A protein produced in living cells, which, even in very low concentration, catalyses specific chemical reactions but is not used up in the reaction. Enzymes are classified into six major groups, according to the type of reaction they catalyse: 1. Oxidoreductase; 2. Transferases; 3. Hydrolases; 4. Lyases; 5. Isomerases; 6. Ligases. The names of most individual enzymes are usually derived from the substrate on which they act, with the suffix -ase. Thus lactase is the enzyme that acts to breakdown lactose; it is classified as a hydrolase.