APPENDIX: Enzymes are Classified on the Basis of the Types of Reactions That They Catalyze

Many enzymes have common names that provide little information about the reactions that they catalyze. For example, a proteolytic enzyme secreted by the pancreas is called trypsin. Most other enzymes are named for their substrates and for the reactions that they catalyze, with the suffix “ase” added. Thus, a peptide hydrolase is an enzyme that hydrolyzes peptide bonds, whereas ATP synthase is an enzyme that synthesizes ATP.

To bring some consistency to the classification of enzymes, in 1964 the International Union of Biochemistry established an Enzyme Commission to develop a nomenclature for enzymes. Reactions were divided into six major groups numbered 1 through 6 (Table 8.8). These groups were subdivided and further subdivided so that a four-number code preceded by the letters EC for Enzyme Commission could precisely identify all enzymes.

Class

Type of reaction

Example

Chapter

1. Oxidoreductases

Oxidation–reduction

Lactate dehydrogenase

16

2. Transferases

Group transfer

Nucleoside monophosphate kinase (NMP kinase)

  9

3. Hydrolases

Hydrolysis reactions (transfer of functional groups to water)

Chymotrypsin

  9

4. Lyases

Addition or removal of groups to form double bonds

Fumarase

17

5. Isomerases

Isomerization (intramolecular group transfer)

Triose phosphate isomerase

16

6. Ligases

Ligation of two substrates at the expense of ATP hydrolysis

Aminoacyl-tRNA synthetase

30

Table 8.8: Six major classes of enzymes

Consider as an example nucleoside monophosphate (NMP) kinase, an enzyme that we will examine in detail in Section 9.4. It catalyzes the following reaction:

NMP kinase transfers a phosphoryl group from ATP to any nucleotide monophosphate (NMP) to form a nucleoside diphosphate (NDP) and ADP. Consequently, it is a transferase, or member of group 2. Many groups other than phosphoryl groups, such as sugars and single-carbon units, can be transferred. Transferases that shift a phosphoryl group are designated 2.7. Various functional groups can accept the phosphoryl group. If a phosphate is the acceptor, the transferase is designated 2.7.4. The final number designates the acceptor more precisely. In regard to NMP kinase, a nucleoside monophosphate is the acceptor, and the enzyme’s designation is EC 2.7.4.4. Although the common names are used routinely, the classification number is used when the precise identity of the enzyme is not clear from the common name alone.