= Geranyl diphosphate
<Geranyl diphosphate> is a ten-carbon prenyl diphosphate, chemically the diphosphate <ester> of <geraniol>. The neutral acid has <molecular formula> $\mathrm{C_{10}H_{20}O_7P_2}$; this formula names a protonation state rather than every ionic form encountered in an <enzyme assay>.<Geranyl diphosphate in ChEBI>[CHEBI:17211, Definition, Formula and net charge]{ref}
A <prenyltransferase> can form <geranyl diphosphate> by joining five-carbon <dimethylallyl diphosphate> to five-carbon <isopentenyl diphosphate>, releasing <diphosphate>. The reaction constructs a longer carbon skeleton while retaining one activated diphosphate group.<Dimethylallyltranstransferase in IUBMB Enzyme Nomenclature>[EC 2.5.1.1, Reaction]{ref}
Consider an ideal isolated reaction supplied with $2\,\mathrm{\mu mol}$ of <dimethylallyl diphosphate> and $3\,\mathrm{\mu mol}$ of <isopentenyl diphosphate>. The one-to-one substrate ratio permits at most $2\,\mathrm{\mu mol}$ of <geranyl diphosphate>, leaving $1\,\mathrm{\mu mol}$ of <isopentenyl diphosphate> if conversion is complete. Carbon accounting gives $2(5+5)=2(10)$ for the material incorporated into product.
This upper bound is a useful assay check, not a measured yield. A signal reporting <diphosphate> release measures a coproduct; chemical identification of the organic product is needed to establish that the intended <geranyl diphosphate> was made. Product identity includes bond geometry as well as atom counts.
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