Sodium-potassium pump: what is it and what are its functions in the cell?

This structure present in the cell membrane is essential for the functioning of the organism.

Active transport is the process required to pump counter-gradient molecules, both electrically and concentration.

In order to move sodium and potassium ions in this way, the sodium-potassium pump exists. the sodium-potassium pump, a transmembrane structure present in cells.. It is involved in several functions fundamental to life and its mechanism of action is quite interesting. Let's see it below.

What is the sodium-potassium pump?

The sodium-potassium pump is a protein structure that can be found in many cell membranes.. As its name implies, its main function is to move sodium and potassium ions across the membrane.

This process occurs in the form of active transport, doing so against the concentration gradient. Inside the cell, sodium (Na+) is less concentrated (12 mEq/L) than outside (142 mEq/L), while the opposite occurs with potassium (K+), with a lower concentration outside (4 mEq/L) than inside (142 mEq/L).while the opposite occurs with potassium (K+), with a lower concentration outside (4 mEq/L) than inside (140 mEq/L).

To do this, the pump uses the energy obtained from the hydrolysis of ATP and, for this reason, it is considered an enzyme of the Na+/K+ATPase type. By expending this energy, it causes the cell to expel sodium while introducing potassium.

This pump belongs to the class of P-class ionic pumps, since it displaces ions. This type of pump consists of at least one transmembrane alpha catalytic subunit, a structure which has a place where an ATP molecule and a smaller beta subunit can bind.

It was discovered in 1957 by Jens Skou (1918-2018), a Danish chemist and university professor who won the Nobel Prize in Chemistry for this discovery.

What is its structure like?

As we have already said, the sodium-potassium pump is a structure with enzymatic function. Its structure is made up of two alpha (α) and two beta (β) protein subunits. Thus, this pump is a tetramer (α2β2), whose integral proteins cross the lipid bilayer, i.e., the cell membrane and also some organelles.

Both types of subunits present variations and, so far, three isoforms for the alpha subunit (α1, α2 and α3) and three for the beta subunit (β1, β2 and β3) have been found.. The α1 is found in the membranes of most cells, while the α2 isoform is characteristic of muscle cells, heart, adipose tissue and brain. The α3 isoform can be found in the Heart and brain.

As for the beta subunits, their distribution is somewhat more diffuse. The β1 can be found in multiple locations, being absent in the vestibular cells of the inner ear and the fast responding glycolytic muscle cells, this absence being occupied by the β2 isoform.

1. Alpha subunits

The alpha subunits are structures that contain the binding sites for the ATP molecule and the Na+ and K+ ions.. These subunits represent the catalytic component of the enzyme, exerting the pump function itself.

Structurally, the alpha subunits consist of large polypeptides, with a molecular weight of 120 kDa (kilodaltons). On their intracellular side (inside the cell) they have binding sites for the ATP molecule and for Na+, whereas the K+ binding site is on the extracellular side (outside the cell).

Beta subunits

The beta subunits do not seem to participate directly in the pumping function, but it has been shown that, in their absence, the sodium-potassium pump does not fulfill its main function.

These subunits each have a molecular weight of 55 kDa, and consist of glycoproteins consist of glycoproteins with a single transmembrane domain.. The glycosidic residues that can be found in these subunits are inserted in the outer region of the cell.

Function of the sodium-potassium pump

The cell can be compared to a balloon filled with fresh water dropped into the sea. Its shell is almost impermeable, and the internal medium has very different chemical properties from the external medium.. The cell has varying concentrations of different substances compared to the surrounding medium, with significant differences with sodium and potassium.

This is related to the main function of the sodium-potassium pump, which is to maintain the homeostasis of the intracellular medium by controlling the concentrations of these two ions. To achieve this goal, fundamental processes are carried out:

1. Ion transport

It introduces K+ ions and expels Na+ ions.. The natural tendency, i.e. without the involvement of the pump, is for sodium to enter and potassium to leave, since they are less and more concentrated inside the cell, respectively.

Na+ is more concentrated outside the cell (142 mEq/L) than inside (12 mEq/L), whereas with K+ it is the other way around, there is less concentration outside (4 mEq/L) than inside (140 mEq/L).

2. Control of cell volume

The outflow and inflow of ions also controls cell volume, controlling the amount of fluid inside the cell itself.

Generation of the membrane potential

The sodium-potassium pump is involved in the generation of the membrane potential. This is because, by expelling three sodium ions for every two potassium ions it introduces, the cell membrane remains negatively charged on its inner side..

This generates charge differences between the inside and outside of the cell, a difference known as the resting potential.

Ions are positively charged, so it should not be possible for them to be introduced and expelled in the way they do. However, the existence of ion channels in the membrane selectively allows a flow against electrochemical gradient when necessary.

Mechanism of action

As mentioned above, the sodium-potassium pump has an enzymatic function and, for this reason, is also called Na+/K+ ATPase. The mechanism of action of this transmembrane structure consists of a catalytic cycle in which a phosphoryl group is transferred to the sodium-potassium pump..

For the reaction to occur, the presence of an ATP molecule and a Na+ ion inside the cell and a K+ ion outside the cell is necessary. The Na+ ions are attached to the transporter to the enzyme, which has three cytosolic binding sites for this ion. This state is referred to as E1 and, upon reaching this state, ATP binds to its site on the molecule, hydrolyzing and transferring it to the enzyme.This state is called E1 and, after reaching it, ATP binds to its site on the molecule, hydrolyzing and transferring a phosphate group to a molecule of aspartate 376, a process from which an acylphosphate is obtained. This induces the change to the next state, E2. This is followed by the expulsion of three sodium ions and the introduction of two potassium ions.

Importance of the sodium-potassium pump

Based on what we have explained, the sodium-potassium pump takes on great importance as it prevents the cell from introducing too many Na+ ions into its interior.. This greater amount of sodium in the cell interior is conditioned by a greater entry of water and, consequently, an increase in the volume of the cell. If this tendency were to continue, and using the example of the balloon, the cell would burst as if it were a balloon. It is thanks to the action of the pump that the cell is prevented from collapsing in this way.

In addition, the pump contributes to the formation of the membrane potential. By introducing two K+ ions for every three Na+ ions that are expelled, the internal electrical charges are decompensated, favoring the production of the membrane potential.The production of the characteristic membrane potential of the cells is favored by the introduction of two K+ ions for every three Na+ ions that are expelled. This importance is even greater if nerve cells are taken into account, in which the action potential is characterized by the reverse process, i.e., sodium input and potassium output.

Renal function

Another interesting aspect of the sodium-potassium pumps is that they are involved in renal function. are involved in renal function and, in fact, without them it would not be possible to. The kidneys filter 180 liters of plasma each day, which contains substances that must be excreted, while others must be reabsorbed so that they are not lost through the urine. The reabsorption of sodium, water and other substances depends directly on the sodium-potassium pumps, which are located in the tubular segments of the kidney nephrons.

Bibliographic references:

• Guyton AC, Hall JE: Substance Transport Across the Cell Membrane, in: Textbook of Medical Physiology, 13th ed, AC Guyton, JE Hall (eds). Philadelphia, Elsevier Inc., 2016.
• Nelson, D. L., Lehninger, A. L., & Cox, M. M. (2008). Lehninger principles of biochemistry. Macmillan.
• Alberts, B., Bray, D., Hopkin, K., Johnson, A. D., Lewis, J., Raff, M., … & Walter, P. (2013). Essential cell biology. Garland Science.