Isomerases are enzymes that catalyze the formation of a substrate's isomer. In other words, they facilitate the transfer of specific functional groups intramolecularly without adding or removing atoms from the substrate. This conversion can be simply represented in the form A → B, where A and B are isomers.
Isomerases are used in many biochemical pathways, including the citric acid cycle and the glycolitic pathway. All isomerases have Enzyme Commission numbers beginning in EC 5. A variety of isomerizations can be carried out, including racemization, cis-trans isomerization, enolization, and many others. Some examples of isomerases include triose phosphate isomerase, bisphosphoglycerate mutase, and photoisomerase.
Isomerases can help prepare a molecule for subsequent reactions such as oxidation-reduction reactions. For example, in the conversion of citrate to isocitrate in the citric acid cycle, isomerization prepares the molecule for subsequent oxidation and decarboxylation by transferring the hydroxyl group of citrate from a tertiary to a secondary position. Additionally, isomerases can catalyze phosphorylation reaction pathways throughout the Krebb Cycle by preparing the molecule for oxidation states. The change in position is facilitated through Isomerases without affecting the overall chemical composition of the substrate or product.