Prokaryotic FtsZ (Tubulin homolog) is a GTPase. Its activity and tendency to polymerize is reduced significantly in the presence of excess of calcium.
Any kind of phosphate with calcium has a very low solubility. Depending on the concentration of calcium your GTP simply could precipitate.
hmm and is there any in vivo mechanism to salvage the precipitated GTP?
Dear Manan,
Unfortunately the answer you have got from Benjamin has nothing to do with your question. You never have in living tissues, except after death, nowhere near concentrations that would allow you for processes like that.
The answer lies in differences between the ions and in availability of these. The main differences are: 1) ionic radius (calcium much larger), 2) associate with it preferred coordination number for ligands (calcium prefers seven), 3) associated with it electronegativity, 4) associated with it polarization power for water conversion (that translates into pKa of the ions and local pH), 5) associated with it availability of an OH species (whether this is water born nucleophile lie in the most of the hydrolytic reactions) or an O- created from the OH group of an alcohol (sugar). So in essence all ATP-ases all GTP-ases, and other nucleotidases would be affected and generally inhibited to a certain degree with calcium.
Evolutionary the Mg was selected as endogenous ligand because of cellular availability. What it means that other similar ions as Mn, Co, sometimes Cu or Fe would do the job but at the diminished rate depending on a local environment and preferences. Proteins are very promiscuous if it comes to metal ions.
Dear Boguslaw,
this topic is not about tissues, it's about bacteria. Since it was not specified which bacteria are meant and in which environment they are, Ca-concetration can be fairly high.
1) High Ca-concentrations can be regulated by polyphosphate (like in ATP/GTP) precipitation (Barat, 2008).
2) Calcium does not only prefer 7 ligands, but 6 to 8. (Kretsinger, Nelson, 1976), the mean number of calcium ligands decreases with increasing calcium concentration (Hewish, 1982).
3) If your argument on electronegativity is compared to magnesium, the electronegativities are quite close (1,0 and 1,2 approx.)
4) You will never ever find a deprotonated alcohol-group of a sugar in an aqueous environment. Simply check the pKa, it's 16.5 in water (Evan's pKa table).
5) Scheffers et al. (2002) shows, that FtsZ polymerization occurs independent of calcium concentrations (0 mM Ca + 10 mM Mg and 10 mM Ca + 5 mM Mg used), so calcium for magnesium exchange is ruled out.
6) Rivas (2000) shows in the methods part, that FtsZ can be purified by precipitation with GTP and Ca, along with other polyphosphates (-> DNA, RNA).
Dear Bnjamin,
You are defending an indefensible answer. Just study more. Even if some of your examples are correct the global image you have is incorrect. Regulation of proteins activity never happens by precipitation with increasing ion concentration. It would have been one of the least selective processes of regulation and the most difficult to reverse. By the way FtzZ belongs to tubulin family and we have quite substantial knowledge of catalytic functioning of it (around 50 structures in the PDB). I agree with you on one point, that noncatalytic binding of Ca stabilizes the protofilaments and prevents recycling therefore "inhibits" the FtzZ function. Such binding must by definition be stoichiometric.
By the way, your ruling out logic is also incorrect and it is reminiscent of many biological conclusions I have seen in the past.
Dear Boguslaw,
as I said it depends on the circumstances:
If there are "normal" to physiologically-high calcium concentrations, I have completely the same opinion as you do.
But under extremely high calcium concentrations - like probably on purpose in the lab - the organism has nothing to regulate, because the whole system cannot compensate the excess calcium ions.
Probably Manan is also looking for kinetics of isolated FtsZ monomers completely lacking cellular mechanisms of calcium homeostatis.
So please don't tell me to study more, when I definitely don't have the narrow view of things as you do.
Afterall we are scientists and should take also non-typical explanations into consideration.
@Benjamin.. im not actually looking at the kinetics of isolated FtsZ protofilament, but am looking from a systems perspective, considering the involvement of two other stabilizing proteins FtsA and ZipA
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