 Transferase

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.^[ref]

A schematic bookkeeping reaction iswhere 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.^[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.^[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.