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Energy coupling occurs when a highly exergonic reaction powers an endergonic reaction. The combined reaction is called a coupled reaction.

Cells use coupled reactions to perform different processes, including biosynthesis, active transport of ions, and mechanical work. Hydrolysis of ATP is a frequently coupled exergonic reaction for powering biological processes.

For example, the endergonic reaction between glutamate and ammonia to form glutamine is coupled with the exergonic hydrolysis of ATP to ADP and inorganic phosphate.

The net free energy change for the coupled reaction is the sum of the individual free energy changes. Because of the large negative ΔG for the ATP hydrolysis, the total standard free energy change is negative 16.3 kJ/mol, and the reaction can proceed.

Coupled biosynthesis reactions often share a common unstable intermediate. In the biosynthesis of glutamine, the transfer of phosphate from ATP to glutamate forms the intermediate glutamyl phosphate, which then reacts with ammonia to form glutamine.