Adenosine-5'-triphosphate (ATP) is comprised of an adenine ring, a ribose sugar, and three phosphate groups. ATP is often used for energy transfer in the cell. The enzyme ATP synthase produces ATP from ADP or AMP + Pi using energy produced from metabolism in the mitochondria. ATP has many uses. It is used as a coenzyme, in glycolysis, for example. ATP is also found in nucleic acids in the processes of DNA replication and transcription. In a neutral solution, ATP has negatively charged groups that allow it to chelate metals. Usually, Mg2+ stabilizes it.

### Introduction

ATP is a molecule which can hydrolyze to ADP and inorganic phosphate when it is in water. The formation of solvated ADP and hydrogen phosphate from solvated ATP and water has a ΔG of -30.5 kJ/mol. The negative ∆G means that the reaction is spontaneous (given an infinite amount of time it will proceed) and produces a net release of energy. However, because it requires energy to rupture the P-O bond connecting the phosphate that leaves ATP, ATP molecules do not instantly fall apart and can be used to transport useful energy around the cell. The energy required to rupture the bond contributes to the activation barrier that prevents the reaction from happening instantly.

Figure 1: Structure of ATP molecule and ADP molecule, respectively. The adenine ring is at the top, connected to a ribose sugar, which is connected to the phosphate groups. Used with permission from Wikipedia Commons.

### Hydrolysis of ATP

At pH 7 the balanced reaction for hydrolysis is:

$ATP ^{4-} + H_2O \rightleftharpoons ADP^{3-} + HPO_4^{2-} + H^+$ ∆G = -30.5 kJ

### Why is ATP hydrolysis an exergonic reaction?

1. The entropy, which is the level of disorder, of solvated ADP + solvated HPO42- is greater than that of solvated ATP (∆SRXN > 0). Additionally, the reaction is exothermic (∆HRXN ≈ -20 kJ/mol).2 Therefore the thermodynamics implies the reaction is spontaneous (∆G = ∆H - T∆S < 0).
2. Electrostatic repulsion of the four negative charges on the oxygens of the ATP molecule. Naturally, like charges repel and opposite charges attract. Therefore, if there are four negative charges in close proximity to one another, they will naturally repel each other. This makes ATP a relatively unstable molecule because it will want to give away its phosphate groups, when given the chance, in order to become a more stable molecule.
3. Resonance stabilization of ADP and of HPO42- is greater than that of ATP. The oxygen molecules of the ADP are sharing electrons. Those electrons are constantly being passed back and forth between the oxygens, creating an effect called resonance. This stables the ADP. Resonance does not occur in ATP; therefore, it is a more unstable molecule.
4. There is a greater degree of solvation of HPO42-, H+, and ADP, relative to ATP. This means that it is easier for ATP to lose one of its phosphate groups. But, it takes a large amount of water to force ADP to lose one of its phosphates.

### ATP in the Cell FIX ME

ATP is the primary energy transporter for most energy-requiring reactions that occur in the cell. The continual synthesis of ATP and the immediate usage of it results in ATP having a very fast turnover rate. This means that ADP is synthesized into ATP very quickly and vice versa. For example, it takes only a few seconds for half of the ATP molecules in a cell to be converted into ADP to be used in driving endergonic (non-spontaneous) reactions and then converted back into ATP using exergonic (spontaneous) reactions.

ATP is useful in many cell processes such as glycolysis, photosynthesis, beta oxidation, anaerobic respiration, active transport across cell membranes (as in the electron transport chain), and synthesis of macromolecules such as DNA.

### References

1. Zubay, Geoffrey. Biochemistry. New York: Macmillan Publishing Company, 1988.
2. Podolsky, R. J.; Morales, M. F., J. Biol. Chem. (218), 945-959, 1956.

### Problems FIX ME

1. In cellular respiration, which process produces the most ATP?
2. True or false: ATP may be used to regulate certain enzymes.
3. From one molecule of glucose, how many molecules of ATP will be produced?
4. Where is ATP synthase located?
5. True or false: ATP is generated through substrate level phosphorylation.

Figure 2. ATP Synthase. Used with permission from Wikipedia Commons.