid transporters (EAATs). Simplified state diagram of the EAAT
transport cycle. After glutamate and coupled ions (step 1) bind to the transporter (T), they are translocated (step 2) and released into the cell
cytosol (step 3). Next K þ binds from the intracellular side (step 4) and reorients the substrate-free transporter (step 5). K þ is released outside
the cell (step 6). (From Jiang & Amara, 2011 . Reproduced with permission from Elsevier.)

After glutamate and coupled ions bind to the transporter, they are translocated and released into the cell cytosol.
Next, K þ binds from the intracellular side and reorients the substrate-free transporter, and finally K þ is released
outside the cell.