Question

Create a thermodynamic argument for how ATP hydrolysis can be coupled to drive endergonic reactions.

Answer 1

In biological organisms, an exergonic reaction can be coupled to an endergonic reaction to drive otherwise unfavorable reactions. Figure below illustrates this principle for the conversion of glucose to glucose 6-phosphate, the first step in the pathway for oxidation of glucose. The simplest way to produce glucose 6-phosphate would be:

Reaction 1: Glucose + Pi ------> glucose 6-phosphate
(endergonic; delta G1 is positive, +13.8 kJ/mol)

Pi is inorganic phosphate here.

This reaction does not occur spontaneously; delta G1 is positive and an endergonic reaction. A second, very exergonic reaction can occur in all cells:

Reaction 2: ATP -------> ADP + Pi
(exergonic; delta G2 is negative, -30.5 kJ/mol)

These two chemical reactions share a common intermediate, Pi, which is consumed in reaction 1 and produced in reaction 2. The two reactions can therefore be coupled in the form of a third reaction, which we can write as the sum of reactions 1 and 2, with the common intermediate, Pi, omitted from both sides of the equation:

Reaction 3: Glucose + ATP -------> glucose 6-phosphate + ADP (delta G3 is -16.7 kJ/mol)

Because more energy is released in reaction 2 than is consumed in reaction 1, the free-energy change for reaction 3, delta G3 is negative, and the synthesis of glucose 6-phosphate can therefore occur by reaction 3.

The coupling of exergonic and endergonic reactions through a shared intermediate is central to the energy exchanges in living systems. As we shall see, reactions that break down ATP release energy that drives many endergonic processes in cells. ATP breakdown in cells is exergonic because all living cells maintain a concentration of ATP far above its equilibrium concentration. It is this disequilibrium that allows ATP to serve as the major carrier of chemical energy in all cells. But it is not the mere breakdown of ATP that provides energy to drive endergonic reactions; rather, it is the transfer of a phosphoryl group from ATP to another small molecule (glucose in the case
above) that conserves some of the chemical potential originally in ATP.