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I am interested in the comparison between the Plasma Membrane Calcium ATPase (PMCA) and the Sodium-Calcium Exchanger (NCX) which are two pumps on the plasma membrane of cells that serve to move calcium out of the cell and maintain relatively low resting calcium concentrations. Typically, I see the statement that the PMCA has high affinity for calcium but a low pumping capacity, whereas the NCX has a low affinity for calcium but a high pumping capacity.

In the literature, I can find relatively simple models of the PMCA as well as data of estimated dissociation constants for it. However, I am having a much harder time finding the concentration of calcium where the NCX becomes relevant. It might be because the NCX also depends on sodium concentrations, membrane potential, etc., which gives rise to much more complicated models. However, there must be some research that backs up the "NCX has a low affinity" claim.

So, what is the typical calcium concentration range where the NCX becomes relevant, such as a calcium dissociation constant where the pump is at 50% of its maximum capacity?

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  • $\begingroup$ Check en.wikipedia.org/wiki/Sodium-calcium_exchanger $\endgroup$
    – PCM
    Sep 1 at 4:06
  • $\begingroup$ @PCM I don't the the answer to my question there. $\endgroup$ Sep 1 at 4:32
  • $\begingroup$ I'm not sure but I think you will find that these are "always on" pumps, so at any concentration they can be pumping. By relevant you mean physiologically? $\endgroup$
    – bob1
    Sep 1 at 21:57
  • $\begingroup$ @bob1 Good point. More quantitatively, I'm asking about something like a dissociation constant where we would expect the pump to operate at 50%. I know that's a huge simplification for the NCX, but I feel like there should still be some sort of estimate. $\endgroup$ Sep 1 at 23:30
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I could not find the $K_d$ of NCX proteins in KEGG BRITE, BRENDA, SuperTarget, DrugBank, Uniprot, or PDSP Ki databases... (Also, I cannot understand the use of the word 'relevant' in the question you asked.)

Still, the requested thermodynamic constants can be found in publications.

Taken from Z.Li et al.(1994)

NCX2 ($Na^+$) : the $K_d$ values for $Na^+$ $(28 \pm 2 mM, n=2)$ are very similar before and after $\alpha$-chymotrypsin treatment and are somewhat higher than those measured for NCX1(e.g. Hilgeman, 1990; Matsuoka et al., 1993)

NCX2 ($Ca^{+2}$) : The fitted $K_d$ values are 1.6 and 1.4 $\mu$M with Hill coefficients of 2.4 and 1.1 respectively. The reason for the shift in the Hill coefficient is unclear.

from Z.Li et al.

Also, B.Linck et al.(1998) studied NCX1,NCX2,NCX3 comprehensively. They reported the $K_d$ of these proteins in various situations and environments. These two results seem conflicting... Maybe because they worked on different types of tissues in their studies...

Taken from B.Linck et. al.(1998)

Exchange currents recorded from NCX2-expressing cells (Fig. 8B) were not as well regulated as those previously reported from oocytes (22) [=> Z.Li et. al. paper] and were smaller than those obtained from NCX1-expressing cells.

Looks like B.Linck's paper is much more reliable, though. But I may not share all the graphs of the paper here.(copyright stuff)

I skimmed through the papers, honestly... So feel free to dive into these papers yourself. Cheers!

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