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+
+
(4) Extrusion of Na into the extracellular space, and uptake of K ;
2+
(5) Dephosphorylation of the enzyme, and release of phosphate and Mg into the intracellular
space;
(6) Conformational rearrangement E 2 → E 1;
+
(7) Release of K into the intracellular space;
2+
2+
2+
(8) Reactivation of the enzyme by uptake of Mg : E 1 + Mg → E 1(Mg ).

P
O O - 3Na+ O O
C C
O - intra
i n n e n
O C
O - ATP
O O C O
C ADP O - O - C
O - C O C
O O -
a u ß e n
extra
E 1 2K+ E 2

+
+
Figure30. Transmembrane transport of Na and K by ATPase. E 1 and E 2 represent different
conformers of the ATPase.

Na + Na +
OH OH
Mg O Mg O
O P O O P OH
ADP
OH H O
2
O O O O + ADP
NH C NH C
H C H C
CH 2 CH 2
C C
O O

Figure 31. Phosphorylation of the ATPase. Phosphatases can be inhibited by the phosphate
-
analogue vanadate (H 2PO 4 ), because vanadium freezes in the trigonal-bipyramidal transition
state (shown on the left).

Ion channels and ionophores

Along with the ATP driven transport, alkaline (and alkaline earth) metal ions can surmount the
membrane also by “passive“ transport along ion channels (hydrophilic transport) or with the
help of transport vehicles, so-called ionophores (hydrophobic transport).

Ion channels are trans-membrane proteins, with their inner surface aligned with
carboxylate (stemming from Glu and Asp) or/and carbonyl groups, allowing for the transport
of ions, which are usually partly or completely deprived of their hydration shell while
transported through weak coordinative interaction with the oxygen functionalities in the
channel. The transport can also occur in the sense of a symport (concomitant transport of two
ionic species) or antiport (counter transport; two ionic species being transported in opposite
directions). The following types of ion channels are distinguished: