If the conversion of reactant A into product B is spontaneous, the reverse reaction converting reactant B into product A is not. The ΔG’s for the forward and reverse reactions have the same absolute value but opposite signs. You might expect that the direction of the reaction would always be from A to B. However, in living organisms, not all chemical reactions are spontaneous. Anabolic reactions are a good example; they require an input of energy to drive them in the right direction. This raises the question: What drives non-spontaneous reactions?

Energetic coupling is a process in which a spontaneous reaction (negative ΔG) drives a non-spontaneous reaction (positive ΔG). It requires that the net ΔG of the two reactions be negative. In addition, the two reactions must occur together. In some cases, this coupling can be achieved if the two reactions share an intermediate.

For example, ATP hydrolysis can be used to drive a non-spontaneous reaction, as shown in Fig. 6.11a. In this case, the phosphate group released during ATP hydrolysis is transferred to glucose to produce glucose 6-phosphate. In addition, the net ΔG for the two reactions is negative. So ATP hydrolysis provides the thermodynamic driving force for the non-spontaneous reaction, and the shared phosphate group couples the two reactions together.

Following ATP hydrolysis, the cell needs to replenish its ATP so that it can carry out additional chemical reactions. The synthesis of ATP from ADP and P_i is an endergonic reaction with a positive ΔG, requiring an input of energy. In some cases, exergonic reactions can drive the synthesis of ATP by energetic coupling (Fig. 6.11b). The sum of the ΔG’s of the two reactions is negative and the reactions share a phosphate group, allowing the two reactions to proceed.

Like ATP, other phosphorylated molecules can be hydrolyzed, releasing free energy. Hydrolysis reactions can be ranked by their free energy differences (ΔG). Fig. 6.12 shows that ATP hydrolysis has an intermediate free energy difference compared with the free energy difference for the hydrolysis of other common phosphorylated molecules. Those reactions that have a ΔG more negative than that of ATP hydrolysis transfer a phosphate group to ADP by energetic coupling, and those reactions that have a ΔG less negative than that of ATP hydrolysis receive a phosphate group from ATP by energetic coupling. Therefore, ADP is an energy acceptor and ATP an energy donor. The ATP–ADP system is at the core of energetic coupling between catabolic and anabolic reactions.

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