SODIUM POTASSIUM PUMP: STRUCTURE, FUNCTION, MECHANISM, IMPORTANCE - BIOLOGY - 2025

The sodium potassium pump is a protein structure included in a broader set of molecules present in many cell membranes, and which are responsible for the active transport of ions or other small molecules against their concentration gradients. They use the energy released by the hydrolysis of ATP and that is why they are generically called ATPases.

The sodium potassium pump is a Na + / K + ATPase because it releases the energy contained in the ATP molecule to move sodium from the inside to the outside of the cell, while introducing potassium.

Schematic of the sodium potassium pump. Exterior and interior of the cell. (Source: Miguelferig, via Wikimedia Commons)

Inside the cell, sodium is less concentrated (12 mEq / L) than outside (142 mEq / L), while potassium is more concentrated outside (4 mEq / L) than inside (140 mEq / L).

ATPase pumps are classified into three large groups:

• Type F and V ion pumps: they are quite complex structures, they can be made up of 3 different types of transmembrane subunits and up to 5 associated polypeptides in the cytosol. They function as proton transporters.
• Superfamily ABC (from English A TP- B inding C assette = ATP binding cassette): made up of more than 100 proteins that can function as transporters of ions, monosaccharides, polysaccharides, polypeptides and even other proteins.

• Ion pumps of the P class: formed by at least one catalytic transmembrane alpha subunit that has a binding site for ATP and a minor β subunit. During the transport process the α subunit is phosphorylated and hence its name "P".

The sodium potassium pump (Na + / K + ATPase) belongs to the group of ionic pumps of class P and was discovered in 1957 by Jens Skou, a Danish researcher, when he was studying the mechanism of action of anesthetics on the nerves of a crab (Carcinus maenas); work for which he was awarded the Nobel Prize in Chemistry in 1997.

Potassium sodium pump. NaKpompe2.jpg: Phi-Gastrein at fr.wikipediaderivative work: sonia / CC BY-SA (http://creativecommons.org/licenses/by-sa/3.0/)

Structure of the sodium potassium pump

The sodium potassium pump is an enzyme that, from the point of view of its quaternary structure, is made up of 2 alpha (α) and two beta (β) type protein subunits.

It is, therefore, a tetramer of the α2β2 type, whose subunits are integral membrane proteins, that is, they cross the lipid bilayer and have both intra- and extra-cytosolic domains.

Alpha and beta subunits of the potassium pump. Rob Cowie / CC BY-SA (http://creativecommons.org/licenses/by-sa/3.0/)

Alpha subunits

The α subunits are those that contain the binding sites for ATP and for the Na + and K + ions and represent the catalytic component of the enzyme and the one that exerts the function of the pump itself.

The α subunits are large polypeptides, with a molecular weight of 120 kDa, 10 transmembrane segments and with their N- and C-terminal ends located on the cytosolic side.

On the intracellular side, they have binding sites for ATP and Na +, as well as an aspartate residue at position 376 that represents the site that undergoes the phosphorylation process during pump activation.

The binding site for K + appears to be on the extracellular side.

Beta subunits

The β subunits do not appear to have a direct participation in the pumping function, but in their absence this function does not occur.

The β subunits have a molecular weight of about 55 kDa each and are glycoproteins with a single transmembrane domain whose carbohydrate residues are inserted in the extracellular region.

They appear to be necessary in the endoplasmic reticulum, where they would contribute to the proper folding of the α subunits, and then, at the membrane level, to stabilize the complex.

Both types of subunits are heterogeneous and α1, α2 and α3 isoforms have been described for the one, and β1, β2 and β3 for the other. Α1 is found in the membranes of most cells, while α2 is present in muscle, heart, adipose tissue, and brain, and α3 in the heart and brain.

The β1 isoform has the most diffuse distribution, although it is absent in some tissues such as the vestibular cells of the inner ear and the rapidly responding glycolytic muscle cells. The latter contain only β2.

The different structures of the subunits that make up the Na + / K + pump in the different tissues may be due to functional specializations that have not yet been elucidated.

Potassium pump function

For any moment considered, the plasma membrane constitutes a separation boundary between the compartment corresponding to the interior of a cell and that which represents the extracellular fluid in which it is immersed.

Both compartments have a composition that can be qualitatively different, since there are substances inside cells that are not found outside them and the extracellular fluid contains substances that are not present intracellularly.

Substances that are present in both compartments can be found in different concentrations, and these differences can have physiological significance. Such is the case with many ions.

Maintenance of homeostasis

The Na + / K + pump plays a fundamental role in the maintenance of intracellular homeostasis by controlling the concentrations of sodium and potassium ions. This maintenance of homeostasis is achieved thanks to:

• Ion transport: introduces sodium ions and ejects potassium ions, a process through which it also drives the movement of other molecules through other transporters that depend on either the electrical charge or the internal concentration of these ions.
• Control of cell volume: the introduction or exit of ions also implies movements of water within the cell, so that the pump participates in the control of cell volume.
• Generation of membrane potential: the expulsion of 3 sodium ions for every 2 potassium ions that are introduced causes the membrane to remain negatively charged on the inside, which generates a difference in charges between the inside and outside of the cell. This difference is known as the resting potential.

Na + has an extracellular concentration of about 142 mEq / L, while its intracellular concentration is only 12 mEq / L; K +, on the other hand, is more concentrated inside the cell (140 mEq / L) than outside it (4 mEq / L).

Although the electrical charge of these ions does not allow their passage through the membrane, there are ion channels that do allow it (selectively), which promote movement if the forces that ordinarily move these ions are also present.

However, these differences in concentration are of great importance in the preservation of the organism's homeostasis and must be maintained in a kind of balance that, if lost, would produce important organic alterations.

Sodium potassium diffusion and pump (Source: BruceBlaus. When using this image in external sources it can be cited as: Blausen.com staff (2014). «Medical gallery of Blausen Medical 2014». WikiJournal of Medicine 1 (2). DOI: 10.15347 / wjm / 2014.010. ISSN 2002-4436.Derivative by Mikael Häggström / CC BY (https://creativecommons.org/licenses/by/3.0) via Wikimedia Commons)

• The difference in concentration for Na + between the inside and outside of the cell creates a chemical gradient that pushes sodium inwards and causes this ion to constantly enter and tend to dissipate that difference, that is, to equalize the concentrations in both sides.
• The potassium gradient is maintained in the opposite direction, that is, from the inside out, allowing the constant exit of the ion and its internal reduction and external increase.

The function of the Na + / K + pump allows the extraction of sodium that has entered by diffusion through channels or other transport routes and the reintroduction of potassium that has diffused out, allowing the conservation of intra and extracellular concentrations of these ions.

Mechanism (process)

The mechanism of action of the Na + / K + ATPase consists of a catalytic cycle that involves transfer reactions of a phosphoryl group (Pi) and conformational changes of the enzyme that go from an E1 state to an E2 state and vice versa.

The operation requires the presence of ATP and Na + inside the cell and K + in the extracellular fluid.

Binding of sodium ions to the transporter

The cycle begins in the E1 conformation state of the enzyme, in which there are 3 cytosolic sites of binding to Na + and of high affinity (Km of 0.6 mM) that are fully occupied because the intra ion concentration (12 mM) allows it.

ATP hydrolysis

In this state (E1) and with Na + attached to its binding sites, ATP binds to its site in the cytosolic sector of the molecule, hydrolyzes and a phosphate group is transferred to aspartate 376, forming a high-energy acylphosphate that induces a conformational change to the E2 state.

Expulsion of 3 sodium ions and introduction of 2 potassium ions

The conformational change to the E2 state implies that the Na + binding sites pass to the outside, their affinity for the ion decreases greatly and it is released into the extracellular fluid, while, at the same time, the affinity of the K + binding sites increases. and these ions are attached to the outside of the pump.

During the E2 state, Na + ions are released to the other side of the membrane.

In turn, this new state of the pump generates affinity for the binding of K + ions.

Reversal from E2 to E1

Once the Na + is released and the K + bound, the hydrolysis of aspartyl phosphate occurs and the conformational change from the E2 state to the E1 state is reverted, with the reintroduction of the empty Na + binding sites and those of occupied K +.

When this change occurs, the sites for Na + regain their affinity and those for K + lose it, with which the K + is released into the cell.

Importance

In the maintenance of cellular osmolarity

The Na + / K + pump is present in most, if not all, mammalian cells, where it is of general importance by helping to maintain their osmolarity and hence their volume.

The continuous entry of sodium ions into the cell determines an increase in the intracellular number of osmotically active particles, which induces the entry of water and the increase in volume that would end up causing the rupture of the membrane and cell collapse.

In the formation of the membrane potential

As these pumps introduce only 2 K + for every 3 Na + they remove, they behave electrogenically, which means that they "decompensate" internal electrical charges, favoring the production of the membrane potential characteristic of body cells.

Its importance is also evident in relation to the cells that make up excitable tissues, in which the action potentials are characterized by the entry of the Na + ion, which depolarizes the cell, and the exit of K +, which repolarizes it.

These ionic movements are possible thanks to the operation of the Na + / K + pumps, which contribute to the production of the chemical gradients that move the ions involved.

Without these pumps, which operate in the opposite direction, the concentration gradients of these ions would dissipate and the excitatory activity would disappear.

In kidney function

Another aspect that highlights the extreme importance of sodium-potassium pumps is related to kidney function, which would be impossible without them.

Kidney function involves the daily filtration of more or less 180 liters of plasma and large amounts of substances, some of which must be excreted, but many have to be reabsorbed so that they are not lost in the urine.

The reabsorption of sodium, water and many of the filtered substances depends on these pumps, which are located in the basolateral membrane of the cells that make up the epithelia of the different tubular segments of the kidney nephrons.

The epithelial cells that line the renal tubules have one face that is in contact with the lumen of the tubule and is called the apical face, and another that is in contact with the interstitium around the tubule and called the basolateral.

The water and substances that are reabsorbed must first pass into the cell through the apical and then into the interstitium through the basolateral.

The reabsorption of Na + is key both in relation to it, as in relation to that of water and that of other substances that depend on it. The apical entry of Na + into the cell requires that there be a gradient that moves it and that implies a very low concentration of the ion inside the cell.

This low intracellular concentration of Na + is produced by the sodium pumps in the basolateral membrane that work intensively to remove the ion from the cells into the interstitium.

References

1. Ganong WF: The General & Cellular Basis of Medical Physiology, in: Review of Medical Physiology, 25th ed. New York, McGraw-Hill Education, 2016.
2. 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.
3. Lodish H, Berk A, Zipursky SL, Matsudaira P, Baltimore D, Darnell J: Transport across the Cells Membranes, In: Molecular and Cell Biology, 4th ed.
4. Nelson, DL, Lehninger, AL, & Cox, MM (2008). Lehninger principles of biochemistry. Macmillan.
5. Alberts, B., Bray, D., Hopkin, K., Johnson, AD, Lewis, J., Raff, M.,… & Walter, P. (2013). Essential cell biology. Garland Science.