= Transferase
{wiki}
A <transferase> is an <enzyme> that transfers a chemical group from a donor to an acceptor. <Methyltransferases> transfer a <methyl group>, while <glycosyltransferases> transfer a <glycosyl group>. The group transferred, the donor and the acceptor all help specify the activity; the transferred group alone does not identify one unique <enzyme>.<Enzyme Nomenclature by the IUBMB>[EC 2 Introduction, donor:acceptor grouptransferase naming scheme]{ref}
A schematic bookkeeping reaction is
$$
\mathrm{D\!\!-X+A\longrightarrow D+A\!\!-X},
$$
where X labels the transferred group. This is an abstract group-transfer scheme rather than a fully balanced molecular equation. Actual reactions can require additional proton or water terms, and a donor is sometimes a <coenzyme>.<Enzyme Nomenclature by the IUBMB>[EC 2 Introduction, donor and coenzyme roles]{ref}
For example, <riboflavin kinase> uses <ATP> in forming <flavin mononucleotide>, whereas <FAD synthase> uses <ATP> to add an <adenylyl group> to <flavin mononucleotide>. Their ATP-derived coproducts differ: the former forms <ADP>, and the latter forms <diphosphate>. Classifying both as <transferases> does not make their reaction balances interchangeable.<Enzyme Nomenclature by the IUBMB>[EC 2.7.1.26 and EC 2.7.7.2, Reactions]{ref}
In a constructed single-pathway experiment, five nanomoles of completed transfers attach five nanomoles of X to acceptor molecules, while the <transferase> can be reused between turnovers. The amount of transferred material is therefore not the amount of <enzyme> consumed. Donor disappearance alone also needs interpretation: a side reaction could consume donor without producing the intended acceptor-bound product. Measuring that product tests the specific transfer being claimed.
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