isomerase

Meaning: Isomerases catalyze intramolecular rearrangements. Nothing is added or removed, so the product has the same molecular formula as the substrate but a different structure. They are one of the six enzyme classes (oxidoreductases, transferases, hydrolases, lyases, isomerases, ligases). Like all enzymes, they lower activation energy and do not change Keq or ΔG of the reaction. Subtypes include epimerases and racemases, which change configuration at a stereocenter, and mutases, which shift a group such as a phosphate to another position in the same molecule.

On the MCAT: Expect glycolysis examples. Phosphoglucose isomerase converts glucose 6-phosphate (an aldose) to fructose 6-phosphate (a ketose). Triose phosphate isomerase interconverts dihydroxyacetone phosphate and glyceraldehyde 3-phosphate. Phosphoglycerate mutase moves the phosphate from carbon 3 to carbon 2. Questions usually give a reaction and ask you to classify the enzyme by noticing that the formula is unchanged, or ask which step lets both halves of a six-carbon sugar continue through the pathway.

Common confusion: A phosphate moving between positions in one molecule is a mutase, a kind of isomerase. A phosphate moving to a different molecule, as with a kinase, is a transferase. Also, an isomerase does not drive a reaction in one direction. The direction follows concentrations, and the reaction can run either way.