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Since Duclaux proposed that all enzymes should give the suffix “ase” for an easy recognition [3], the suffix “ase” has been added to the name of many enzymes according to their substrate or to a word or phrase for describing the activity. For example, glucose oxidase catalyzes the oxidation of glucose to produce gluconolactone, and cellulase catalyzes the hydrolysis of cellulose to form glucose. However, enzymes such as pepsin and trypsin have names that do not relate with their substrates or functions. Because more and more enzymes are discovered accompanied with the progress of scientific researches, the name of new enzyme may have two or more names, or two different enzymes may be given the same name. To avoid the ambiguity for naming enzymes, a systematic method for naming and classifying enzymes should be used and agreed globally.

In 1960s, the Commission on Enzyme Nomenclature was formed by International Union of Biochemistry (IUB) to classify enzymes into six major classes according to the type of reaction catalyzed as indicated in Table 1.4 [2, 9, 30]. Each of the six major classes is further divided into subclasses and subgroups.

Table 1.4 Six major classes of enzyme.

Source: Based on Armstrong [2]; Nelson and Cox [9]; Kula [30].

No.	Class	Catalytic function or reaction
1	Oxidoreductases	Transfer of electrons (hydride ions or H atoms), e.g.
2	Transferases	Group‐transfer reactions, e.g.
3	Hydrolases	Hydrolysis reactions (transfer of functional groups to water), e.g.
4	Lyases	Addition of groups to double bonds, or formation of double bonds by removal of groups, e.g.
5	Isomerases	Transfer of groups within molecules to yield isomeric forms, e.g.
6	Ligases	Formation of C–C, C–S, C–O, and C–N bonds by condensation reactions coupled to ATP cleavage, e.g.

By international agreement, the catalytic reaction is assigned and identified by a group of four‐digit number according to the enzyme classification system. For example, the enzyme catalyzes the transfer of a phosphoryl group from ATP to D‐glucose is named as ATP:glucose phosphotransferase. The enzyme is classified as

Transferase	Main class 2
Phosphotransferase	Subclass 7
Using a hydroxyl group as acceptor	Subgroup 1
D‐Glucose as the phosphoryl‐group acceptor	The serial number 1

The serial number of the last digit of an enzyme is identified by the first three entrees. Therefore, the Enzyme Commission number (E.C. number) of this enzyme is 2.7.11 denoted as E.C. 2.7.1.1. However, a trivial name, hexokinase, is more commonly used for this enzyme.