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

[jaumzaum]

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.

 

[Ygggdrasil]

Is bond breakage the only thing that occurs during the hydrolysis of ATP? Consider the other reactants involved in the reaction.

 

[jaumzaum]

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.

 

[Ygggdrasil]

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.

 

[snorkack]

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.