The Periodic Table
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What is the chemical basis for the abnormally high energy of hydrolysis
of these phosphate bonds? Why is ATP so unstable relative to ADP
and phosphate? The answer lies in the charges on the polyphosphate
chain. The triphosphate group has three to four negative charges,
and the mutual repulsion of these charges makes the ATP molecule
less stable than expected. The single- and double-bond structure
of the phosphates drawn at the left is only schematic. In reality,
the doublebond electrons are spread over the entire triphosphate
group, thereby giving every P-O bond a partial double bond character.
The negative charges also are delocalized over the entire chain.
The drawings at the top of the opposite page are an attempt to illustrate
a dynamic and constantly changing situation by one singly and doubly
bonded structure, and are only approximately right. For example,
in the roughly neutral conditions of a cell, half of the phosphate
is found as |
When ATP is hydrolyzed and one phosphate group is split off by
water, the charges on the ATP group are separated from one another,
as shown at the middle of the sequence of drawings above. Less charge
is left on ADP, and the two ions repel one another. Extra energy
is given off - the energy that originally was needed to bring a
negatively charged phosphate group up to the already negatively
charged ADP and make them bond together. Another gain in stability
is obtained when the repulsions of the remaining negative charges
on ADP are relieved by splitting it into AMP and phosphate. The
hydrolysis energy of the second phosphate bond likewise is correspondingly
high: |
and half as the 
ions drawn above. The ATP, ADP, and AMP structures drawn will be
correct for approximately 50% of the molecules, and the others will
have lost one more 
each. Nevertheless, the molecules as shown illustrate the principles
of charge repulsion and energy in ATP.
AMP + phosphate + 7.3 kcal 
