The International System Classification of Enzymes proposed by the Enzymology Committee of the International Society of Biochemistry divides all known enzymes into six categories according to the types of reactions they catalyze, which are represented by numbers 1, 2, 3, 4, 5, and 6, respectively. Next, let's take a closer look at the enzyme classification of these six different enzymes.

1. Oxidoreductases
All enzymes that can catalyze the oxidation-reduction reaction of a substrate are called oxidoreductases. In organic reactions, dehydrogenation and oxygenation are usually regarded as oxidation, and hydrodeoxygenation is regarded as reduction. Such enzymes include dehydrogenases, oxygenases (oxygenases), oxidases (oxidases), reductases (reductase), peroxidases (peroxidases), etc., among which dehydrogenases are the most numerous. The reaction catalyzed by dehydrogenase can be expressed by the general formula:
AH2 + B →A + BH2
AH2 represents the substrate, and B is the original hydrogen acceptor. In the dehydrogenation reaction, it is the coenzyme (base) that directly obtains hydrogen atoms from the substrate. After the coenzyme (base) obtains the hydrogen atom from the substrate, it undergoes a certain transfer process, and finally it is combined with oxygen to form water.

The reaction catalyzed by oxidase can be expressed as:
AH2 + O2→A + H2O
In this type of reaction, the hydrogen atoms removed from the substrate molecule directly react with oxygen to form water without being transferred. Most of the reactions catalyzed by oxidase are irreversible.

2. Transferases
All enzymes that can catalyze the transfer or exchange of groups of substrates are called transferases. According to the different types of groups transferred, the common transferases are aminotransferases, transmethylases, acyltransferases, kinases and phosphorylases. The reaction catalyzed by the transferase can be expressed by the general formula:
A-R + B →A + B-R
In the above formula, R is the group to be transferred.

Many transferases are binding proteins, and the transferred group is first combined with the coenzyme, and then transferred to another receptor. For example, the coenzyme of aminotransferase is pyridoxal phosphate. In the process of transamination, the transferred amino group first combines with pyridoxal phosphate to form pyridoxamine phosphate, and then pyridoxamine phosphate transfers this amino group to another substance.

3. Hydrolases
All enzymes that can catalyze the hydrolysis of the substrate are called hydrolases. Common hydrolases include amylase, maltase, protease, peptidase, esterases and phosphatase. The general formula of the enzymatic reaction of this kind of enzyme is expressed as:
A-B + H2O → AH + BOH
Extracellular hydrolases are simple proteases. Most of the reactions catalyzed by hydrolases are irreversible.

4. Lyase
Any enzyme that can catalyze the cleavage of the C-C (or C-O, C-N, etc.) chemical bond in a substrate molecule, and the conversion of one molecule of the substrate into two molecules of products after the cleavage, are called lyases. The general formula of the enzymatic reaction of this type of enzyme is:
A-B → A + B
Most of the reactions catalyzed by these enzymes are reversible. The reaction proceeding from left to right is a cleavage reaction, and from right to left is a synthesis reaction.

Aldolase (aldolases) is a very important enzyme in the process of sugar metabolism. It is widely present in various biological cells and is a relatively common lyase. It catalyzes the cleavage of fructose 1,6-diphosphate into glyceraldehyde phosphate and dihydroxyacetone phosphate. In addition, common lyases include decarboxylases, citrate lyase, dehydratases, and deaminase.

5. Isomerase
Isomerases can catalyze the geometric or structural isomeric changes of substrate molecules. Geometrical changes include cis-trans isomerism, epimerism, and molecular configuration changes; structural changes include intramolecular group variation and intramolecular redox. Common isomerases are cistrans-isomerases, epimerase, mutase and racemases. The general formula of the enzymatic reaction is:
A → B
The reactions catalyzed by isomerases are all reversible. The isomerases in glycolysis include glucose phosphate mutase, triose phosphate isomerase and phosphoglycerate mutase

6. Ligases
Ligases is a type of enzyme that catalyzes the connection of two molecules together and is accompanied by the breaking of the high-energy phosphate bond in the ATP molecule. The general formula of the enzymatic reaction can be expressed as:
A + B +ATP→ A-B + ADP + Pi
or A + B + ATP→ A-B + AMP + PPi
Most of these reactions are irreversible. Pi or PPi in the reaction formula represent inorganic phosphoric acid and pyrophosphoric acid, respectively. ATP (or GTP, etc.) must be involved in the reaction. Common synthetic enzymes such as pyruvate carboxylase, glutamine synthetase, glutathione synthetase, etc.

Enzyme Naming System
In each major class of enzymes, according to the nature of the group or bond in the substrate molecule is divided into several sub-categories, and each sub-category is further divided into several sub-subcategories. Then arrange the enzymes belonging to this sub-subclass in order. In this way, all the enzymes are sorted into a table called enzyme table. The position of each enzyme in the table can be represented by a uniform number. This number includes 4 numbers. It is preceded by E C (abbreviation of International Enzymology Committee).

For example, lactate dehydrogenase (E C catalyzes the dehydrogenation of lactate to produce pyruvate. In its number:
The first 1 means that the enzyme belongs to the first category, that is, oxidoreductases.
The second 1 means that the enzyme belongs to the first subclass of oxidoreductases and catalyzes the oxidation of alcohol.
The third 1 indicates that the enzyme belongs to the first subclass of the first subclass of oxidoreductases; the hydrogen acceptor is NAD+; "27" indicates the sequence number of lactate dehydrogenase in this subclass.

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