 | |
 Phosphorus, P Oxygen, O | |
| Names | |
|---|---|
| IUPAC name Diphosphate | |
| Systematic IUPAC name Dipolyphosphate | |
| Other names Pyrophosphate Phosphonatophosphate | |
| Identifiers | |
| |
3D model (JSmol) | |
| ChEBI | |
| ChemSpider |
|
| DrugBank |
|
| E number | E450(thickeners, ...) |
| 26938 | |
| UNII | |
| |
| |
| Properties | |
| P2O4−7 | |
| Molar mass | 173.941 g·mol−1 |
| Conjugate acid | Pyrophosphoric acid |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Inchemistry,pyrophosphates arephosphorusoxyanions that contain two phosphorus atoms in aP−O−P linkage. A number of pyrophosphate salts exist, such asdisodium pyrophosphate (Na2H2P2O7) andtetrasodium pyrophosphate (Na4P2O7), among others. Often pyrophosphates are calleddiphosphates. The parent pyrophosphates are derived from partial or complete neutralization ofpyrophosphoric acid. Thepyrophosphate 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 newP−O−P bond, and which mirrors the nomenclature foranhydrides of carboxylic acids. Pyrophosphates are found inATP and othernucleotide triphosphates, which are important in biochemistry. The term pyrophosphate is also the name ofesters formed by the condensation of a phosphorylated biological compound withinorganic phosphate, as fordimethylallyl pyrophosphate. This bond is also referred to as ahigh-energy phosphate bond.
Pyrophosphoric acid is a tetraprotic acid, with four distinctpKas:[1]
The pKas occur in two distinct ranges because deprotonations occur on separate phosphate groups. For comparison, the pKas forphosphoric acid are 2.14, 7.20, and 12.37.
At physiologicalpHs, pyrophosphate exists as a mixture of doubly and singly protonated forms.
Disodium pyrophosphate is prepared by thermal condensation ofsodium dihydrogen phosphate or by partial deprotonation of pyrophosphoric acid.[2]
Pyrophosphates are generally white or colorless. Thealkali metal salts are water-soluble.[3] 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 ofpolyphosphates.[4]
The anionP2O4−7 is abbreviatedPPi, standing forinorganicpyrophosphate. It is formed by thehydrolysis ofATP intoAMP incells.
For example, when a nucleotide is incorporated into a growingDNA orRNA strand by apolymerase, pyrophosphate (PPi) is released. Pyrophosphorolysis is the reverse of thepolymerization reaction in which pyrophosphate reacts with the 3′-nucleosidemonophosphate (NMP or dNMP), which is removed from theoligonucleotide to release the corresponding triphosphate (dNTP from DNA, orNTP from RNA).
The pyrophosphate anion has the structureP2O4−7, and is anacid anhydride ofphosphate. It is unstable inaqueous solution andhydrolyzes 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.[5]
(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 PPiirreversible, and biochemical reactions coupled to this hydrolysis are irreversible as well.
From the standpoint ofhigh energy phosphate accounting, the hydrolysis of ATP to AMP and PPi requires two high-energy phosphates, as to reconstitute AMP into ATP requires twophosphorylation reactions.
The plasma concentration of inorganic pyrophosphate has a reference range of 0.58–3.78 μM (95% prediction interval).[6]
Isopentenyl pyrophosphate converts togeranyl pyrophosphate, the precursor to tens of thousands ofterpeness andterpenoids.[7][8]
PPi is an important inhibitor ofhydroxyapatite precipitation present inextracellular fluid (ECF) (includingblood plasma,[9]synovial fluid, andurine.[10][better source needed]). ECF issupersaturated in Ca2+ and PO43- 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 asmetastatic calcification).[9]
The levels of PPi are regulated by at least three molecules - notablytissue-nonspecific alkaline phosphatase (TNAP) which is secreted byosteoblasts locally into theosteoid to degrade PPi and thus allowbone mineralization to proceed during bone growth orremodeling. Congenital deficiency of TNAP in animal models results in offspring with soft, inadequately calcified bones.[9]
Osteoblasts also secretenucleotide pyrophosphatase phosphodiesterase 1 (NPP1) which causes extracellular PPi production, andprogressive ankylosis protein homolog (ANK) which promotes PPi transport out of cells. Deficiencies of NPP1 and ANK result in deficient extracellular PPi concentrations and are clinically associated withmetastatic calcification and excessive calcification of bone (manifesting clinically e.g. asosteophytes).[9]
Cells may channel intracellular PPi into ECF.[11][better source needed]ANK is a non-enzymatic plasma-membrane PPi channel that supports extracellular PPi levels.[11][better source needed] Defective function of the membrane PPi channel ANK is associated with low extracellular PPi and elevated intracellular PPi.[10]Ectonucleotide pyrophosphatase/phosphodiesterase (ENPP) may function to raise extracellular PPi.[11][better source needed]
Various diphosphates are used asemulsifiers,stabilisers,acidity regulators,raising agents,sequestrants, andwater retention agents in food processing.[12] They are classified in theE number scheme under E450:[13]
In particular, various formulations of diphosphates are used to stabilizewhipped cream.[14]
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