Pyrophosphate

In chemistry, the anion, the salts, and the esters of pyrophosphoric acid are called pyrophosphates. Any salt or ester containing two phosphate groups is called a diphosphate. As a food additive, diphosphates are known as E450.

Chemistry
Pyrophosphates were originally prepared by heating phosphates (pyro from the Greek, meaning "fire"). Pyrophosphates generally exhibit the highest solubilities among the phosphates; moreover, they are good complexing agents and have many uses in industrial chemistry. Pyrophosphate is the first member of an entire series of polyphosphates.

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.

The synthesis of tetraethyl pyrophosphate was first described in 1854 by Philip de Clermount at a meeting of the French Academy of Sciences.

In biochemistry
Pyrophosphates are very important in biochemistry. The anion P2O74− is abbreviated PPi and is formed by the hydrolysis of ATP into AMP in cells.
 * ATP → AMP + PPi

For example, when a nucleotide is incorporated into a growing DNA or RNA strand by a polymerase, pyrophosphate (PPi) is released. Pyrophosphorolysis is the reverse of the polymerization reaction in which pyrophosphate reacts with the 3'-nucleotidemonophosphate (NMP or dNMP), which is removed from the oligonucleotide to release the corresponding triphosphate (dNTP from DNA, or NTP from RNA).

The pyrophosphate anion has the structure P2O74&minus;, and is an acid anhydride of phosphate. It is unstable in aqueous solution and hydrolyzes into inorganic phosphate:
 * P2O74− + H2O → 2 HPO42&minus;

or in biologists' shorthand notation:
 * PPi + H2O → 2 Pi

In the absence of enzymic catalysis, hydrolysis reactions of simple polyphosphates such as pyrophosphate, linear triphosphate, ADP, and ATP normally proceed extremely slowly in all but highly acidic media.

(The reverse of this reaction is a method of preparing pyrophosphates by heating phosphates.)

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.

PPi occurs in synovial fluid, blood plasma, and urine at levels sufficient to block calcification and may be a natural inhibitor of hydroxyapatite formation in extracellular fluid (ECF). Cells may channel intracellular PPi into ECF. ANK is a nonenzymatic plasma-membrane PPi channel that supports extracellular PPi levels. Defective function of the membrane PPi channel ANK is associated with low extracellular PPi and elevated intracellular PPi. Ectonucleotide pyrophosphatase/phosphodiesterase (ENPP) may function to raise extracellular PPi.

From the standpoint of high energy phosphate accounting, the hydrolysis of ATP to AMP and PPi requires two high-energy phosphates, as to reconstitute AMP into ATP requires two phosphorylation reactions.
 * AMP + ATP → 2 ADP
 * 2 ADP + 2 Pi → 2 ATP