Question

Which of the following has more free energy: glutamic acid + ammonia + ATP or glutamine + ADP + Pi? Explain.

Short answer

The first set of combinations of glutamic acid + ammonia + ATP has more energy because it is an exergonic process.

Step-by-step solution

Glutamic acid to glutamine

The conversion of glutamic acid to glutamine is an endergonic process. This reaction has a positive Gvalue which means the reaction requires energy to proceed. Thus, the reaction is not spontaneous and energetically unfavorable.

As a result, the reaction is coupled with ATP hydrolysis.ATP hydrolysis drives this endergonic reaction and synthesizes glutamine from glutamic acid and ammonia.

Reaction coupled with ATP hydrolysis

ATP transfers a phosphate group to glutamic acid to form a phosphorylated intermediate in the first couple reaction. This intermediate has more free energy than glutamic acid and is thereby less stable.

In the second couple reaction, glutamine is formed along with ADP and Pi. Thus, the products of the second set of reactions do not have high free energy.

 Step 3: The couple reaction is an exergonic process

In a coupled reaction, the first set of combinations transforms into the second set of combinations. This happens when the ammonia displaces the phosphate group from the phosphorylated intermediate resulting in the formation of glutamine.

As a result, the overall reaction is an exergonic process as ATP hydrolysis provides the reaction energy. This exhibits the first set of combinations has more free energy.