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A sulfotransferase is an enzyme that catalyses biochemical sulfation by transferring a group from a donor to an acceptor. The IUBMB places these activities in EC 2.8.2, within sulfur-group-transferring transferases. Many familiar biochemical examples use PAPS, with adenosine 3',5'-bisphosphate as the donor-derived product.^[ref]

The acceptor and the modified atom distinguish particular activities. Chondroitin 4-sulfotransferase and chondroitin 6-sulfotransferase modify different positions of N-acetylgalactosamine in chondroitin. Heparan sulfate glucosamine N-sulfotransferase instead concerns a nitrogen position in heparan sulfate. The shared donor does not make these acceptor reactions identical.^{[ref][ref][ref]}

A constructed assay example shows why an activity label needs a measured product. Suppose two samples each consume ten nanomoles of PAPS, but one transfers the group to acceptor A and the other to acceptor B. A donor-consumption measurement can agree while the acceptor products differ. If a third sample consumes donor through a side reaction, donor loss alone would not even establish the intended transfer.

Testing a sulfotransferase therefore joins three questions: which donor is consumed, which acceptor changes, and where the new group is attached. A total incorporation signal addresses an amount; identifying the modified position supplies structural information. This distinction makes a broad enzyme class useful without erasing the chemical specificity of its members.

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.