Classification of Enzymes

Enzymes are grouped into six major classes according to the general type of reaction they catalyse:

1. Oxidoreductases catalyse oxidation–reduction reactions by transferring hydrogen and/or oxygen between molecules — e.g. alcohol dehydrogenase (alcohol + NAD⁺ → aldehyde + NADH₂).
2. Transferases catalyse the transfer of a chemical group from one molecule to another — e.g. glucokinase (glucose + ATP → glucose-6-phosphate + ADP).
3. Hydrolases catalyse hydrolysis reactions — the breakdown of a molecule by addition of water. This class includes the amylases, proteases and lipases — e.g. sucrase (sucrose + H₂O → glucose + fructose).
4. Lyases catalyse elimination reactions that remove a group of atoms from a substrate, leaving behind a double bond; the class includes the aldolases, decarboxylases and dehydratases — e.g. histidine decarboxylase (histidine → histamine + CO2), and fumarate hydratase.
5. Isomerases catalyse a structural rearrangement within a single molecule, converting it into one of its isomers; depending on the exact kind of isomerism involved, such enzymes are called racemases, epimerases, isomerases or mutases — e.g. xylose isomerase, or glucose-6-phosphate isomerase (glucose-6-phosphate → fructose-6-phosphate). …