To reduce confusion in the naming of enzymes, the Enzyme Commission (EC) of the International Union of Biochemistry and Molecular Biology (IUBMB) devised a naming and numbering system. In it, enzymes are divided into six major classes, with sub-groups and sub-subgroups to define their functions more precisely. The major classes are as follows (Table 11.7):

1. Oxidoreductases catalyze oxidation - reduction reactions.

2. Transferases catalyze transfer of functional groups from one molecule to another.

3. Hydrolases catalyze hydrolytic cleavage.

4. Lyases catalyze removal of a group from or addition of a group to a double bond, or other cleavages involving electron rearrangement.

5. Isomerases catalyze intramolecular rearrangement.

6. Ligases catalyze reactions in which two molecules are joined.

The EC of the IUBMB has given each enzyme a number with four parts, such as EC 3.4.21.5. The first three numbers define major class, subclass, and sub-subclass, respectively. The last is a serial number in the sub-subclass, indicating the order in which each enzyme is added to the list, which is continually growing. For example, triose phosphate isomerase is listed as EC 5.3.1.1. Thus, it is an isomerase and in the third subclass (enzymes that involve an oxidation in one part of the substrate molecule and reduction in another). It is in the first sub-subclass (those that interconvert aldoses and ketoses) and is the first entry (of 19 so far) in this sub-subclass.