Classification of Enzymes (EC Number System)

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Classification of Enzymes (EC Number System)

Enzymes are classified based on the types of reactions they catalyze. The EC (Enzyme Commission) Number System is a widely accepted method of classifying enzymes. This system was established by the International Union of Biochemistry and Molecular Biology (IUBMB) to provide a standardized way of naming and categorizing enzymes according to their specific actions.

The EC system assigns a unique EC number to each enzyme based on the type of reaction it catalyzes, and the number consists of four digits, each representing a specific level of classification.

The EC Number System

The EC number consists of four digits, which provide information about the enzyme’s class, subclass, sub-subclass, and a serial number. The structure of the EC number is as follows:

  • EC 1.x.x.x – First digit: Class (Type of reaction)
  • EC 1.1.x.x – Second digit: Subclass (Specific type of reaction)
  • EC 1.1.1.x – Third digit: Sub-subclass (More specific classification)
  • EC 1.1.1.1 – Fourth digit: Serial number (Unique identifier for the enzyme)

Enzyme Classes

The first digit in the EC number indicates the class of the enzyme, which is based on the general type of reaction the enzyme catalyzes. There are six main classes of enzymes:

  1. Oxidoreductases (EC 1.x.x.x)
    • Reaction type: Catalyze oxidation-reduction reactions, where electrons are transferred between molecules.
    • Common cofactors: NAD⁺, NADP⁺, FAD, FMN, heme groups.
    • Example: Alcohol dehydrogenase (EC 1.1.1.1), which catalyzes the oxidation of alcohol to aldehyde.
  2. Transferases (EC 2.x.x.x)
    • Reaction type: Transfer functional groups (e.g., methyl, acyl, amino groups) from one molecule to another.
    • Example: Aminotransferase (EC 2.6.1.1), which transfers an amino group between amino acids.
  3. Hydrolases (EC 3.x.x.x)
    • Reaction type: Catalyze the hydrolysis of chemical bonds by adding water, often breaking down large molecules into smaller components.
    • Example: Lipase (EC 3.1.1.3), which breaks down fats into fatty acids and glycerol.
  4. Lyases (EC 4.x.x.x)
    • Reaction type: Catalyze the breaking of chemical bonds by means other than hydrolysis or oxidation, often forming a double bond or ring structure.
    • Example: Aldolase (EC 4.1.2.13), which catalyzes the cleavage of a sugar molecule, forming a new aldehyde and a product with a double bond.
  5. Isomerases (EC 5.x.x.x)
    • Reaction type: Catalyze the rearrangement of atoms within a molecule, converting it from one isomeric form to another.
    • Example: Glucose-6-phosphate isomerase (EC 5.3.1.9), which converts glucose-6-phosphate into fructose-6-phosphate.
  6. Ligases (EC 6.x.x.x)
    • Reaction type: Catalyze the joining of two molecules, usually coupled with the hydrolysis of ATP or another nucleotide triphosphate.
    • Example: DNA ligase (EC 6.5.1.1), which joins two DNA strands by catalyzing the formation of a phosphodiester bond.

Subclasses and Sub-Subclasses

The second and third digits of the EC number provide further specificity within each enzyme class:

  • Second Digit (Subclass): Represents the type of substrate or functional group involved in the reaction.
    • For example, in EC 1.1.1.x, the second digit 1 indicates that the enzyme acts on alcohols.
    • In EC 2.6.1.x, the second digit 6 refers to aminotransferases, enzymes that transfer amino groups.
  • Third Digit (Sub-subclass): Refers to a more specific substrate or reaction type. For example:
    • In EC 1.1.1.x, the third digit specifies whether the enzyme uses NAD⁺ or NADP⁺ as a cofactor for the oxidation reaction.
  • Fourth Digit (Serial Number): A unique identifier for the enzyme within its subclass. It is assigned in order as enzymes within a subclass are discovered.

Examples of EC Numbers

Here are some examples of enzymes and their EC numbers:

  1. EC 1.1.1.1 – Alcohol dehydrogenase
    • Class 1: Oxidoreductases (electron transfer)
    • Subclass 1.1: Acting on alcohols
    • Sub-subclass 1.1.1: Using NAD⁺ or NADP⁺ as acceptor
    • Serial Number 1: Unique identifier
  2. EC 2.6.1.1 – Alanine aminotransferase (ALT)
    • Class 2: Transferases (group transfer)
    • Subclass 2.6: Aminotransferases (transfers amino groups)
    • Sub-subclass 2.6.1: Transfers an amino group to an alpha-keto acid
    • Serial Number 1: Unique identifier
  3. EC 3.1.1.3 – Lipase
    • Class 3: Hydrolases (cleavage by water)
    • Subclass 3.1: Acting on ester bonds
    • Sub-subclass 3.1.1: Phosphoric-monoester hydrolases
    • Serial Number 3: Unique identifier
  4. EC 4.1.2.13 – Aldolase
    • Class 4: Lyases (cleavage without water or oxidation)
    • Subclass 4.1: Acting on carbon-carbon bonds
    • Sub-subclass 4.1.2: Aldolase-type lyases
    • Serial Number 13: Unique identifier

The EC Number Format

An EC number is used internationally to refer to enzymes and their reactions. The full EC number for a specific enzyme provides a concise and standardized way of identifying the enzyme’s class and reaction type. For example:

  • EC 3.4.23.18: A protease that cleaves peptides at specific sites.
    • Class 3: Hydrolases
    • Subclass 3.4: Acting on peptide bonds
    • Sub-subclass 3.4.23: Endopeptidases (cleaving within the peptide chain)
    • Serial Number 18: The unique identifier.

Conclusion

The EC number system provides a systematic and comprehensive way to classify enzymes based on the types of chemical reactions they catalyze. The classification allows scientists to easily understand the type of enzymatic activity an enzyme performs, what substrate it acts on, and the type of reaction it catalyzes. The six main enzyme classes—oxidoreductases, transferases, hydrolases, lyases, isomerases, and ligases—cover a broad range of biochemical processes, and the EC system helps organize and standardize enzyme nomenclature for research, medicine, and biotechnology applications.

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