High-energy phosphate

High-energy phosphate

High-energy phosphate can mean one of two things:

* The phosphate-phosphate bonds formed when compounds such as adenosine diphosphate and adenosine triphosphate are created.

* The compounds that contain these bonds, which include the nucleoside diphosphates and nucleoside triphosphates, and the high-energy storage compounds of the muscle, the phosphagens. When people speak of a high-energy phosphate pool, they speak of the total concentration of these compounds with these high-energy bonds.

High-energy phosphate bonds are pyrophosphate bonds, acid anhydride linkages, formed by taking phosphoric acid derivatives and dehydrating them. As a consequence, the hydrolysis of these bonds is exothermic under physiological conditions, releasing energy.

Energy released by high energy phosphate reactions
ReactionΔG [kJ/mol]
ATP + H2O → ADP + Pi -36.8
ADP + H2O → AMP + Pi -36.0
ATP + H2O → AMP + PPi -40.6
PPi → 2 Pi -31.8
AMP + H2O → A + Pi -12.6

Except for PPi → 2 Pi, these reactions are, in general, not allowed to go uncontrolled in the human cell but are instead coupled to other processes needing energy to drive them to completion. Thus, high-energy phosphate reactions can:
* provide energy to cellular processes, allowing them to run;
* couple processes to a particular nucleoside, allowing for regulatory control of the process;
* drive the reaction "to the right", by taking a reversible process and making it irreversible.

The one exception is of value because it allows a single hydrolysis, ATP + 2H2O → AMP + PPi, to effectively supply the energy of hydrolysis of two high-energy bonds, with the hydrolysis of PPi being allowed to go to completion in a separate reaction. The AMP is regenerated to ATP in two steps, with the equilibrium reaction ATP + AMP ↔ 2ADP, followed by regeneration of ATP by the usual means, oxidative phosphorylation or other energy-producing pathways such as glycolysis.

Often, high-energy phosphate bonds are denoted by the character '~'. In this notation, ATP becomes A-P~P~P.

References

* McGilvery, R. W. and Goldstein, G., "Biochemistry - A Functional Approach", W. B. Saunders and Co, 1979, 345-351.


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