Active transport is the process required to pump counter-gradient molecules, both electrical and concentration.
Being able to displace sodium and potassium ions in this way exists the sodium-potassium pump, a transmembrane structure found in cells. It is involved in several fundamental functions of life and its mechanism of action is quite interesting. Let’s see below.
What is the sodium-potassium pump?
The sodium-potassium pump is a protein structure found in many cell membranes. As the name suggests, its main function is to move sodium and potassium ions across the membrane.
This process occurs in the form of active transport, which works against the concentration gradient. Inside the cell, sodium (Na +) is less concentrated (12 mEq / L) than outside (142 mEq / L), While it is the opposite of potassium (K +), having 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 is therefore considered to be an enzyme of the Na + / K + ATPase type. By spending this energy, the cell expels sodium while introducing potassium.
this bomb belongs to the class of P-class ion pumps, as they move the ions. These types of pumps are made up of at least one transmembrane alpha catalytic subunit, a structure that has a site 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.
How is its structure?
As we said, in the sodium-potassium pump there is an enzymatic-functioning structure. Its structure consists of two protein subunits of alpha type (α) and two beta types (β). Thus, this pump is a tetramer (α2β2), the integral proteins cross the lipid bilayer, that is to say the membrane of the cells and also certain organelles.
Both types of subunits have variations and so far 3 isoforms could be found for the alpha subunit (α1, α2 and α3) and three for the beta (β1, β2 and β3). 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 is found in the heart and brain.
As for the beta subunits, their distribution is a little more diffuse. Β1 can be found in multiple sites, being absent from inner ear vestibular cells and rapidly responding glycolytic muscle cells, this absence being occupied by the β2 isoform.
1. Alpha subunits
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, performing the function of the pump itself.
Structurally, alpha subunits are made up of large polypeptides, with a molecular weight of 120 kDa (kilodaltons). In their intracellular side (inside the cell), they have binding sites for the ATP molecule and for Na +, while the K + binding site is on the extracellular side (outside of the cell).
2. Beta subunits
Beta subunits do not appear to be directly involved in the pumping function, but it has been shown that in their absence, the sodium-potassium pump does not perform its main function.
These subunits have a molecular weight of 55 kDa each, and they are made up of glycoproteins with a single transmembrane domain. The carbohydrate residues that can be found in these subunits are inserted into the outer region of the cell.
Sodium-potassium pump function
The cell can be compared to a balloon filled with fresh water thrown into the sea. Its layer is almost waterproof and the internal environment has very different chemical properties from the external environment.. The cell has varying concentrations of different substances compared to the surrounding environment, with significant differences with sodium and potassium.
This is linked to the main function of the sodium-potassium pump, which is to maintain the homeostasis of the intracellular environment, by controlling the concentrations of these two ions. To achieve this goal, perform key processes:
1. Ion transport
It introduces K + ions and expels Na + ions. The natural tendency, that is, without the intervention of the pump, is that sodium goes in and potassium goes out, as they are increasingly 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 opposite, there is less concentration outside (4 mEq / L) than inside (140 mEq / L)
2. Cell volume control
As ions move out and in, the volume of the cell is also controlled, controlling the amount of fluid inside the cell itself.
3. Generation of membrane potential
The sodium-potassium pump is involved in the generation of membrane potential. This is because, by expelling 3 sodium ions for every two potassium ions it introduces, the cell membrane remains negatively charged on its internal face.
This generates differences in charge between the interior and exterior of the cell, a difference known as the quiescent potential.
Ions have a positive charge, so it shouldn’t be possible for them to be introduced and expelled as they do. However, the existence of ion channels in the membrane selectively allows electrochemical counter-gradient flow when needed.
As we said, the sodium-potassium pump has an enzymatic function and for this reason is also called ATPase Na + / K +. 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 + outside is necessary. Na + ions bind to the enzyme transporter, which has three cytosolic binding sites for this ion. This state is called E1 then reached, ATP is fixed in place of the molecule, Hydrolysis and transfer of a phosphate group to an aspartate 376 molecule, a process from which an acylphosphate is obtained. This induces the transition to the next state, E2. After that comes the expulsion of three sodium ions and the introduction of two of potassium.
Importance of the sodium-potassium pump
Based on what we have explained, the sodium-potassium pump acquires great importance because it prevents the cell from introducing too many Na + ions inside. This greater quantity of sodium inside the cell is conditioned by a greater influx of water and, consequently, an increase in the volume of the cell. If you followed this trend, and taking the previous example of the bubble, the cell would explode as if it were one. It is thanks to the action of the pump that the cell is thus prevented from collapsing.
In addition, the pump contributes to the formation of the membrane potential. The introduction of two K + ions for three expelled Na + ions decomposes the internal electric charges, Promoting the production of the characteristic membrane potential of cells. This importance is even greater if we take into account the nerve cells, 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 without them it would not be possible. 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 in the urine. Reabsorption of sodium, water and other substances directly depends on sodium-potassium pumps, which are found in the tubular segments of renal nephrons.
- Guyton AC, Hall JE: Transport of substances across the cell membrane, in: Textbook of Medical Physiology, 13th ed., AC Guyton, JE Hall (eds). Philadelphia, Elsevier Inc., 2016.
- Nelson, DL, Lehninger, AL and Cox, MM (2008). Lehninger’s principles of biochemistry. Macmillan.
- Alberts, B., Bray, D., Hopkin, K., Johnson, AD, Lewis, J., Raff, M., … and Walter, P. (2013). Essential cell biology. Garland Science.