= Inorganic pyrophosphatase
An <inorganic pyrophosphatase> is an <enzyme> that catalyses <hydrolysis> of free inorganic <pyrophosphate> into two <orthophosphate> units. One explicitly balanced choice of ionic forms is
$$
\mathrm{P_2O_7^{4-}+H_2O\longrightarrow 2HPO_4^{2-}}.
$$
Actual protonation and metal binding depend on <pH> and solution conditions. <Hydrolysis> removes <pyrophosphate> generated by other reactions, helping couple those reactions to product removal.
In 1952, <M. Kunitz> isolated crystalline <inorganic pyrophosphatase> from baker's yeast. The preparation hydrolysed free <pyrophosphate>, required activating metal <ions> such as <magnesium>, and did not interchangeably hydrolyse the tested organic diphosphate-containing compounds. Purifying a catalyst and comparing substrates connected the reaction to a specific <protein> activity.<Crystalline inorganic pyrophosphatase isolated from baker's yeast by M. Kunitz>[Abstract: purification, specificity and metal-ion dependence]{ref}
For a stoichiometric assay example, complete <hydrolysis> of $10\,\mu\mathrm M$ <pyrophosphate> adds $20\,\mu\mathrm M$ <orthophosphate>, provided no other reaction consumes or generates it. Pre-existing <phosphate> must be measured as background. Stoichiometry predicts product amount; <enzyme kinetics>, <pH> and metal availability determine the time required. The yeast preparation's preferred conditions should not be assigned to every <inorganic pyrophosphatase>.
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