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 electrical and concentration.
In order to displace sodium and potassium ions in this way, there is the sodium-potassium pump, a transmembrane structure present in cells. It is involved in several fundamental functions for life and its mechanism of action is quite interesting. Let’s see it next.
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 suggests, its main function is to move sodium and potassium ions through the membrane.
This process occurs in the form of active transport, doing it 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 within (140 mEq / L).
To do this, the pump uses the energy obtained from the hydrolysis of ATP and, therefore, it is considered an enzyme of the Na + / K + ATPase type. By expending that energy, it causes the cell to expel sodium while introducing potassium.
This pump belongs to the class of P-class ion pumps, since they displace ions. These types of pumps are made up of at least one transmembrane alpha catalytic subunit, a structure which has a place where an ATP molecule and a minor 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 thanks to this find.
How is its structure?
As we already said, the sodium-potassium pump is a structure with an enzymatic function. Its structure is made up of two protein subunits of the alpha (α) type and two of the beta (β) type. Thus, this pump is a tetramer (α2β2), whose integral proteins cross the lipid bilayer, that is, the cell membrane and also some organelles.
Both types of subunits show variations and, so far, three isoforms have been found for the alpha subunit (α1, α2 and α3) and three for the beta (β1, β2 and β3). 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.
Regarding the beta subunits, their distribution is somewhat more diffuse. The β1 can be found in multiple places, being absent in the vestibular cells of the inner ear and the glycolytic muscle cells of rapid response, 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, acting as a pump itself.
Structurally, the alpha subunits are made up 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 +, while the K + binding site is found on the extracellular side (outside the cell).
2. Beta subunits
The beta subunits do not seem to participate directly in the pumping function, but it has been seen 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 with a single transmembrane domain. The carbohydrate residues that can be found in these subunits are found inserted in the external region of the cell.
Function of the sodium-potassium pump
The cell can be compared to a balloon filled with fresh water thrown into the sea. Its layer is almost impervious, and the internal environment has very different chemical properties than the external environment. The cell has variable concentrations of different substances in comparison with the environment that surrounds it, with significant differences with sodium and potassium.
This is related to the main function of the sodium-potassium pump, which consists in maintaining homeostasis of the intracellular medium, controlling the concentrations of these two ions. To achieve this objective, carry out fundamental processes:
1. Ion transport
It introduces K + ions and expels Na + ions. The natural tendency, that is, without the implication of the pump, is that sodium enters and potassium leaves, 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), while with K + it occurs the other way around, there is less concentration outside (4 mEq / L) than inside ( 140 mEq / L)
2. Cell volume control
As ions leave and enter, the cell volume is also controlled, controlling the amount of liquid within the cell itself.
3. Generation of membrane potential
The sodium-potassium pump participates in the generation of the membrane potential. This is due to the fact that, by expelling three sodium ions for every two potassium ions it introduces, the cell membrane remains negatively charged on its inside.
This generates charge differences between the inside and outside of the cell, a difference which is known as the resting potential.
Ions are positively charged, so it shouldn’t be possible for them to be pushed in and out the way they do. However, the existence of ion channels in the membrane allows, selectively, that there is a flux against electrochemical gradient when necessary.
Mechanism of action
As we already said, the sodium-potassium pump has an enzymatic function and, for this reason, it 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.
For the reaction to take place, the presence of an ATP molecule and a Na + ion inside the cell and a K + ion outside the cell is necessary. Na + ions bind to the enzyme transporter, which has three cytosolic binding sites for this ion. This state is called E1 and, after reaching it, ATP binds to its site on the molecule, hydrolyzing and transferring a phosphate group to an aspartate 376 molecule, a process from which an acylphosphate is obtained. This induces the change to the next state, E2. After this, comes 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 acquires a great importance considering that it prevents the cell from introducing too many Na + ions inside it. 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 I followed this trend, and using the previous case of the balloon as an example, the cell would explode as if it were one. It is thanks to the action of the pump that the cell is prevented from collapsing like this.
In addition, the pump contributes to the formation of the membrane potential. Introducing two K + ions for every three Na + that are expelled, the internal electrical charges are decompensated, favoring the production of the cell’s characteristic membrane potential. This importance is even greater if nerve cells are taken into account, in which the action potential is characterized by the reverse process, that is, the entry of sodium and the exit of potassium.
Another interesting aspect of sodium-potassium pumps is that they are involved in kidney function and, in fact, it would not be possible without them. The kidneys filter 180 liters of plasma every 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 depend directly on the sodium-potassium pumps, which are found in the tubular segments of the kidney nephrons.
- 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, DL, Lehninger, AL, & Cox, MM (2008). Lehninger principles of biochemistry. Macmillan.
- Alberts, B., Bray, D., Hopkin, K., Johnson, AD, Lewis, J., Raff, M.,… & Walter, P. (2013). Essential cell biology. Garland Science.