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

• ATP → AMP + PP_i

The pyrophosphate anion has the structure P₂O₇^4-, and is an acid anhydride[?] of phosphate. It is unstable in aqueous solution and rapidly hydrolyzes[?] into inorganic phosphate:

• P₂O₇^4- + H₂O → 2 HPO₄^2-

or in shorthand notation:

• PP_i + H₂O → 2 P_i

This hydrolysis to inorganic phosphate effectively renders the hydrolysis of ATP to AMP and PP_i 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 PP_i will cost 2 high energy phosphates, as to reconstitute AMP from ATP will require 2 phosphorylation reactions.

• AMP + ATP → 2 ADP
• 2 ADP + 2 P_i → 2 ATP

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