Sodium-potassium Pump: What It Is And What Are Its Functions In The Cell
Active transport is the process that is required to pump molecules against the gradient, both electrical and concentration.
In order to move sodium and potassium ions in this way, there is 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.
The sodium-potassium pump is a protein structure that can be found in many cell membranes As its name indicates, 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 (140 mEq/L).
To carry out 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. Expending that energy, it causes the cell to expel sodium while introducing potassium.
This bomb belongs to the class of class P ion pumps, since they displace ions This type of pump is 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 discovery.
What is its structure like?
As we already said, the sodium-potassium pump is a structure with an enzymatic function. Its structure is made up of two alpha-type (α) and two beta-type (β) protein subunits. 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 present variations and, until now, Three isoforms have been 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 can be found in the heart and brain.
As for the beta subunits, their distribution is somewhat more diffuse. β1 can be found in multiple places, being absent in the vestibular cells of the inner ear and fast-response glycolytic muscle cells, this absence being occupied by the β2 isoform.
1. Alpha subunits
The alpha subunits are structures that contain binding sites for the ATP molecule and Na+ and K+ ions These subunits represent the catalytic component of the enzyme, performing the pump function 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 pump function, but it has been seen that, in their absence, the sodium-potassium pump does not fulfill its main function.
These subunits have a molecular weight of 55 kDa each, and consist of glycoproteins with a single transmembrane domain The carbohydrate residues that can be found in these subunits are 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 lying in the sea. Its layer is almost impermeable, and the internal medium has very different chemical properties than the external medium The cell has variable concentrations of different substances compared to 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 of maintaining the homeostasis of the intracellular medium, controlling the concentrations of these two ions. To achieve this objective, carry out fundamental processes:
1. Ion transport
Introduces K+ ions and expels Na+ ions The natural tendency, that is, without the involvement of the pump, is for sodium to enter and potassium to leave, given that 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 happens the other way around, there is a lower concentration outside (4 mEq/L) than inside ( 140 mEq/L)
2. Cell volume control
When ions enter and leave, cell volume is also controlled, controlling the amount of liquid inside the cell itself.
3. Generation of membrane potential
The sodium-potassium pump participates in the generation of the membrane potential. This is due to, By expelling three sodium ions for every two potassium ions introduced, the cell membrane remains negatively charged on its inner side
This generates charge differences between the inside and outside of the cell, a difference which is known as the resting potential.
Ions have a positive charge, so it should not be possible for them to be introduced and expelled the way they do. However, the existence of ion channels in the membrane selectively allow for flow against an 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 occur, the presence of an ATP molecule and a Na+ ion inside the cell and a K+ ion outside the cell are necessary. Na+ ions are attached to the transporter to the enzyme, 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.
Based on what we have explained, The sodium-potassium pump becomes very important considering that it prevents the cell from introducing too many Na+ ions into its interior This greater amount of sodium inside the cell is conditioned by a greater entry of water and, consequently, an increase in the volume of the cell. If this trend were to continue, and using the previous case of the balloon as an example, the cell would burst 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. By introducing two K+ ions for every three Na+ that are expelled, the internal electrical charges are decompensated, favoring the production of the characteristic membrane potential of cells. This importance is even greater if we take into account nerve cells, in which the action potential is characterized by the inverse process, that is, the entry of sodium and the exit of potassium.
Renal function
Another interesting aspect of sodium-potassium pumps is that 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 through urine. The reabsorption of sodium, water and other substances depends directly on the sodium-potassium pumps, which are found in the tubular segments of the nephrons of the kidney.
