ATP as Free Energy Currency

ATP is metastable (a thermodynamically unstable compound that does not rapidly break down in absence of a catalyst) and is commonly referred to as "free energy currency." Like monetary currency, ATP is used to provide energy in a wide variety of metabolic reactions and is universal among cells. Nevertheless, the energy content of ATP is not significantly different from other nucleoside di- and tri-phosphates. For whatever reason, however, evolution has created an array of enzymes that preferentially bind ATP and use its free energy of hydrolysis to drive endergonic reactions. Hydrolysis of either phosphoanhydride bond in ATP has a of about -31 kJ/mol. Be aware, however, that utilization of that energy to drive endergonic reactions usually does NOT involve hydrolysis of ATP. Instead, ATP breakdown is usually coupled with a thermodynamically unfavorable reaction. In glycolysis, for example, ATP energy is used to synthesize glucose-6-phosphate from glucose. In this case, the phosphate is transferred directly from ATP to glucose to form glucose-6-phosphate.

Because ATP can transfer a phosphate group, we say that ATP has a high "phosphoryl group transfer potential" rather than calling it a high energy compound. The phosphate anhydride bonds of ATP, ADP, or pyrophosphate have relatively high values. In fact, they are roughly twice as high as the phosphate ester bonds of glucose-6-phosphate or AMP (see also - Figure 3.8). There are, however, cellular compounds with even higher phosphoryl group transfer potentials than ATP. For example, the for breakdown of phosphoenolpyruvate (PEP), 1,3-bisphosphoglycerate, and creatine phosphate are -62, -49, and -43 kJ/mol, respectively. Although the breakdown of "super-high-energy" compounds, such as PEP, is not used routinely in cells to drive endergonic reactions, these compounds are still important because they can be used to drive the synthesis of ATP from ADP + Pi. In fact, this coupling, called substrate level phospohorylation, is the process by which ATP is synthesized in glycolysis.

ATP hydrolysis under cellular conditions yields ADP + Pi or AMP + PPi: The energy available from ATP hydrolysis is probably not -31 kJ/mol (see above), however, for several reasons:

> 1. values for ATP hydrolysis do not represent actual G' values under likely biological conditions;
> 2. G' depends on temperature;
> 3. is defined at pH = 7.0, but actual pH may vary from 6.5 to 8.0;
> 4. Varying amounts of magnesium ion will change G in complicated ways;
> 5. Actual concentrations of ATP, ADP, and Pi in cells are very different from the 1M value of the standard state of ;
> 6. Effective G' values in cells may be close to -50 kJ/mol; and
> 7. Thus, ATP hydrolysis is very effective in driving cellular processes.

See also: Substrate Level Phosphorylation, Oxidation as a Metabolic Energy Source, Factors Contributing to Large Energies of Hydrolysis of Phosphate Compounds (from chapter 3)