High energy phosphate

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High energy phosphate can mean one of two things:

It can mean the phosphate-phosphate bonds formed when compounds such as adenosine diphosphate and adenosine triphosphate are created.

It can mean the compounds which 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.

Energetic of High Energy Phosphate Reactions
Reaction	Δ G in kilojoules per mole
ATP + H₂O → ADP + P_i	-36.8
ADP + H₂O → AMP + P_i	-36.0
ATP + H₂O → AMP + PP_i	-40.6
PP_i → 2 P_i	-31.8
AMP + H₂O → A + P_i	-12.6

Except for PP_i → 2 P_i, these reactions are generally not allowed to go uncontrolled in the human cell, but generally are coupled to other processes needing energy to drive them to completion. So, high energy phosphate reactions can

provide energy to cellular processes, to allow them to run
by coupling processes to a particular nucleoside, allow 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 → ADP + 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 notation, ATP becomes A-P~P~P.

Reference:

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

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