In chemistry, pyrophosphates are phosphorus oxyanions that contain two phosphorus atoms in a linkage. A number of pyrophosphate salts exist, such as disodium pyrophosphate () and tetrasodium pyrophosphate (), among others. Often pyrophosphates are called diphosphates. The parent pyrophosphates are derived from partial or complete neutralization of pyrophosphoric acid. The pyrophosphate bond is also sometimes referred to as a phosphoanhydride bond, a naming convention which emphasizes the loss of water that occurs when two phosphates form a new bond, and which mirrors the nomenclature for anhydrides of carboxylic acids. Pyrophosphates are found in ATP and other nucleotide triphosphates, which are important in biochemistry. 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.

Acidity

Pyrophosphoric acid is a tetraprotic acid, with four distinct pK<sub>a</sub>s:

:, pK<sub>a1</sub> = 0.85

:, pK<sub>a2</sub> = 1.96

:, pK<sub>a3</sub> = 6.60

:, pK<sub>a4</sub> = 9.41

The pKas occur in two distinct ranges because deprotonations occur on separate phosphate groups. For comparison, the pK<sub>a</sub>s for phosphoric acid are 2.14, 7.20, and 12.37.

At physiological pHs, pyrophosphate exists as a mixture of doubly and singly protonated forms.

Preparation

Disodium pyrophosphate is prepared by thermal condensation of sodium dihydrogen phosphate or by partial deprotonation of pyrophosphoric acid.

Pyrophosphates are generally white or colorless. The alkali metal salts are water-soluble. They are good complexing agents for metal ions (such as calcium and many transition metals) and have many uses in industrial chemistry. Pyrophosphate is the first member of an entire series of polyphosphates.

Biochemistry

The anion is abbreviated PP<sub>i</sub>, standing for inorganic pyrophosphate. It is formed by the hydrolysis of ATP into AMP in cells.

:ATP → AMP + PP<sub>i</sub>

For example, when a nucleotide is incorporated into a growing DNA or RNA strand by a polymerase, pyrophosphate (PP<sub>i</sub>) is released. Pyrophosphorolysis is the reverse of the polymerization reaction in which pyrophosphate reacts with the 3′-nucleosidemonophosphate (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 , and is an acid anhydride of phosphate. It is unstable in aqueous solution and hydrolyzes into inorganic phosphate:

:

or in biologists' shorthand notation:

:

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 PP<sub>i</sub> 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<sub>i</sub> requires two high-energy phosphates, as to reconstitute AMP into ATP requires two phosphorylation reactions.

:AMP + ATP → 2 ADP

:2 ADP + 2 P<sub>i</sub> → 2 ATP

The plasma concentration of inorganic pyrophosphate has a reference range of 0.58–3.78&nbsp;μM (95% prediction interval).

Terpenes

Isopentenyl pyrophosphate converts to geranyl pyrophosphate, the precursor to tens of thousands of terpeness and terpenoids.

thumb|center|400px|[[Isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMAPP) condense to produce geranyl pyrophosphate, precursor to all terpenes and terpenoids.]]

Physiological role

Hydroxyapatite precipitation inhibitor

PP<sub>i</sub> is an important inhibitor of hydroxyapatite precipitation present in extracellular fluid (ECF) (including blood plasma, synovial fluid, and urine.). ECF is supersaturated in and ions – in other words, the concentration of these ions is far in excess of the limit beyond which hydroxyapetite crystals would normally begin to precipitate from the solution (a pathological condition known as metastatic calcification). ANK is a non-enzymatic plasma-membrane PP<sub>i</sub> channel that supports extracellular PP<sub>i</sub> levels. They are classified in the E number scheme under E450:

  • E450(a): disodium dihydrogen diphosphate; trisodium diphosphate; tetrasodium diphosphate (TSPP); tetrapotassium diphosphate
  • E450(b): pentasodium and pentapotassium triphosphate
  • E450(c): sodium and potassium polyphosphates

In particular, various formulations of diphosphates are used to stabilize whipped cream.

See also

  • Adenosine monophosphate
  • Adenosine diphosphate
  • Adenosine triphosphate
  • ATPase
  • ATP hydrolysis
  • ATP synthase
  • Biochemistry
  • Bone
  • Calcium pyrophosphate
  • Calcium pyrophosphate dihydrate deposition disease
  • Catalysis
  • DNA
  • High energy phosphate
  • Inorganic pyrophosphatase
  • Nucleoside triphosphate
  • Nucleotide
  • Organophosphate
  • Oxidative phosphorylation
  • Phosphate
  • Phosphoric acid
  • Phosphoric acids and phosphates
  • RNA
  • Sodium pyrophosphate
  • Superphosphate
  • Thiamine pyrophosphate
  • Tooth
  • Zinc pyrophosphate

References

Further reading