Biomedical Engineering Reference
In-Depth Information
Fig. 4.12
A model diagram
for a linear
β
-helix located
in the membrane.
a
Chan-
nel length is larger than the
bilayer thickness.
b
Channel
length matches the bilayer
thickness.
c
Channel length
is smaller than the bilayer
thickness. To hydrophobically
match a bilayer and a channel
at the two channel edges, the
bilayer deformation, channel
orientation, stretching, etc.,
can be possible. Due to the
higher stiffness of the pro-
tein (channel) structure, it is
hypothesized that the bilayer
(due to its elastic properties)
is more likely to undergo
necessary deformations. The
previously discussed grami-
cidin A channel (Sect.
4.1.1
)
falls in this category
K
+
-ATPase, originally described more than a half century ago [
49
], is a membrane-
bound ion pump belonging to the family of P-type ATPases. By using energy derived
from ATP hydrolysis, the Na
+
,K
+
-ATPase generates electrochemical gradients for
Na
+
and K
+
across the plasma membranes of animal cells, as required for electrical
excitability, cellular uptake of ions, nutrients, and neurotransmitters, and the regu-
lation of cell volume and intracellular pH. The transport function is accomplished
by enzyme conformational changes between two states, E1 and E2, that selectively
bind three Na
+
and two K
+
ions, respectively (see Fig.
4.13
); the ions become tran-
siently 'occluded', that is, inaccessible to the medium on either side of the membrane
[
23
,
40
]. The pump is sensitive to the membrane potential—the major voltage-
dependent steps being associated with the binding and release of one of the three
Na
+
ions [
4
,
19
].
The Na
+
,K
+
-ATPase consists of
α
- and
β
-subunits. The
α
-subunit has binding
sites for Na
+
,K
+
, and ATP, and is homologous to single-subunit P-type ATPases
such as the Ca
2
+
-ATPase. The
β
-subunit is unique to the K
+
-counter-transporting
P-type ATPases, Na
+
,K
+
-ATPase and H
+
,K
+
-ATPase. The
β
-subunit is required
for routing the
α
-subunit to the plasma membrane and for occlusion of the K
+
ions
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