Question

Phosphoenolpyruvate (PEP) has an extremely high energy phosphoryl bond, meaning that its dephosphorylation produces a substantially greater amount of energy than does ATP. Why is ATP the “energy currency” of the cell but not PEP?

Answer 1

Within cells, energy is provided by oxidation of “metabolic fuels” such as carbohydrates, lipids, and proteins.The oxidation process results in free energy production that can be stored in phosphoanhydrine “high-energy bonds” within molecules such as nucleoside diphosphate and nucleoside triphosphate (i.e., adenosine 5′ diphosphate and adenosine 5′ trisphosphate, ADP, and ATP, respectively), phosphoenolpyruvate, carbamoyl phosphate, 2,3-bisphosphoglycerate, and other phosphagens like phosphoarginine, or phosphocreatine. Among them, ATP is the effective central link—the exchange coin—between energy-producing and the energy-demanding processes that effectively involve formation, hydrolysis, or transfer of the terminal phosphate group.

Phosphoenol pyruvate, the product of enolase, is unable to tautomerize to the keto form because of the phosphate group. Enols are usually not as stable as keto compounds. (Recall from organic chemistry that tautomers are compounds that react as if they were made up of two components, differing only in the placement of a substituent, like a hydrogen atom.) Therefore, there is a large negative free energy change associated with release of the phosphate; phosphate release allows the formation of the keto tautomer—that, is, of pyruvate. This free energy change is more than enough to phosphorylate ADP to make ATP in the reaction catayzed by pyruvate kinase.