Editing Pyrophosphate (section)

== In biochemistry ==
The anion {{chem2|P2O7(4−)}} is abbreviated '''PP<sub>i</sub>''', standing for '''''i'''norganic '''p'''yro'''p'''hosphate''. It is formed by the [[hydrolysis]] of [[Adenosine triphosphate|ATP]] into [[Adenosine monophosphate|AMP]] in [[Cell (biology)|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 ([[Nucleoside triphosphate|NMP]] or dNMP), which is removed from the [[oligonucleotide]] to release the corresponding triphosphate ([[nucleoside triphosphate|dNTP]] from DNA, or [[nucleoside triphosphate|NTP]] from RNA).

The pyrophosphate anion has the structure {{chem2|P2O7(4−)}}, and is an [[acid anhydride]] of [[phosphate]]. It is unstable in [[aqueous solution]] and [[hydrolysis|hydrolyze]]s into inorganic phosphate:
:{{chem2|P2O7(4−) + H2O → 2 HPO4(2−)}}
or in biologists' shorthand notation:
:{{chem2|PP_{i} + H2O → 2 P_{i} + 2 H+}}

In the absence of enzymic catalysis, hydrolysis reactions of simple polyphosphates such as pyrophosphate, linear triphosphate, [[Adenosine diphosphate|ADP]], and ATP normally proceed extremely slowly in all but highly acidic media.<ref name="Wazer">{{ cite journal |vauthors=Van Wazer JR, Griffith EJ, McCullough JF |title=Structure and Properties of the Condensed Phosphates. VII. Hydrolytic Degradation of Pyro- and Tripolyphosphate |journal=J. Am. Chem. Soc. |date=Jan 1955 |volume=77 |issue=2 |pages=287–291 |doi=10.1021/ja01607a011}}</ref>

(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> [[hydrolysis|irreversible]], and biochemical reactions coupled to this hydrolysis are irreversible as well.

PP<sub>i</sub> 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).<ref name=Ho>{{ cite journal |vauthors=Ho AM, Johnson MD, Kingsley DM |title=Role of the mouse ank gene in control of tissue calcification and arthritis |journal=Science |date=Jul 2000 |volume=289 |issue=5477 |pages=265–70 |pmid=10894769 |doi=10.1126/science.289.5477.265 |bibcode=2000Sci...289..265H}}</ref> Cells may channel intracellular PP<sub>i</sub> into ECF.<ref name=Rutsch>{{ cite journal |vauthors=Rutsch F, Vaingankar S, Johnson K, Goldfine I, Maddux B, Schauerte P, Kalhoff H, Sano K, Boisvert WA, Superti-Furga A, Terkeltaub R |title=PC-1 nucleoside triphosphate pyrophosphohydrolase deficiency in idiopathic infantile arterial calcification |journal=Am J Pathol |date=Feb 2001 |volume=158 |issue=2 |pages=543–54 |pmid=11159191 |pmc=1850320 |doi=10.1016/S0002-9440(10)63996-X}}</ref> [[ANKH|ANK]] is a nonenzymatic plasma-membrane PP<sub>i</sub> channel that supports extracellular PP<sub>i</sub> levels.<ref name=Rutsch/> Defective function of the membrane PP<sub>i</sub> channel ANK is associated with low extracellular PP<sub>i</sub> and elevated intracellular PP<sub>i</sub>.<ref name=Ho/> [[Ectonucleotide pyrophosphatase/phosphodiesterase 1|Ectonucleotide pyrophosphatase/phosphodiesterase]] (ENPP) may function to raise extracellular PP<sub>i</sub>.<ref name=Rutsch/>

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 [[Adenosine diphosphate|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;[[Molarity|μM]] (95% prediction interval).<ref>{{cite journal |vauthors=Ryan LM, Kozin F, McCarty DJ |title=Quantification of human plasma inorganic pyrophosphate. I. Normal values in osteoarthritis and calcium pyrophosphate dihydrate crystal deposition disease |journal=Arthritis Rheum. |volume=22 |issue=8 |pages=886–91 |year=1979 |pmid=223577 |doi= 10.1002/art.1780220812 |doi-access=}}</ref>

===Terpenes===
[[Isopentenyl pyrophosphate]] converts to [[geranyl pyrophosphate]], the precursor to tens of thousands of [[terpene|terpenes]]s and [[terpenoid|terpenoids]].<ref>{{cite book|title=Terpenes: Flavors, Fragrances, Pharmaca, Pheromones|url=https://archive.org/details/terpenes00brei|url-access=limited|pages=[https://archive.org/details/terpenes00brei/page/n18 10]–23|chapter=Hemi- and Monoterpenes|author=Eberhard Breitmaier|year= 2006|doi=10.1002/9783527609949.ch2|isbn=9783527609949}}</ref><ref name=KO>{{cite book |doi=10.1002/0471238961.2005181602120504.a01.pub2|chapter=Terpenoids |title=Kirk-Othmer Encyclopedia of Chemical Technology |year=2006 |last1=Sell |first1=Charles S. |isbn=0471238961 }}</ref> 
[[Image:Synthesis of geranyl pyrophosphate.png|thumb|center|400px|[[Isopentenyl pyrophosphate]] (IPP) and [[dimethylallyl pyrophosphate]] (DMAPP) condense to produce [[geranyl pyrophosphate]], precursor to all terpenes and terpenoids.]]