Jens Christian Skou was awarded the Nobel Prize in Chemistry in 1997 for being the first person to describe an ion pump. An ion pump is an enzyme that transports ions across a cell membrane.

Skou described the sodium-potassium pump as early as 1957, but a number of enzymes with similar properties have subsequently been detected.

• Function of the sodium-potassium pump in the body
• The road to the pump
• The pump’s cycle
• More information

Function of the sodium-potassium pump in the body

Inside the cells, the sodium-potassium pump does not appear particularly significant. Each pump measures approximately65 x 75 x 150 angstrom units, and an angstrom is one ten-millionth of a millimetre. In spite of its inferior size, however, this little pump is very important in our lives because it acts as a generator that provides the cells with electricity and thus entirely controls both our movement apparatus and our thought apparatus. We use less than a quarter of our energy to drive the activity of the little pump.

The sodium-potassium pump is anchored in the cell membrane and pumps sodium ions (Na ⁺ ) and potassium ions (K ⁺ ) out of and into the cell, respectively. The intracellular Na ⁺ concentration is lower than the extracellular. To equalise the difference, Na ⁺ automatically flows into the cell via channels in the cell membrane, but it is continuously pumped out again by means of the sodium-potassium pump. For each round of this pumping action, a high-energy molecule called ATP is broken down (ATPase activity takes place), whereby three Na ⁺ are transported out of the cell and two K ⁺ in.The difference in the intracellular and extracellular concentration forms the basis for the electrical potential difference across the cell membrane that is released by nerve impulses in muscles and the brain, for example.

The sodium-potassium pump is the key to functions such as cardiac and renal activity, as well as all general transport processes into and out of the cell. The pump thus forms the basis for our ability to absorb a considerable number of nutrients, excrete waste products from the kidneys and regulate the water balance in the cells. If this little pump stopped pumping sodium ions out of the cells, the latter would rapidly swell up because of the infiltration of water and finally burst.

Illustration: Kjell Lundin.

The intracellular Na + concentration is lower than the extracellular.To equalise the difference, Na + automatically flows into the cell via channels in the cell membrane, but it is continuously pumped out again by means of the sodium-potassium pump. It is very important that the pump continuously maintains the (unequal) intracellular and extracellular Na + balance because the flow of Na + into a nerve cell forms the basis for the nerve impulses that make it possible for us to move.

The road to the pump

It has been well known since the 1870s that the intracellular composition of ions is different from the cell’s surroundings.The concentration of Na ⁺ is lower inside the cell and the concentration of K ⁺ is higher than in the extracellular fluid.

In 1939, scientists demonstrated that muscle cell membrane can be penetrated by Na ⁺ – but how was the unequal Na ⁺ balance maintained all the time? This question was given a theoretical answer in the early 1940s, when the proposal was made that maintaining this uneven distribution of ions was caused by a type of “pump” in the cell membrane.

During the 1950s, it was discovered that Na ⁺ always flows into a nerve cell when the nerve is stimulated. Scientists observed that the difference arising in the Na ⁺ concentration was equalled by Na ⁺ one way or another being transported out again. It was also considered likely that this ion transport required ATP (an important energy carrier in the cells), because the scientists demonstrated that it was possible to obstruct the transport of Na ⁺ and K ⁺ by inhibiting the formation of ATP.

In connection with a number of studies on the influence of local anaesthetics on the structure of cell membranes, Skou wanted to measure how an ATPase activity was affected by these anaesthetic agents. It turned out that the ATPase he had chosen – extracted from crab nerves – had enzymatic characteristics not previously described in the literature. Skou particularly found that both Na ⁺ and K ⁺ were necessary for ATPase activity. This led to a proposal that this ATPase was involved in Na ⁺ and K ⁺ transport.

In the article he initially published, Skou did not use the term “pump” regarding his discovery. He tentatively just stated that the enzyme he described (chemically indicated as Na ⁺ ,K ⁺ -ATPase), had properties that made it reasonable to assume that it was involved in the active transport of Na ⁺ and K ⁺ across the cell membrane. It was soon proved that Skou was right in his assumption, and the article earned him the Nobel Prize in 1997.

The pump’s cycle

Illustration: Kjell Lundin.

The enzyme opens up towards the inner side of the cell with binding sites for three Na + . Once the Na + are bound, an ATP molecule is bound to the enzyme and one of its phosphate groups (P) is transferred,i.e. there is hydrolysis of ATP, in which energy is transferred to the enzyme. The enzyme changes shape, opens up towards the outer side of the cell and releases Na + . There are now binding sites for two K + on the inner side of the cell. When K + is bound, the phosphate group (P) is released. When K + has been released within the cell, the enzyme returns to its first stage – ready to receive new Na + , so the cycle can begin all over again.

More information

• The influence of some cations on an adenosine triphosphatase from peripheral nerves , Jens Christian Skou’s Nobel article from 1957.
• The Identification of the Sodium-Potassium Pump , Jens Christian Skou’s Nobel lecture.