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Are the phosphate groups of ATP protonated at pH = 7?

  1. Sep 16, 2011 #1
    1. The problem statement, all variables and given/known data

    The problem states: "Draw the chemical structure of ATP at a pH of 7.

    2. Relevant equations



    3. The attempt at a solution

    The textbook diagrams the phophate groups as unprotonated, but since H3PO4 has a pKa of <7, I was thinking that maybe each phosphate group would have lost one hydrogen atom?
     
  2. jcsd
  3. Sep 19, 2011 #2

    epenguin

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    You mean lost a hydrogen ion, i.e. a proton, so they are deprotonated as you say.
    I think inorganic phosphate is a good guide: it is a strongish acid for the first dissociation (pK ca. 2) then the second dissociation is at near neutral pH's pK ca. 6.8.

    In ATP correspondingly 3 protons are always lost at physiological pH's. The fourth pK I have seen given as various values 6.6 up to 6.95. So at intracellular pH of about 7 rather more than half should be quadruply deprotonated. Then in life it is all complexed with Mg2+ anyway. As far as I know there is not a single reaction involving phosphate or pyrophosphate transfer, which must be at least half of all important reactions, that occurs in the absence of magnesium.
     
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