March 21, 2003
A: FROM MENTOR AMY MCMILLAN IN NY
Hi Stacy - I googled ATP and got some interesting websites... this one
is particularly technical about the physics and chemistry involved in the
bonds between the phosphate groups of the ATP molecule. I hope they help.
http://www.shodor.org/succeed/projects/compchem/cchem97/atp/
these 2 give more basic background - but I thought they were useful and interesting.
http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookATP.html
http://www.brooklyn.cuny.edu/bc/ahp/LAD/C7/C7_atp.html

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A: FROM MENTOR JOAN LUSK IN RI
Actually, hydrolysis of the bond between the first and second
phosphates releases as much free energy as hydrolysis of the bond
between the second and third, 31 kJoule/mol in the standard state for
either ATP + H20 = ADP + Pi or ADP + H20 = AMP + Pi. Hydrolysis of
the first phosphate, AMP + H20 = Adenosine + Pi, releases only 14
kJoule/mole. (Data from a biochem textbook.) So you are right to
wonder why the last phosphate should be different from the middle one
- it's not so different energetically. In cells, however, the
concentration of ATP is much higher than that of ADP, so under these
conditions hydrolysis or phosphoryl transfer from ATP is more
effective at driving reactions than ADP would be, at its lower
concentration (by mass action.)

Why should the first phosphate bond (to adenosine) be different?
Aside from the fact that it's oxygen is bonded to carbon and not
another phosphorus, one factor may be that when that the only charge
on AMP is on the phosphate itself. There's no electrostatic
repulsion between adenosine and the phosphoryl group. At neutral pH
there will be some negative charge on each of the phosphoryl groups
of ATP (or both of ADP), and the loss of the terminal phosphate
removes its charge from close proximity of the remaining phosphoryl
groups.