2.1.2: Enzyme Commission Number

Introduction

Enzymes are biological catalysts that speed up chemical reactions in living organisms. Scientists needed a way to organize the growing list of enzymes, leading to the creation of the Enzyme Commission (EC) number system. This system works like the Dewey Decimal System or the Library of Congress classification, but for enzymes. Each enzyme is assigned a unique four-part EC number based on the type of reaction it catalyzes.

The system, developed by the International Union of Biochemistry and Molecular Biology (IUBMB), provides a standardized way to classify and study enzymes.

The Seven Major Classes of the Enzyme Commission

EC Number	Class Name	Reaction Catalyzed	Example Reaction	Enzyme Examples (Trivial Names)	Enzyme Count (as of March 2025)
EC 1	Oxidoreductases	Catalyze oxidation-reduction (redox) reactions by transferring electrons or hydrogen atoms.	\( \text{AH} + \text{B} \rightarrow \text{A} + \text{BH} \) (reduced) \( 2\text{A} + \text{O}_2 \rightarrow 2\text{AO} \) (oxidized)	dehydrogenase, oxidase	2,010
EC 2	Transferases	Transfer functional groups (methyl, acyl, amino, phosphate, etc.) between molecules.	\( \text{AB} + \text{C} \rightarrow \text{A} + \text{BC} \)	transaminase, kinase	2,069
EC 3	Hydrolases	Break chemical bonds by adding water (hydrolysis).	\( \text{AB} + \text{H}_2\text{O} \rightarrow \text{AOH} + \text{BH} \)	lipase, amylase, peptidase, phosphatase	1,357
EC 4	Lyases	Break bonds without hydrolysis or oxidation, often forming double bonds.	\( \text{X-A} + \text{B-Y} \rightarrow \text{A=B} + \text{X-Y} \)	decarboxylase	773
EC 5	Isomerases	Rearrange atoms within a molecule without changing its molecular formula.	\( \text{ABC} \rightarrow \text{BCA} \)	isomerase, mutase	320
EC 6	Ligases	Join two molecules by forming new bonds (C-O, C-S, C-N, or C-C) with ATP hydrolysis.	\( \text{X} + \text{Y} + \text{ATP} \rightarrow \text{XY} + \text{ADP} + \text{P}_i \)	synthetase	249
EC 7	Translocases	Move ions or molecules across membranes.	—	transporter	98

The Six Major Classes of Enzyme Reactions

EC 1: Oxidoreductases

Oxidoreductases catalyze oxidation-reduction (redox) reactions by transferring electrons or hydrogen atoms from one molecule to another. In these reactions, at least one substrate is oxidized while another is reduced. These enzymes are crucial in metabolic pathways such as cellular respiration.

Common oxidoreductases include dehydrogenases, reductases, and oxidases. A well-known example is lactate dehydrogenase, which plays a key role in glycolysis by converting pyruvate into lactate.

Example Reaction: Lactate Dehydrogenase

Pyruvate is reduced to lactate while NADH is oxidized to NAD⁺:

\[ \text{Pyruvate} + \text{NADH} + H^+ \rightleftharpoons \text{Lactate} + \text{NAD}^+ \]

EC 2: Transferases

Transferases move functional groups, such as methyl, acyl, amino, or phosphate groups, from one molecule to another. These enzymes often require cofactors that act as donors.

One important subclass of transferases is the kinases, which transfer phosphate groups. For example, hexokinase phosphorylates glucose, enabling it to enter metabolic pathways like glycolysis.

Example Reaction: Hexokinase

Glucose is phosphorylated using ATP:

\[ \text{Glucose} + \text{ATP} \rightarrow \text{Glucose-6-phosphate} + \text{ADP} \]

EC 3: Hydrolases

Hydrolases break chemical bonds by adding water in a process called hydrolysis. They typically act on C-O, C-N, and C-S bonds, breaking down larger molecules into smaller components.

Hydrolases are widely involved in digestion and metabolism. A key example is trypsin, a proteolytic enzyme that breaks down proteins in the digestive system.

Example Reaction: Dipeptide Hydrolysis

Alanine dipeptide hydrolysis into two alanine molecules:

\[ \text{Ala-Ala} + H_2O \rightarrow \text{Ala} + \text{Ala} \]

EC 4: Lyases

Lyases catalyze the removal or addition of functional groups to form or break double bonds, without using hydrolysis or oxidation. These enzymes can break bonds such as C=C, C=O, C=N, and C=S.

An example of a lyase is aldolase, which is essential in glycolysis for breaking down fructose-1,6-bisphosphate into smaller sugar molecules.

Example Reaction: Aldolase

Cleavage of fructose-1,6-bisphosphate into two three-carbon sugars:

\[ \text{Fructose-1,6-bisphosphate} \rightarrow \text{Glyceraldehyde-3-phosphate} + \text{Dihydroxyacetone phosphate} \]

EC 5: Isomerases

Isomerases rearrange atoms within a molecule to form an isomer. The chemical formula remains unchanged, but the structure is altered, allowing for different biochemical functions.

A common example is glucose-6-phosphate isomerase, which converts glucose-6-phosphate into fructose-6-phosphate in the second step of glycolysis.

Example Reaction: Glucose-6-Phosphate Isomerase

Conversion of glucose-6-phosphate to fructose-6-phosphate:

\[ \text{Glucose-6-phosphate} \rightleftharpoons \text{Fructose-6-phosphate} \]

EC 6: Ligases

Ligases catalyze the joining of two molecules by forming new chemical bonds, often powered by ATP hydrolysis. They are responsible for the formation of carbon-carbon, carbon-nitrogen, carbon-oxygen, and carbon-sulfur bonds.

An essential ligase is DNA ligase, which is crucial in DNA replication and repair, joining broken DNA strands together.

Example Reaction: Pyruvate Carboxylase

Formation of oxaloacetate from pyruvate:

\[ \text{Pyruvate} + \text{CO}_2 + \text{ATP} \rightarrow \text{Oxaloacetate} + \text{ADP} + \text{P}_i \]

EC 7: Translocases – An Extension to the System

In 2018, the Enzyme Commission introduced EC 7: Translocases, a new category for enzymes that facilitate the movement of molecules, ions, or macromolecules across membranes or within cellular compartments. Unlike other enzyme classes, translocases do not chemically modify their substrates. Instead, they catalyze translocation, often utilizing energy sources like ATP hydrolysis or ion gradients to drive transport.

The classification of translocases follows the general principles of enzyme nomenclature set by the Enzyme Commission:

• Only single catalytic entities should be classified as enzymes. Multi-enzyme systems that participate in transport should include “system” in their names (e.g., proton pump system).
• Translocases should not be confused with enzymes that modify their substrates. Unlike oxidoreductases (EC 1) or transferases (EC 2), translocases move substances without altering their chemical structure.
• Systematic names should reflect the molecule transported and the driving energy source. Examples include ATP-driven ion pumps and symporters using electrochemical gradients.

As research progresses, new translocases continue to be discovered and classified. Major enzyme databases, including Expasy, now recognize EC 7 translocases as an established extension to enzyme classification.

Understanding the EC Numbering System

The Enzyme Commission (EC) numbering system organizes enzymes into a hierarchy based on the reactions they catalyze. Each enzyme is assigned a unique four-part EC number (e.g., EC 3.4.11.4), where each level provides more specificity. Let’s explore how this system is structured.

Four Parts of the EC Number

Each enzyme’s EC number consists of four parts, separated by periods:

EC Number Component	Meaning	Description
First digit	Main enzyme class (EC X)	Indicates the broad category of reaction catalyzed (e.g., oxidation-reduction, transfer, hydrolysis).
Second digit	Subclass (EC X.Y)	Specifies the type of reaction within the main class, often based on the type of bond or chemical group involved.
Third digit	Sub-subclass (EC X.Y.Z)	Further defines the reaction mechanism or the specific type of chemical transformation.
Fourth digit	Enzyme identifier (EC X.Y.Z.W)	Uniquely identifies the specific enzyme that catalyzes a reaction within the given subclass.

Let's examine the classification of a specific enzyme, EC 3.4.11.4, to see how it fits into the system:

• EC 3 - Hydrolases (enzymes that use water to break bonds)
• EC 3.4 - Hydrolases that act on peptide bonds
• EC 3.4.11 - Peptidases that cleave the first amino acid from a polypeptide
• EC 3.4.11.4 - An enzyme that removes the first amino acid from a tripeptide

Example 1: Exploring an Entire Class (EC 2 - Transferases)

Now, let’s look at an entire enzyme class, EC 2 (Transferases), which move functional groups between molecules. This class includes many subclasses, each defined by the type of group being transferred:

EC Number	Subclass Name	Description
EC 2.1	Transferring One-Carbon Groups	Transfers single-carbon units such as methyl groups
EC 2.2	Transferring Aldehyde or Ketone Groups	Transfers aldehyde or ketone groups between molecules
EC 2.3	Acyltransferases	Transfers acyl groups (e.g., acetyl, fatty acyl)
EC 2.4	Glycosyltransferases	Transfers sugar (glycosyl) groups
EC 2.5	Transferring Alkyl or Aryl Groups	Transfers alkyl or aryl groups (except methyl groups)
EC 2.6	Transferring Nitrogenous Groups	Transfers amino or related nitrogen-containing groups
EC 2.7	Transferring Phosphorus-Containing Groups	Transfers phosphate groups (e.g., kinases)
EC 2.8	Transferring Sulfur-Containing Groups	Transfers sulfur-based functional groups
EC 2.9	Transferring Selenium-Containing Groups	Transfers selenium-based functional groups

Example 2: A Deep Dive into a Subclass (EC 6.3 - Carbon-Nitrogen Bond Formation)

Finally, let’s zoom into a specific subclass, EC 6.3 (Ligases that form carbon-nitrogen bonds). This subclass includes several enzyme sub-subclasses:

EC Number	Sub-subclass Name	Description
EC 6.3.1	Acid-Ammonia (or Amine) Ligases (Amide Synthases)	Form amide bonds using ammonia or amines
EC 6.3.2	Acid-Amino-Acid Ligases (Peptide Synthases)	Form peptide bonds between amino acids
EC 6.3.3	Cyclo-Ligases	Form cyclic molecules via carbon-nitrogen bonds
EC 6.3.4	Other Carbon-Nitrogen Ligases	Form C-N bonds but don’t fit into other categories
EC 6.3.5	Carbon-Nitrogen Ligases with Glutamine as Amido-N-Donor	Use glutamine as a nitrogen donor for C-N bond formation

History of the EC Number System

The Road to Systematic Naming

• Early enzyme names were inconsistent, often based on the substrate or reaction type (e.g., urease, maltase).
• By the 1950s, biochemists realized the need for a standardized classification system.
• In 1956, the International Commission on Enzymes was formed, and in 1961, the first edition of the EC system was published.

Growth of the EC System Over Time

Year	Publication	Number of Enzymes Classified
1961	Report of the Enzyme Commission	712
1964	Enzyme Nomenclature	875
1972	Enzyme Nomenclature	1,770
1978	Enzyme Nomenclature	2,122
1984	Enzyme Nomenclature	2,477
1992	Enzyme Nomenclature	3,196
2013	—	5,212
2020	—	6,520
2023	—	6,754
2025	—	6,876

As research expands, new enzymes continue to be classified, with the list regularly updated by the IUBMB.

Additional Resources

For up-to-date enzyme classifications, visit:

• IUBMB Enzyme Database – Official classification list.
• Expasy Enzyme Database – Searchable enzyme database.
• The Enzyme Database – Comprehensive enzyme classification and nomenclature.
• Wikipedia - EC Number – General overview.