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How do O2 molecules from solution get into ATP?

  1. Aug 9, 2017 #1
    This was a practice exam Q:
    ATP is added to the myosin ATPase domain in water labeled with an oxygen isotope. After 50% of the ATP has been hydrolyzed, the remaining ATP is isolated and found to contain 18O. Explain.

    I get how H2O is used in order to regenerate ATP from the Pi and ADP, but on all the diagrams I've seen online I can't see where the oxygen from that H2O goes.
     
  2. jcsd
  3. Aug 9, 2017 #2

    Andrew Mason

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    I assume that the water is H218O (18O being the isotope of O with atomic mass 18). The water hydrolyses the ATP releasing a phosphate molecule. That phosphate will contain an 18O atom and the now ADP molecule takes an H atom from the H218O. There must be a free H.

    AM
     
    Last edited: Aug 9, 2017
  4. Aug 10, 2017 #3

    I don't really understand the explanation. How can water hydrolyse the ATP when hydrolysis is a reaction... It's not like water is a catalyst?
     
  5. Aug 11, 2017 #4

    Andrew Mason

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    The bond strength between the outer phosphates in the ATP is weak because of the strain due to strong repulsion forces between them. That strain is enough so that the bonds can be broken by hydrogen bonding with water molecules. ATPase enzymes act as a catalyst. There is a great deal of energy released once the bonds are broken because of large the repulsive coulomb forces between these phosphates.

    AM
     
    Last edited: Aug 11, 2017
  6. Aug 14, 2017 #5

    Andrew Mason

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    The actual mechanism by which ATP hydrolysis occurs is the subject of this paper, which you may find interesting.

    AM
     
  7. Aug 27, 2017 #6

    epenguin

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    I'm sorry I saw this thread so late. I think that reference, part of vast, important and fascinating story or stories may be too complicated and specialised for the general question and principle here.

    What is happening is this: O18 labelled water attacks the enzyme-bound ATP creating enzyme-bound ADP and enzyme-bound O18-labelled Pi. While bound to the enzyme, this reaction is reversible. If it were reversed exactly, then the same labelled oxygen atom that had been Incprporated would be expelled of course. But all four O atoms of Pi are chemically equivalent. The enzyme-bound phosphate ion can rotate while sitting in the enzyme, and so the oxygen atom expelled in the reverse reaction will often be not the same one as entered and thus the ATP is reconstituted containing O18.
     
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