lyase

Meaning: Lyases cleave a bond by elimination rather than by hydrolysis or oxidation-reduction. One product usually ends up with a new double bond (or ring). Because the enzyme speeds both directions, the reverse reaction, adding a group across a double bond, is also lyase chemistry. Examples: Aldolase splits fructose-1,6-bisphosphate into DHAP and glyceraldehyde-3-phosphate in glycolysis. Fumarase interconverts fumarate and malate by adding or removing water across a double bond. Pyruvate decarboxylase removes CO₂. Carbonic anhydrase interconverts CO₂ + H₂O and H₂CO₃. Enzymes called synthases often belong here because they join molecules without using ATP.

On the MCAT: A passage gives a reaction and asks you to classify the enzyme among the six classes: oxidoreductase, transferase, hydrolase, lyase, isomerase, ligase. Look for a molecule splitting in two with no water consumed and no electron transfer, or for a double bond forming or disappearing. Questions may also place aldolase or fumarase in a metabolic pathway. Like any enzyme, a lyase lowers activation energy and does not change ΔG or Keq.

Common confusion: Students call every bond-breaking enzyme a hydrolase. A hydrolase cleaves a bond by adding water across it, and the water ends up in the products. A lyase cleaves without that water addition. Also keep synthase and synthetase apart: a synthetase is a ligase that uses ATP, while a synthase does not need ATP.