Pyrophosphate
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In chemistry, pyrophosphate is the name of the anion, the salts, and the esters of pyrophosphoric acid. The anion is abbreviated PPi and is formed by the hydrolysis of ATP into AMP in cells. This hydrolysis is called pyrophosphorolysis.
- ATP → AMP + PPi
The pyrophosphate anion has the structure P2O74-, and is an acid anhydride of phosphate. It is unstable in aqueous solution and rapidly hydrolyzes into inorganic phosphate:
- P2O74- + H2O → 2 HPO42-
or in shorthand notation:
- PPi + H2O → 2 Pi
This hydrolysis to inorganic phosphate effectively renders the cleavage of ATP to AMP and PPi irreversible, and biochemical reactions coupled to this hydrolysis are irreversible as well.
From the standpoint of high energy phosphate accounting, the hydrolysis of ATP to AMP and PPi will require two high energy phosphates, as to reconstitute AMP from ATP will require two phosphorylation reactions.
- AMP + ATP → 2 ADP
- 2 ADP + 2 Pi → 2 ATP
The term pyrophosphate is also the name of esters formed by the condensation of a phosphorylated biological compound with inorganic phosphate as for dimethylallyl pyrophosphate. This bond is also referred to as a high energy phosphate bond.