life11e_ch08

ATP couples exergonic and endergonic reactions

As you have just seen, the hydrolysis of ATP is exergonic and yields ADP, P_i, and more free energy (or AMP, PP_i, and more free energy). The reverse reaction, the formation of ATP from ADP and P_i, is endergonic and consumes as much free energy as is released by the hydrolysis of ATP:

ADP + P_i + free energy → ATP + H₂O

Many exergonic reactions in the cell can provide the energy to convert ADP into ATP. For eukaryotes and many prokaryotes, the most important of these reactions is cellular respiration, in which some of the energy released from fuel molecules is captured in ATP. The formation and hydrolysis of ATP constitute what might be called an “energy-coupling cycle,” in which ADP picks up energy from exergonic reactions to become ATP, which then donates energy to endergonic reactions. ATP is the common component of these reactions and is the agent of coupling, as illustrated in Figure 8.6.

Figure 8.6 Coupling of Reactions Exergonic cellular reactions release the energy needed to make ATP from ADP. The energy released from the conversion of ATP back to ADP can be used to fuel endergonic reactions.

Coupling of exergonic and endergonic reactions is very common in metabolism. Free energy is captured and retained in the P~O bonds of ATP. ATP then diffuses to another place in the cell, where its hydrolysis releases the free energy to drive an endergonic reaction. For example, the formation of glucose 6-phosphate from glucose (Figure 8.7), which has a positive ΔG (is endergonic), will not proceed without the input of free energy from ATP hydrolysis, which has a negative ΔG (is exergonic). The overall ΔG for the coupled reactions (when the two ΔGs are added together) is negative. Hence the reactions proceed exergonically when they are coupled by occurring at the same time and place, and glucose 6-phosphate is synthesized. As you will see in Chapter 9, this is the initial reaction in the catabolism of glucose.

Figure 8.7 Coupling of ATP Hydrolysis to an Endergonic Reaction The addition of phosphate derived from the hydrolysis of ATP to glucose forms the molecule glucose 6-phosphate (in a reaction catalyzed by hexokinase). ATP hydrolysis is exergonic and the energy released drives the second reaction, which is endergonic.

Question

Q: What would be the magnitude of the ΔG of a different conversion that could drive the synthesis of ATP?

The synthesis of ATP has a ΔG of +7.3 kcal/mol. So any reaction driving this synthesis must have a ΔG of at least –7.3 kcal/mol. Because of the second law of thermodynamics, the transfer of energy is not efficient (entropy increases), so the ΔG of the driving reaction would be more than +7.3 kcal/mol.

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An active cell requires the production of millions of molecules of ATP per second to drive its biochemical machinery. From previous chapters, you may already be familiar with some of the activities in the cell that require energy from the hydrolysis of ATP:

Active transport across a membrane (see Figure 6.14)
Condensation reactions that use enzymes to form polymers (see Figure 3.4A)
Modifications of cell signaling proteins by protein kinases (see Figure 7.15)
Motor proteins that move vesicles along microtubules (see Figure 5.19)

An ATP molecule is typically consumed within a second of its formation. At rest, an average person produces and hydrolyzes about 40 kg of ATP per day—as much as some people weigh. This means that each ATP molecule undergoes about 10,000 cycles of synthesis and hydrolysis every day!