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Encyclopedia of Physical Science and Technology EN008K-353 June 29, 2001 12:41

Ion Transport Across Biological Membranes 107

VII. INORGANIC ION TRANSPORT transmembrane membrane ion transport that requires the
AGAINST A CONCENTRATION hydrolysis of ATP. (2) Fast activated channels. For activa-
GRADIENT AT EXPENSE tion, these just require the binding of a ligand to the chan-
OF ATP HYDROLYSIS nel protein or a change in transmembrane voltage. The
examples given are the neurotransmitter-activated chan-
+
Most cells have a high concentration of K andalow nels and the voltage-activated K channel.
+
concentration of Na inside the cell membrane relative to The development of techniques using crystallography,
+
the concentration of these ions bathing the cell. As dis- NMR, electron diffraction, and molecular biology to pro-
cussed, the rapid flow of these ions along their concentra- duce specific proteins in large amounts to determine the
tion gradient across the cell membrane is used in signal structureoftransmembranechannelsformedbyproteinsis
transduction between the cells of an organism, and plays a very active field. It is expected that an increasing num-
an important role in life. To maintain the concentration ber of high-resolution transmembrane structures will be
gradient of sodium and potassium ions and thus the rest- forthcoming in the next few years. These structures, to-
ing membrane potential of the cells, energy is needed. In gether with kinetic measurements, are expected to give
1979, Skou and Norby discovered an enzyme that reestab- detailed information about the mechanism by which in-
lishes the original concentration gradient. It is called the organic ions are transported across the cell membrane.
+
Na –K ATPase where ATP stands for adenosine triphos- The single-channel current-recording technique is ideally
+
phate, an energy source. Sodium ions are moved from in- suited for studying channels that open because of the con-
side of the cell to the outside and potassium ions are moved centration gradient of inorganic ions and the resulting
in the opposite direction. With each cycle of the enzyme, voltage changes. The change in transmembrane voltage
one molecule of ATP is hydrolyzed and two potassium determines whether or not a signal is propagated. Rapid
ions are moved into the cell and three sodium ions moved chemical kinetic techniques with a 100-µsec time res-
out. olution, and suitable for investigations of ligand-gated
+
+
Na –K ATPase is a membrane protein with two sub- ion channels on cell surfaces, are also now available.
units spanning the membrane. The current hypothesis is They are expected to provide additional information about
that the enzyme reacts with ATP to give a phosphorylated ligand-gated ion channels and their mechanism of ac-
enzyme and ADP (adenosine diphosphate). The conver- tion. The ability to determine the effect of neurotransmit-
sion of ATP to ADP (adenosine diphosphate) and P i (in- ter concentration on the rate of transmembrane ion flux
organic phosphate has been formulated as follows: and, therefore, the change in transmembrane voltage is
expected to provide important insight into how cells per-
ATP + Enzyme → ADP
ceive, store, and transmit information. It is also expected
+ P-Enzyme (phosphorylated enzyme) to indicate how the receptor mechanism is changed by dis-
eases of the nervous system and by the hundreds of drugs
P-Enzyme + H 2 O → Enzyme + P i
that affect the mechanism of these proteins. This informa-
The resulting enzyme has a high affinity for K and tion is expected to be essential in devising strategies for
+
+
alow affinity for Na . Hydrolysis of the phosphorylated curing mental diseases and overcoming drug addiction.
enzymeresultsintheliberationofinorganicphosphateand
the regeneration of the enzyme form with high affinity
+
for Na + and low affinity for K . The net result is the SEE ALSO THE FOLLOWING ARTICLES
+
movement of two K ions into the cell and three Na ions
+
out of the cell for each ATP hydrolyzed. About 25% of the BIOENERGETICS • BIOINORGANIC CHEMISTRY • LIPO-
energy consumption of a human at rest is used to maintain PROTEIN/CHOLESTEROL METABOLISM • MEMBRANE
the resting concentration of sodium and potassium ions in STRUCTURE • NUCLEIC ACID SYNTHESIS • PHOTOCHEM-
cells. ISTRY,MOLECULAR • PROTEIN STRUCTURE • PROTEIN
SYNTHESIS
VIII. CONCLUSION AND OUTLOOK
BIBLIOGRAPHY
It should be mentioned that examples of two types of ion
Doyle, D. A., Morais, C. J., Pfuetzner, R. A., Kuo, A., Gulbis, J. M.,
channels have been given. (1) So-called slow channels in-
Cohen, S. L., Chait, B. T., and MacKinnon, R. (1988). “ The struc-
volve second messengers. The examples given here are the ture of the potassium channel: Molecular basis of K conduction and
+
light-activated channels that are opened by cGMP and the selectivity,” Science 280, 669–677.