Biomedical Engineering Reference
In-Depth Information
is used to fuel the Na-K pump at rest. This pump is vital to maintain the cell's resting mem-
brane potential. In this section, we focus on the enzyme reactions. The Na-K pump was dis-
covered by Jens Skou in 1957, who subsequently received a Nobel Prize for his work in 1997.
Using radioactive ions, Skou showed that the concentrations of the ions are interdependent,
implying the involvement of a common mechanism using an ATP-ase carrier.
The overall reaction for the Na-K pump is given by
Na
i
þ
K
o
!
ATP
ase
Na
o
þ
K
i
ATP
þ
3
2
ADP
þ
P
i
þ
3
2
where the subscripts
i
for inside the cell and
o
for outside the cell are used as before.
Na
þ
binding sites and 2
K
þ
binding sites in its two conformations. There
The pump has 3
Na
þ
outside the cell than inside and a higher concentration of
K
þ
inside the cell than outside; left unchecked by the pump, this gradient would drive
Na
þ
into the cell and
is a higher concentration of
K
þ
out of the cell, and thus change the resting membrane potential.
Any change in the concentration gradient of K
þ
and Na
þ
is prevented by the Na-K pump.
The pump transports a steady stream of Na
þ
out of the cell and K
þ
into the cell.
The Na-K pump uses six steps to move 3
K
þ
ions into the
cell at a total cost of 1 ATP molecule. Figure 8.22 illustrates the six steps that are continually
repeated at a rate of 100 s
1
:
1.
Three
Na
þ
ions out of the cell and 2
Na
i
ions in the cytosol move into and bind to the carrier in the pump,
ð
Þ
i
:
Na
C
3
Note that the pump has a bound ATP molecule.
2.
ATP is hydrolyzed. ADP is released, and the inorganic phosphate,
P,
binds with
ð
Na
C
Þ
i
3
.
3.
Using the energy gained by the hydrolyzation, a conformational change occurs in the
pump that moves
to create
ð
Na
CP
Þ
i
3
Na
o
ð
Na
3
CP
Þ
o
to the outside of the cell membrane, exposing
ions to the
Na
o
ions exit the pump.
4.
On the outside of the cell, 2
outside. The 3
K
o
ions then bind to the carrier and inorganic phosphate in
.
5.
Dephosphorylation of the pump occurs, releasing the inorganic phosphate. Following
this, a conformational change occurs in the pump that moves
the pump, creating
ð
K
2
CP
Þ
o
K
i
ð
K
2
C
Þ
i
, exposing 2
ions
to the inside of the cell.
6.
ATP binds to the pump, and the 2
K
i
ions are released into the cell.
The following equations list the reactions that describe the Na-K pump, where
C
is the
carrier and
is the inorganic phosphate. Note that we have eliminated the reverse reactions
as they are relatively small.
P
K
1
!
ATP
i
K
2
Na
i
þ
C
i
!
3
ð
Na
3
C
Þ
i
K
3
ð
Na
3
C
Þ
i
þ
ATP
i
!
ð
Na
3
CP
Þ
i
þ
ADP
i
K
4
K
5
Na
o
þ
C
ð
o
ð
Na
CP
Þ
i
!
ð
Na
CP
Þ
o
!
3
3
3
K
6
K
7
K
o
!
C
ð
o
þ
2
ð
K
CP
Þ
o
!
P
i
þ
K
ð
C
Þ
i
2
2
K
8
K
i
þ
C
i
ð
K
C
Þ
i
!
2
2
K
9
ADP
i
!
