It is the first letter of each class. Each class in turn is subdivided into many classes, which are further divided. A four-digit enzyme commission (E.C) number is assigned to each enzyme representing class (first digit shows which of six main classes in OTHLIL), subclass (second digit), sub-sub class (third digit), and individual enzyme (fourth…

Enzymes catalysing the synthetic reactions, where two molecules are joined together and ATP is used. A + B → A − B

Enzymes involved in all the isomerisation reactions. Inter-conversion of isomers: A → A′

Enzymes specialised in the addition or removal of water, ammonia, CO2, and so on. A − B + X − Y → AX − BYAddition → Elimination

Enzymes that bring about hydrolysis of various compound. A − B + H2O → AH − B − OH

Enzymes that catalyse the transfer of functional group. A − X + B → A + B − X

Enzymes involved in oxidation–reduction reactions. AH2 + B → A + BH2

In 1964, the IUB of enzyme classified into six major groups. Each class on its own represents the general type of reaction brought about by the enzymes of that class.

Based on their chemical composition, the enzymes have been classified into three categories:

Few enzymes have been named based on the name of the substance synthesised. Example: Rhodoase.