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In chemistry, pyrophosphate, or PPi is an anion observed in living systems, usually formed by the hydrolysis of ATP into AMP.

The pyrophosphate anion has the structure P2O74-, and is an acid anhydride of phosphate. It is unstable in aqueous solution and rapidly hydrolyzess into inorganic phosphate:

or in shorthand notation:

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 cost 2 high energy phosphates, as to reconstitute AMP from ATP will require 2 phosphorylation reactions.

In biology, the term pyrophosphate may also be used as the name of the bond formed by the condensation of a phosphorylated biological compound with inorganic phosphate. This bond is also referred to as a high energy phosphate bond.