Re: [ccp4bb]: summary of sulphate binding at ATP binding sites

[Mischa Machius <Mischa.Machius@utsouthwestern.edu>]

Hi y'all,


My 2 cents: sometimes, ATP (or any other charged ligand) won't bind to a protein if the ionic strength is high. This is due to the Debye-Huckel shielding effect and has nothing to do with the relative affinities of sulfate (NOT sulphate !!!!!) vs. ATP as any other salt would result in the same effect. I know of cases where ATP binding is reduced at fairly moderate ionic strengths already. One can easily check this by doing ATP binding studies with varying ionic strength.

Pre-forming the complex, then crystallizing it and hoping that the ATP doesn't come out, seems to be a good try. 

MM


> > I am trying to get cocrystals with ATP bound in the active
> > site.  But the electron density maps show no evidence of ATP binding. I
> > have ~200mM ammonium sulphate, PEG and 4mM ATP in the crystallization
> > conditions and most probably ammonium sulphate competes with the ATP,
> > preventing its binding.


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Mischa Machius, PhD
Assistant Professor
UT Southwestern Medical Center at Dallas
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