Most of these enzymes were previously listed as sub-subclass EC 2.7.5, under the heading: 'Phosphotransferases with regeneration of donors, apparently catalysing intramolecular transfers'. The reaction for these enzymes was written in the form:
X-(P)2 + AP = BP + X-(P)2.
In fact, since phosphorylation of the acceptor produces a bisphosphate that is identical to the donor, the overall reaction is an isomerization of AP into BP, with the bisphosphate acting catalytically. It has been shown in some cases that the enzyme has a functional phosphate group, which can act as the donor. Phosphate is transferred to the substrate, forming the intermediate bisphosphate; the other phosphate group is subsequently transferred to the enzyme:
E-P + AP = E + X-(P)2
X-(P)2 + E = BP + E-P.
The bisphosphate may be firmly attached to the enzyme during the catalytic cycle, or, in other cases, may be released so that free bisphosphate is required as an activator. Under these circumstances, it was agreed in 1983 that all of these enzymes should be listed together in this sub-subclass based on the overall isomerase reaction.