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Classification of enzymes- More than 2000 different enzymes are currently known. The commonly used names for most enzymes describe the type of reaction catalyzed, followed by the suffix -ase. For example, dehydrogenases remove hydrogen atoms, proteases hydrolyze proteins, and isomerases catalyze rearrangements in configuration. Modifiers may precede the name to indicate the substrate (xanthine oxidase), the source of the enzyme (pancreatic ribonuclease), its regulation (hormone-sensitive lipase) etc.

To address ambiguities, the International Union of Biochemists (IUB) developed an unambiguous system of enzyme nomenclature in which each enzyme has a unique name and code number that identify the type of reaction catalyzed and the substrates involved. Enzymes are grouped into six classes:

1) The oxidoreductases (class 1) catalyze the transfer of reducing equivalents(Hydrogen and electrons)from one redox system to another.

2) The transferases (class 2) catalyze the transfer of other groups from one molecule to another. Oxidoreductases and transferases  generally require coenzymes

3) The hydrolases (class 3) hydrolases cause cleavage of bond using water

4) Lyases (class 4, often also referred to as“synthases”) catalyze reactions involving either the cleavage or formation of chemical bonds, with double bonds either arising or disappearing.(See figure- reversible reaction is shown). Cleavage of bond does not require water.

5) The isomerases (class 5) move groups within a molecule, without changing the gross composition of the substrate.

6) The ligation reactions catalyzed by ligases (“synthetases,” class 6) are energy-dependent and are therefore always coupled to the hydrolysis of nucleoside triphosphates(See figure)


Each enzyme is entered in the Enzyme Catalogue with a four-digit Enzyme Commission number (EC number). The first digit indicates membership of one of the six major classes. The next two indicate subclasses and subsubclasses. The last digit indicates where the enzyme belongs in the subsubclass.

For example, The IUB name of hexokinase is ATP:D-hexose 6-phosphotransferase E.C. This name identifies hexokinase as a member of class 2 (transferases), subclass 7 (transfer of a phosphoryl group), sub-subclass 1 (alcohol is the phosphoryl acceptor), and “hexose-6” indicates that the alcohol phosphorylated is on carbon six of a hexose. However, it is still called as hexokinase.

Figure- Showing the classification of enzymes with examples of each class of enzymes


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