19
K
Potassium
39.098
Essential: all life
Potassium
Major functions in cells: (1)
-Major cation in all cells
- Electrical signaling
-Variety of cellular processes
Reduce:
- Substitution: in B. subtilis, positively charged amino acids can partially take over function of potassium (4)
- Substitution: In plants, Na or other monovalent cations can substitute for K under K-limiting conditions (5)
- Substitution: In plants, halophiles, and mammals, Rb and Cs may be able to partially substitute for K (6)
Learn More!
(1) Potassium: A Major Cation
Potassium is a major cation that is required for all known organisms, for a variety of different cellular processes.
In bacteria, K is required for maintaining turgor, regulating the pH within the cell, and activating a large array of enzymes (Shramke, et al., 2017).
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In plants, K performs many of those same functions, as well as participates in other physiological processes like stomata opening and photosynthesis. K creates the ionic environment that is necessary for many of these cellular processes to occur, and plants thus have a very high requirement for K in the cell.
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In animal cells, K is also essential due to its role in the sodium-potassium pump, which maintains the resting potential of the cell (Grider et al., 2020). This pump is necessary for a variety of cellular processes, such as nervous system signaling, kidney function, and maintaining blood pressure.
(2) Potassium-Solubizing Bacteria
Although K is a prevalent ion in soil, much of it is in a form that is unusable to plants, which require K in high concentrations for many cellular functions. Potassium-solubizing bacteria have the ability to convert this insoluble K into a form that plants can uptake. Biological fertilizers that contain these bacteria are being developed as an alternative to the environmentally unsustainable chemical fertilizers currently used extensively in agriculture (Etesami et al., 2017).
(3) K Transporter as Antimicrobial Target
A. baumannii, a deadly human pathogen, has a high affinity K transporter, KdpE, that is synthesized under K-limited conditions. This transport system is essential for the pathogenesis of the bacterium and thus provides a possible potential target for antimicrobials for this deadly pathogen (Samir et al., 2016).
(4) Substitution of Positively Charged Amino Acids for K
B. subtilis has 3 potassium transporters. In the absence of these transporters, the bacterium is unable to survive due to potassium limitation, unless it acquires a mutation that affects amino acid metabolism, increasing production of positively charged amino acids like ornithine, citrulline, and arginine. It has been suggested that these amino acids are able to partially substitute for the functions of potassium in the cell (Gundlach et al., 2017).
(5) Substitution of K with Na in Plants
K is the major monovalent cation in plants and performs a variety of essential functions. Under K-limiting conditions, other monovalent cations, such as Na and to some extent Rb and Cs, can functionally substitute for K and thus reduce the cell's requirement for K (Subbarao, et al., 2003). K can be substituted by other monovalent cations in the following biological roles:
1. Internal osmoticum- K is the most common ion used to maintain osmotic pressure in the plant cell, but under K-limitation, Na, Mg, and Ca can replace K.
2. Stomatal function- In many species, K is primarily responsible for the change in turgor pressure of the guard cells during movement of the stomata. In certain species, such as Commelina communis, Na was able to replace K in this biological role, and was in fact even more effective than K.
3. Photosynthesis- In certain plants, like the red beet, photosynthesis on leaves remains at a high level under K limitation if there is a high Na concentration.
4. Counter-ion in long distance transport- In the red beet, Na may also be able to functionally replace K as a counter-ion in long distance transport.
5. Enzyme activation- Potassium activates a number of enzymes in the plant cell. Na is generally able to functionally substitute, though it less effective, in activating these enzymes.
(6) Substitution of K with Rb and Cs
Rb and Cs may be able to partially substitute for K in certain organisms. For example, the archaeal halophile Haloarcula marismortui accumulates Rb and Cs in low K conditions (Jensen et al., 2015). This seems to present strong evidence for a conditionally beneficial role for both Rb and Cs, at least in this specific condition.
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The effects of Rb have also been characterized in mammals. At the cellular level, Rb can displace a fraction of intracellular K, but the sum of the two ions remains relatively constant (Kirk et al., 1984). Replacement of K by Rb, and even Cs in rats, is dramatic in muscle tissue, where Rb or Cs can replace more than half of intracellular K (Relman et al., 1957).