If a bond breaking absorbs energy, why does ATP hydrolysis release it?

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Discussion Overview

The discussion centers on the energy dynamics involved in ATP hydrolysis, specifically addressing why breaking a bond in ATP releases energy despite the general principle that bond breaking absorbs energy. The scope includes theoretical and conceptual aspects of biochemical reactions.

Discussion Character

  • Conceptual clarification
  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants assert that while bond breaking typically absorbs energy, the hydrolysis of ATP releases a significant amount of energy, prompting questions about the underlying mechanisms.
  • One participant suggests that the hydrolysis reaction involves more than just bond breakage, hinting at the role of other reactants in the process.
  • Another participant describes the reaction in terms of bond formation and breakage, expressing confusion about the energy implications and questioning the notion that bonds can "store" energy.
  • It is proposed that the hydrolysis of ATP is enthalpically favorable due to the relief of electrostatic repulsion between negatively charged phosphate groups, which contributes to the energy release.
  • Additionally, the reaction is noted to be entropically favorable due to the high ATP/ADP ratio maintained in cells.
  • One participant emphasizes that the Pi-ADP bond is unstable and that breaking it leads to the formation of more stable bonds, resulting in energy release.
  • Concerns are raised about the reactivity of free radicals formed during bond breaking, suggesting that while hydrolysis releases energy, the initial bond's weakness prevents spontaneous reactions without catalysts.

Areas of Agreement / Disagreement

Participants express differing views on the nature of energy storage in bonds and the mechanisms of ATP hydrolysis. There is no consensus on the interpretation of energy dynamics involved in the reaction.

Contextual Notes

Some statements reflect assumptions about bond energy and stability that may not be universally accepted. The discussion includes unresolved questions about the role of reactants and the implications of bond formation and breakage in energy release.

jaumzaum
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Hi. We know that bond formation releases energy and bond break absorbs energy. I still don't figure out why ATP hydrolysis (that breaks a bond between oxygen and phosphorous) releases energy, and it releases a LOT.
 
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Is bond breakage the only thing that occurs during the hydrolysis of ATP? Consider the other reactants involved in the reaction.
 
Hello @Ygggdrasil
You mean the hydroxyl group formation?
I would say the reaction is something like this (I suppressed part of the molecule, and I don't know if Pi and ADP are ionized)
P-O-P + H-O-H -> P-O-H + H-O-P
I really forgot the other reactants. So there is 2 P-O and 2 O-H broken,, and 2 P-O and 2 O-H formed. That confused me even more rsrsrsrs. Didnt the energy released have to be near zero?
Also majority of the books says that there is a huge amount of energy "stored" in the Pi-ADP bound. Is that correct to say? For me bounds don't store energy, they require energy to be broken.
 
Essentially, there are two effects going on.
1) Breaking a phosphodiester bond between the Pi and ADP to form a new P-OH bond. Although the bonds being broken are somewhat chemically similar, this step is enthalpically favorable in large part because you are relieving the electrostatic repulsion between the negative charge on the terminal gamma phosphate and the neighboring two negatively charged phosphate groups in ATP.

2) The reaction is also entropically favorable because the cell maintains a relatively high ATP/ADP ratio.

I would agree with you that it is incorrect to say that the Pi-ADP bond stores energy. It is more correct to say that the Pi-ADP bond is a very unstable bond (high potential energy), so breaking that bond and replacing it with another more stable bond (low potential energy) results in the release of energy. In organic chemistry terms, the gamma phosphate is a good leaving group.
 
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Right.
You would need a bond to store energy well.
You might accumulate even more energy by breaking bonds completely.
This might give you a free radical of .PO(OH)2.
But the problem with completely breaking bonds is that free radicals are too reactive and too easily liable to spontaneous and unwanted reactions.
The weak P-O-P bond hampers spontaneous formation of new bonds. Hydrolysis releases a lot of energy by forming new and stronger bonds, but the initial weaker bond hampers spontaneous formation of new bonds, so that ATP hydrolyses under influence of catalysts but not spontaneously.
 

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