Question

Explain how ATP hydrolysis can be coupled to endergonic reactions to make them energetically more favorable.

Answer 1

Adenosine triphosphate is regarded as the energy currency of the cell. It is a high energy compound as there are two phosphoanhydride bonds which release energy when they are hydrolysed

The general ATP hydrolysis reaction is:

ATP + H₂O --------> ADP + Pi + 7.3 cal of energy

Pi- Inorganic phosphate

This is a very spontaneous reaction and as it releases energy, it is called as exergonic reactions.

Since ATP is shortlived due to its instability and as it is the prime currency in the cell, various mechanisms interplay in the cell to conserve the liberated energy and turns the liberated energy into most profitable channels. The energy of this hydrolysis is utilized to drive the endergonic (reactions which require energy and non spontaneous) and thus making the required product even though it is against the odds.

To prevent the loss of energy during ATP hydrolysis in the form of heat dissipation, ATP hydrolysis is always colloborated with a exergonic reaction, which are called as coupled reactions to increase the energy efficiency and conservation

The ATP-ADP cycle is shown in the following figure:

Let us see this in terms of an example of conversion of Glyceraldehyde 3 phosphate to 1,3 bisphosphoglycerate:

The reaction proceeds as follows:

                                                                             G3P dehydrogenase

Glyceraldehyde 3 phosphate(G3P) + NAD+ Pi <-------------------------------> 1, 3 bisphosphoglycerate + NADH + H⁺

The conversion of G3P to 1, 3 bisphosphoglycerate is highly unfavorable and an endergonic reaction, ATP hydrolysis is coupled with the above reaction which acts as the fuel to drive the reaction in the forwarding direction maintaing the Gibbs free energy in the negative values and thus making the reaction, energetically favorable

When two reactions are coupled together, total free energy is the sum of individual free energies of the reactions. ATP hydrolysis showing negative values, utilises formation of intermediates and maintains same negative free energy value of the overall reaction to make it favorable. When the sum of free energies of two reactions are negative, the reaction proceeds

Conclusion:

ATP hydrolysis is always coupled such that the liberated energy can be used into other endergonic reactions which are nonspontaneous and energetically unfavorable, thus making the energy conservative and effective to fuel up the reactions that usually do not occur without the drive.