 Inorganic diphosphatase

Inorganic diphosphatase catalyses diphosphate hydrolysis, producing two phosphate products per substrate molecule. Its classification is EC 3.6.1.1. Substrate specificity and activation by metal ions depend on the source of the enzyme.^[ref]

An explicitly balanced representation for chosen ionic forms isBoth sides contain two phosphorus atoms, eight oxygens, two hydrogens and net charge $-4$. If the substrate is instead $\mathrm{H}{\mathrm{P}}_2{\mathrm{O}}_7^{3-}$, an additional proton appears on the product side. These balances specify forms without asserting which dominates at an unspecified pH.

A constructed assay gives a practical consequence of the two-to-one product ratio. If measured phosphate increases by $12\mathrm{nmol}$ while the matched blank increases by $2\mathrm{nmol}$, the net signal is 10 nmol phosphate, corresponding to 5 nmol diphosphate hydrolysed, provided no other reaction contributes.

In 1952, M. Kunitz isolated a crystalline yeast enzyme and characterized phosphate production, metal activation and reaction-condition dependence.^[ref] The experiment ties a purified protein preparation to a measured chemical reaction. Crystallinity alone would not supply that activity assignment, and its properties do not automatically describe every soluble or membrane-associated diphosphatase.