In chemistry, pyrophosphates are phosphorusoxyanions that contain two phosphorus atoms in a P−O−P linkage. A number of pyrophosphate salts exist, such as disodium pyrophosphate (Na2H2P2O7) and tetrasodium pyrophosphate (Na4P2O7), 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 P−O−P 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 pKa's:[1]
H4P2O7 ⇌ [H3P2O7]− + H+, pKa1 = 0.85
[H3P2O7]− ⇌ [H2P2O7]2− + H+, pKa2 = 1.96
[H2P2O7]2− ⇌ [HP2O7]3− + H+, pKa3 = 6.60
[HP2O7]3− ⇌ [P2O7]4− + H+, pKa4 = 9.41
The pKa's occur in two distinct ranges because deprotonations occur on separate phosphate groups. For comparison with the pKa's for phosphoric acid are 2.14, 7.20, and 12.37.
At physiological pH's, 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.[2]
Pyrophosphates are generally white or colorless. The alkali 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 of polyphosphates.[4]
In biochemistry
The anion P2O4−7 is abbreviated PPi, standing for inorganic pyrophosphate. It 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′-nucleosidemonophosphate (NMP or dNMP), which is removed from the oligonucleotide to release the corresponding triphosphate (dNTP from DNA, or NTP from RNA).
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 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.
^Van Wazer JR, Griffith EJ, McCullough JF (Jan 1955). "Structure and Properties of the Condensed Phosphates. VII. Hydrolytic Degradation of Pyro- and Tripolyphosphate". J. Am. Chem. Soc. 77 (2): 287–291. doi:10.1021/ja01607a011.
^Ryan LM, Kozin F, McCarty DJ (1979). "Quantification of human plasma inorganic pyrophosphate. I. Normal values in osteoarthritis and calcium pyrophosphate dihydrate crystal deposition disease". Arthritis Rheum. 22 (8): 886–91. doi:10.1002/art.1780220812. PMID223577.
