Biology Reference
In-Depth Information
discovery, ouabain was routinely used to treat atrial fibrillation and congestive heart
failure in humans. More recently, ouabain has been replaced by digoxin, a structurally
related, but more lipophilic cardiac glucoside.
There are several important observations about the Na
þ
/K
þ
ATPase that had to be
factored in before a mechanism of action could be proposed. These include:
1. Na
þ
/K
þ
ATPase is an example of primary active transport and active antiport.
2. Na
þ
/K
þ
ATPase is inhibited by ouabain, a cardiac glycoside.
3. Ouabain binds to the outer surface of the Na
þ
/K
þ
ATPase and blocks K
þ
transport into
the cell.
4. Na
þ
binds better from the inside.
5. K
þ
binds better from the outside.
6. ATP phosphorylates an aspartic acid on the enzyme from the inside.
7. Phosphorylation is related to Na
þ
transport.
8. Dephosphorylation is related to K
þ
transport.
9. Dephosphorylation is inhibited by ouabain.
10. 3Na
þ
s are pumped out of the cell as 2 K
þ
s are pumped in.
11. The Na
þ
/K
þ
ATPase is electrogenic.
The mechanism of the Na
þ
/K
þ
ATPase
[19]
is based on toggling back and forth between
two conformational states of the enzyme, ENZ-1 and ENZ-2. Three Na
þ
s bind from the inside
to the Na
þ
/K
þ
ATPase in one conformation (ENZ-1). This becomes phosphorylated by ATP
causing a conformation change to ENZ-2~P. ENZ-2~P does not bind Na
þ
but does bind
2K
þ
s. Therefore 3 Na
þ
s are released to the outside and 2 K
þ
s are bound generating ENZ-
2~P (2K
þ
). Upon hydrolysis of ~P, the Na
þ
/K
þ
ATPase reverts back to the original ENZ-1
conformation that releases 2 K
þ
s and binds 3 Na
þ
s from the inside. Ouabain blocks the
dephosphorylation step.
Mechanism of the Na
þ
/K
þ
ATPase
Secondary Active Transport
Secondary active transport (also known as co-transport) systems are composed of two
separate functions. The energy-dependent movement of an ion (e.g. H
þ
,Na
þ
or K
þ
)
generates an electrochemical gradient of the ion across the membrane. This gradient is
coupled to the movement of a solute in either the same direction (Symport), or in the
