Encyclopedia > Pyrophosphate

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

  • 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 hydrolysis 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.

  • AMP + ATP → 2 ADP
  • 2 ADP + 2 Pi → 2 ATP

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