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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 exothermic under physiological conditions, releasing energy.
Except for PPi → 2 Pi, 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 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. ---- Reference: McGilvery, R. W. and Goldstein, G., Biochemistry - A Functional Approach, W. B. Saunders and Co, 1979, 345-351.
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