Explain why ATP hydrolysis releases so much energy and why this is useful for the cell.

Question

The primary reason why ATP hydrolysis releases so much energy is that the three phosphate groups on ATP have a strong negative charge. This will cause the phosphate groups to repel each other and thus release energy once the triphosphate breaks. This large release of energy can be used to drive other endergonic reactions in the cell.

Answer 1

The explanation offered in the question stem offers a decent answer as to why ATP hydrolysis releases so much energy, but I was hoping to expand upon this.

First, it is important to take a step back and understand the structure of ATP. It is a nucleotide made of adenine, ribose, and a unit of three phosphates. This triphosphate part is what makes it so energy rich since it has two phosphoanhydride bonds (bonds formed between two phosphoryl groups with the loss of a water molecule). So, with ATP hydrolysis, a good amount of energy is released.

1) ATP + H₂O ⇔ ADP + Pi

2) ATP + H₂O ⇔ AMP + PPi

Note that the free energy of ATP hydrolysis under standard conditions (ΔG°) is -30.5 kJ/mol for reaction 1. Reaction 2 has a ΔG° of -45.6kJ/mol. For these reactions in a cellular environment (nonstandard conditions), the ΔG is even more negative. This free energy is what is used to drive other things, like muscle contraction. Things that are unfavorable can be coupled with ATP hydrolysis to drive the reaction forward.

You might be wondering why other compounds containing a phosphoryl group are not used as fuel for the cell. If you compare the free energy, you will see that many reactions have a smaller magnitude of ΔG° compared to ATP hydrolysis. Aside from the free energy, you can reasonably say that the structure of ATP and the products of hydrolysis (ADP, Pi) also play a huge role. The repulsion of the negative charges of the triphosphate is reduced after hydrolysis, making it favorable. There is also a higher resonance stabilization with ADP, driving the reaction forward. Lastly, water can bind ADP more effectively compared to ATP, thus stabilizing it by hydration. Our cells have an aqueous environment, so it makes sense that water would be readily available to stabilize the structure of ADP.

Further, there is a cycle going on in our bodies where the reverse reaction takes place (ADP and Pi form ATP). There is a balance of ATP hydrolysis and the phosphorylation of ADP. This is the basic way of exchanging energy in biological systems.

Hope this helps! Happy studying!