= Dimethylallyl diphosphate
<Dimethylallyl diphosphate> is a five-carbon isoprenoid building block carrying an activated <diphosphate> group. Its carbon skeleton is $\mathrm{(CH_3)_2C{=}CH{-}CH_2{-}}$, attached through oxygen to diphosphate. The neutral-acid <molecular formula> is $\mathrm{C_5H_{12}O_7P_2}$. ChEBI calls this specific compound prenyl diphosphate; the title here distinguishes it from longer prenyl diphosphates.<Dimethylallyl diphosphate in ChEBI>[CHEBI:16057, Definition, Formula, structure and synonyms]{ref}
The placement of its <double bond> differs from that in <isopentenyl diphosphate>. Matching <molecular formulas> therefore does not make these substrates interchangeable: an <enzyme> encounters a specific arrangement of bonds, not just five carbons and two <phosphorus> atoms.
In the reaction catalysed by dimethylallyltranstransferase, <dimethylallyl diphosphate> supplies the donor group, <isopentenyl diphosphate> is the other substrate, and ten-carbon <geranyl diphosphate> is produced with release of <diphosphate>.<Dimethylallyltranstransferase in IUBMB Enzyme Nomenclature>[EC 2.5.1.1, Reaction and Comments]{ref} Mixing $1\,\mathrm{\mu mol}$ of each substrate can produce at most $1\,\mathrm{\mu mol}$ of geranyl product in the ideal isolated reaction. Summing both substrate amounts and predicting two micromoles of product would count molecules without respecting the two-substrate reaction stoichiometry.
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