Classification and Nomenclature of Enzymes

Thousands of enzymes have been found, purified and studied, and to bring order to this variety they are sorted into groups according to the kind of chemical reaction each one drives. Under this scheme enzymes fall into six broad classes. Each class is further divided into four to thirteen subclasses, and every enzyme is given a characteristic four-digit number that locates it precisely within the system.

The six classes, each defined by the type of reaction it catalyses, are:

• Oxidoreductases (dehydrogenases). These carry out oxidation-reduction reactions between two substrates. In effect one substrate is reduced while the partner substrate is oxidised, so electrons or hydrogen are handed from one to the other.

S(reduced) + S'(oxidised) → S(oxidised) + S'(reduced)

• Transferases. These move a particular chemical group (any group other than hydrogen) from one substrate to another, transferring the group G from one molecule onto its partner.

S-G + S' → S + S'-G

• Hydrolases. These bring about hydrolysis, splitting bonds with the addition of water. They act on a wide range of bond types, including ester, ether, peptide, glycosidic, carbon-carbon, carbon-halide and phosphorus-nitrogen bonds.
• Lyases. These remove groups from their substrates by routes other than hydrolysis, and in doing so they typically leave behind double bonds.

X–C–C–Y → X-Y + C=C

• Isomerases. These interconvert isomers of a molecule, handling changes between optical, geometric or positional isomers. …