Pyrophosphate (${\mathrm{P}}_2{\mathrm{O}}_7^{4-}$)

Pyrophosphate is the two-phosphorus member of the condensed phosphate family, with a phosphorus–oxygen–phosphorus bridge joining its two phosphate units. The fully deprotonated anion is ${\mathrm{P}}_2{\mathrm{O}}_7^{4-}$; its acid is ${\mathrm{H}}_4{\mathrm{P}}_2{\mathrm{O}}_7$. The name also occurs in salts and partially protonated forms, so an actual chemical solution must be described with its pH and protonation state.

Cleaving the bridge by hydrolysis produces two separate phosphate units. One balanced representation isBoth the atoms and the total charge balance: the two products together carry charge $-4$. The equation distinguishes chemical cleavage from merely releasing an intact pyrophosphate ion from a larger molecule.

A classic experiment isolated crystalline inorganic pyrophosphatase from yeast and tested its activity on pyrophosphate. The enzyme accelerated hydrolysis and required activating metal ions, including magnesium. Its failure to hydrolyze the tested organic compounds containing related phosphate linkages demonstrated reaction substrate specificity: recognizing free pyrophosphate is not equivalent to attacking every phosphorus–oxygen–phosphorus linkage.^[ref]