Essential Cell Biology 5th edition

398
CYTOSOL
CHAPTER 12 Transport Across Cell Membranes
plasma membrane
Na +
Figure 12–11 The Na + pump uses the
+ +
energy of ATP hydrolysis to pump Na +
out of animal cells and K + in. In this
+ +
+
way, the pump helps keep the cytosolic
+ + +
concentrations of Na + 1
low BINDS
and K + high.
K +
6
Na +
electrochemical
gradient
+
plasma
membrane
+ + + +
– – – –
2 K +
P
3 Na +
EXTRACELLULAR
SPACE
+ + + +
– – – –
CYTOSOL
ATP
+
+
PUMP RETURNS
TO ORIGINAL
CONFORMATION The Na + pump is very efficient: the whole pumping cycle takes only 10
AND K + IS EJECTED
milliseconds. Furthermore, the tight coupling between steps in the cycle
ensures that the pump operates only when the appropriate ions—both
Na + and K + —are available to be transported, thereby avoiding a wasteful
hydrolysis of ATP.
The Na + Pump Generates a Steep Concentration
Gradient of Na + Across the Plasma Membrane
The Na + pump functions like a bilge pump in a leaky ship, ceaselessly
expelling the Na + that is constantly slipping into the cell through other
ECB5 e12.11/12.11
ADP
+
K +
electrochemical
gradient
+
+
K +
+
P
+
4
+
K + BINDS
+
K +
K + PUMP RETURNS TO
PUMP DEPHOSPHORYLATES
Na +
EXTRACELLULAR
plasma membrane
SPACE
PHOSPHORYLATION TRIGGERS
PUMP PHOSPHORYLATES
ITSELF, HYDROLYZING ATP
CONFORMATIONAL CHANGE
AND Na + IS EJECTED
Na +
2 3
CYTOSOL
P
P
Na +
phosphate in
high-energy
4 K + BINDS
Na + BINDS 1
linkage
P
K +
6
5
ORIGINAL CONFORMATION
ITSELF
AND K + IS EJECTED
P
Figure 12−12 The Na + pump undergoes a series of conformational changes as it exchanges Na + ions for K + .
The binding of cytosolic Na + (1) and the subsequent phosphorylation by ATP of the cytosolic face of the pump (2)
induce the protein to undergo conformational changes that transfer the Na + across the membrane and release
it outside the cell (3). The high-energy linkage of the phosphate to the protein provides the energy to drive the
conformational changes. The binding of K + from the extracellular space (4) and the subsequent dephosphorylation
(5) allow the protein to return to its original conformation, which transfers the K + across the membrane and releases
it into the cytosol (6).
The cycle is shown in Movie 12.2. The changes in conformation are analogous to those shown for the
glucose transporter in Figure 12−9, except that here the Na + -dependent phosphorylation and K + -dependent
dephosphorylation of the protein cause the conformational changes to occur in an orderly fashion, enabling the
protein to do useful work. For simplicity, only one binding site is shown for each ion. The real pump in mammalian
cells contains three binding sites for Na + and two for K + . The net result of one cycle of the pump is therefore the
transport of three Na + out and two K + in. Ouabain inhibits the pump by preventing K + binding (4).