High-energy phosphate

High-energy phosphate can mean one of two things:

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.

Energy released by high energy phosphate reactions
Reaction ΔG [kJ/mol]
ATP + H2O → ADP + Pi -36.8
ADP + H2O → AMP + Pi -36.0
ATP + H2O → AMP + PPi -40.6
PPi + H2O → 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:

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 "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.[1] It emphasizes the special nature of these bonds.[2] Stryer states:

ATP is often called a high energy compound and its phosphoanhydride bonds are referred to as high-energy bonds. There is nothing special about the bonds themselves. They are high-energy bonds in the sense that free energy is released when they are hydrolyzed, for the reasons given above. Lipmann’s term “high-energy bond” and his symbol ~P (squiggle P) for a compound having a high phosphate group transfer potential are vivid, concise, and useful notations. In fact Lipmann’s squiggle did much to stimulate interest in bioenergetics.

.

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, as with the breaking of any bond, is an endergonic step (i.e., it absorbs energy, not releases it). The negative free energy change comes instead from the increased resonance stabilisation and solvation of the products relative to the reactants.

References

  1. ^ Lipmann F (1941). Adv. Enzymol. 1: 99–162. ISSN 0196-7398. 
  2. ^ Lubert Stryer BIOCHEMISTRY, 3rd edition, 1988. Chapter 13, p. 318

Further reading