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.

Description

High-energy phosphate bonds are usually 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 Gibbs free energy.

Reaction

ATP + H₂O → ADP + P_i

ADP + H₂O → AMP + P_i

ATP + H₂O → AMP + PP_i

PP_i + H₂O → 2 P_i

Except for PP_i → 2 P_i, these reactions are, in general, not allowed to go uncontrolled in cells 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 a reaction out of equilibrium by promoting one direction of the reaction faster than the equilibrium can relax.

The one exception is of value because it allows a single hydrolysis, ATP + H₂O → AMP + PP_i, to effectively supply the energy of hydrolysis of two high-energy bonds, with the hydrolysis of PP_i 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 "squiggle" notation, ATP becomes A-P~P~P. The squiggle notation was invented by Fritz Albert Lipmann, who first proposed ATP as the main energy transfer molecule of the cell, in 1941. Lipmann's notation emphasizes the special nature of these bonds. Stryer states:
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. The breaking of these bonds, like the breaking of most bonds, is endergonic and consumes energy rather than releasing it. The negative free energy change comes instead from the fact that the bonds formed after hydrolysis - or the phosphorylation of a residue by ATP - are lower in energy than the bonds present before hydrolysis.. This effect is due to a number of factors including increased resonance stabilization and solvation of the products relative to the reactants, and destabilization of the reactants due to electrostatic repulsion between neighboring phosphorus atoms.