EC 5. Introduction

EC 5. Isomerases

These enzymes catalyse changes within one molecule.

EC 5.1 Racemases and Epimerases

These enzymes, catalysing either racemization or epimerization of a centre of chirality, are subdivided according to their substrates; amino acids (EC 5.1.1), hydroxy acids (EC 5.1.2), carbohydrates and derivatives (EC 5.1.3), or other compounds (EC 5.1.99).

EC 5.2 cis-trans-Isomerases

These enzymes rearrange the geometry at double bonds.

EC 5.3 Intramolecular Oxidoreductases

These enzymes bring about the oxidation of one part of a molecule with a corresponding reduction of another part. They include the enzymes converting, in the sugar series, aldoses to ketoses and vice-versa (sugar isomerases, EC 5.3.1), enzymes catalysing a keto-enol equilibrium (tautomerases, EC 5.3.2), enzymes shifting a carbon-carbon double bond from one position to another (EC 5.3.3), enzymes transposing S-S bonds (EC 5.3.4), and a group of miscellaneous enzymes (EC 5.3.99).

EC 5.4 Intramolecular Transferases

These enzymes transfer acyl (EC 5.4.1), phospho (EC 5.4.2), amino (EC 5.4.3), hydroxy (EC 5.4.4) or other groups (EC 5.4.99) from one position to another.

EC 5.4.2 Phosphotransferases (Phosphomutases)
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 identical with 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 the following sub-subclass based on the overall isomerase reaction.

These catalyse reactions in which a group can be regarded as eliminated from one part of a molecule, leaving a double bond, while remaining covalently attached to the molecule.

EC 5.99 Other Isomerases

A subclass of miscellaneous enzymes.