High-energy phosphate can mean one of two things:
- 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 exergonic under physiological conditions, releasing energy.
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.
The term 'high energy' with respect to these bonds can be misleading, because the negative free energy change is not due directly to the breaking of the bonds themselves, which is, as with the breaking of any bond, an endergonic step. The negative free energy change comes instead from the increased resonance stabilisation and solvation of the products relative to the reactants.
- McGilvery, R. W. and Goldstein, G., Biochemistry - A Functional Approach, W. B. Saunders and Co, 1979, 345-351.
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