Enzyme Definition and Classification
Definition
An enzyme is a protein or RNA produced by living cells, which is highly specific and highly catalytic to its substrates. Enzymes are a very important type of macromolecular biological catalysts. Due to the action of enzymes, chemical reactions in organisms can also be carried out efficiently and specifically under mild conditions.
Nomenclature
The nomenclature of enzymes is derived from their substrates or the catalyzed chemical reactions, and "ase" is usually added as a suffix. Enzymes can be indexed with letters and numbers according to International Union of Biochemistry and Molecular Biology: the letter EC plus four numbers representing four elements. The first number represents enzymes that are classified according to the mechanism of enzymatic reaction.
Classification
According to the type of reactions that the enzymes catalyze, enzymes are classified into seven categories, which are oxidoreductases, transferases, hydrolases, lyases, isomerases, ligases, and translocases. Oxidoreductases, transferases and hydrolases are the most abundant forms of enzymes. Individual enzyme classes are further classified systematically based on the chemical name of the substrate and its reaction mechanism.
| Enzyme class | Reaction type | Description |
|
EC 1 Oxidoreductases |
|
Catalyze redox reaction and can be categorized into oxidase and reductase. |
|
EC 2 Transferases |
|
Catalyze the transfer or exchange of certain groups among some substrates |
|
EC 3 Hydrolases |
|
Accelerate the hydrolysis of substrates |
|
EC 4 Lyases |
|
Promote the removal of a group from the substrate to leave a double bond reaction or catalyze its reverse reaction |
|
EC 5 Isomerases |
|
Facilitate the conversion of isoisomers, geometric isomers or optical isomers. |
|
EC 6 Ligases |
|
Catalyze the synthesis of two molecular substrates into one molecular compound with the release energy |
|
EC 7 Translocases |
Catalyze the movement of ions or molecules across membranes or their separation within membranes |
According to the unified classification principle of enzymes published by the International Society of Biochemistry, each group of enzymes in the above seven categories can be further divided into several subgroups according to the characteristics of the functional groups or bonds in the substrates. In order to show the properties of substrates or reactants more accurately, each subclass is further divided into subclasses and directly contains a quantity of enzymes.
Moreover, on the basis of the molecular composition, enzymes can be divided into pure enzymes and binding enzymes. Enzymes containing only protein are called pure enzymes. Binding enzymes are composed of proteins and cofactors. Only when the two components are combined, can the enzyme have catalytic activity.
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