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membrane is usually impermeable for the hydrated, hydrophilic sodium and potassium ions,
transport mediators, so-called ionophores can be employed. Alternatively, and more efficiently,
the transport can occur along ion channels in the membrane.

+
+
+
+
The active transport for Na and K is achieved by a Na /K -specific pump, an ATPase,
abbreviated E (for enzyme) in the following discussion. The energy necessary for the transport
+
+
+
+
Na (in) → Na (ex) // K (ex) → K (in) against a concentration gradient is provided by the
hydrolysis of ATP. E consists of two glycoprotein subunits of molecular mass 131 and 62 kDa,
the larger of which is the transport unit. In the course of the ion transport, coupled to
+
phosphorylation/dephosphorylation of E, this unit switches between the conformations E 1 (Na
+
+
+
sensitive) und E 2 (K sensitive). Per MgATP hydrolysed two 2 K are locked in and 3 Na are
locked out:

-
+
-
+
+
+
MgATP + 3Na in + 2K ex → MgADP + P i + 3Na ex + 2K in

extracellular intracellular
+ -
+
+
c(Na ) = 140 mM c(Na ) = 10 mM
+
+
c(K ) = 5 mM c(K ) = 150 mM
3Na +
Active transport
2K + by Na,K-ATPase
+
K , Na +
Passive transport

2+
+
The charge imbalance thus produced is balanced, in part, by a Na ,Ca -ATPase, and in part by
passive transport. The various steps of the transport as catalysed by the Na,K-ATPase are
assembled in Fig. 29; for the conformational changes of E see Fig. 30.
MgATP
+
2K + 3Na
E 1 (Na, Mg, ATP)

E 1 (K) E 1 (Na) ADP
E 1 (Na, Mg, P)
Mg 2+ ,P i i n n e n
E

a u ß e n E 2 (Na, Mg, P)
E 2 (K)
Figure 29. Function of the Na,K
E 2 (Mg, P) pump (Na,K-dependent ATPase),
E 2 (K, Mg, P) 3Na +
E.
2K +

Individual events (see Fig. 29):
+
2+
(1) Uptake of intracellular Na , Mg and ATP by E 1;
(2) Phosphorylation of the enzyme (cf. Fig. 31);
(3) Conformational change E 1 → E 2 (cf. Fig. 30);