How Enzymes are Named and Classified

When scientists first started discovering enzymes, they faced a simple problem: what should we call these remarkable biological catalysts? Early researchers took an intuitive approach-they named enzymes based on what they did or what they acted upon, often adding the suffix “-ase” to create names like lipase (acts on lipids) or maltase (acts on maltose). While this system worked for a handful of enzymes, it quickly became inadequate as more enzymes were discovered and the field of biochemistry expanded rapidly.

The chaos of early enzyme nomenclature 🔗

By the late 1950s, enzyme nomenclature had become problematic. Without any guiding authority, the same enzyme often received multiple names from different researchers, while identical names were sometimes applied to completely different enzymes. Some names gave no indication of the reaction catalyzed, and similar names were used for enzymes with entirely different functions. This confusion made scientific communication difficult and hindered progress in the field.

The early naming conventions fell into several patterns. Some enzymes retained names ending in “-in” from their discovery period, such as pepsin, trypsin, and ptyalin. Others were named after their source-papain from papaya, bromelain from pineapple. Still others followed the newer “-ase” convention, but inconsistently. The result was a patchwork system that served neither precision nor practicality.

The birth of systematic enzyme classification 🔗

Recognizing this problem, the International Union of Biochemistry (IUB) established an International Commission on Enzymes in 1956, following a decision at the third International Congress of Biochemistry in Brussels. This commission worked for five years, circulating 52 formal documents and consulting with experts worldwide. In 1961, they presented their comprehensive report at the General Assembly in Moscow, introducing a systematic classification that would revolutionize enzyme nomenclature.

The system introduced two key innovations: systematic names that precisely described enzyme function, and a numerical coding system-the EC (Enzyme Commission) number-that uniquely identified each enzyme based on the reaction it catalyzes rather than its structure or source.

Understanding the EC number system 🔗

The EC number consists of four digits separated by periods, with each position conveying specific information. The first digit indicates which of the main enzyme classes the enzyme belongs to. The second digit specifies the subclass, typically describing the type of chemical group involved in the reaction. The third digit provides further details about the substrate or reaction mechanism. The fourth digit is simply a serial number identifying the specific enzyme within its sub-subclass.

For example, hexokinase has the EC number 2.7.1.1. The “2” indicates it’s a transferase, “7” shows it transfers phosphate groups, “1” specifies that an alcohol group receives the phosphate, and “1” is its unique identifier within that category. This systematic approach means that anyone familiar with the system can understand an enzyme’s basic function just from its number.

The six main enzyme classes 🔗

The classification system organizes enzymes into six primary categories based on the type of chemical reaction they catalyze. A seventh class, translocases, was added in 2018 to include enzymes that move molecules across membranes, but the original six remain the foundation of enzyme classification.

Class 1: Oxidoreductases 🔗

Oxidoreductases catalyze oxidation-reduction reactions, where electrons or hydrogen atoms are transferred from one molecule to another. The substrate that loses electrons (or hydrogen) is considered the donor, while the molecule accepting them is the acceptor. These enzymes include dehydrogenases, which transfer hydrogen to coenzymes like NAD+ or NADP+, oxidases that use oxygen as the final electron acceptor, and reductases that catalyze reduction reactions. Common examples include alcohol dehydrogenase and cytochrome oxidase.

Class 2: Transferases 🔗

Transferases move functional groups from one molecule (the donor) to another (the acceptor). These groups can include methyl groups, acyl groups, phosphate groups, or amino groups. Kinases, a important subgroup of transferases, transfer phosphate groups from ATP to various substrates, playing crucial roles in cellular signaling and metabolism. Transaminases, which move amino groups between molecules, are also classified here despite their complex mechanisms.

Class 3: Hydrolases 🔗

Hydrolases catalyze the cleavage of bonds through the addition of water molecules. This class includes many digestive enzymes that break down large molecules into smaller units. Esterases hydrolyze ester bonds, glycosidases break glycosidic linkages in carbohydrates, and peptidases cleave peptide bonds in proteins. Enzymes in this class often have names that simply add “-ase” to their substrate name-for instance, lipase for lipids or protease for proteins.

Class 4: Lyases 🔗

Lyases cleave carbon-carbon, carbon-oxygen, carbon-nitrogen, and other bonds without using water or oxidation. Instead, they create double bonds or ring structures, or conversely, add groups to double bonds. Common names for these enzymes include decarboxylases (removing CO2), dehydratases (removing water), and aldolases. When the reverse reaction is emphasized, these enzymes may be called synthases.

Class 5: Isomerases 🔗

Isomerases catalyze structural or geometric rearrangements within a single molecule. They convert compounds into their isomers without adding or removing atoms. This class includes racemases (which invert configuration at a single asymmetric center), epimerases (which invert configuration at one of several asymmetric centers), and mutases (which transfer a group from one position to another within the same molecule). These enzymes are essential for metabolic pathways that require specific molecular configurations.

Class 6: Ligases 🔗

Ligases join two molecules together, coupling this synthesis with the breakdown of ATP or another energy-rich molecule. The systematic names follow the pattern “X:Y ligase (ADP-forming)” to indicate both the molecules being joined and the energy source. These enzymes are crucial for biosynthetic reactions, including DNA replication and repair. DNA ligase, for instance, seals breaks in DNA strands by forming phosphodiester bonds.

The practical value of systematic classification 🔗

This classification system does more than organize knowledge-it facilitates clear communication across languages and disciplines. When researchers discover a new enzyme, they can assign it a systematic name following established rules, though only the official Nomenclature Committee assigns EC numbers. The system also acknowledges that enzymes from different organisms catalyzing the same reaction receive the same classification, recognizing that function matters more than source or structure.

The International Union of Biochemistry and Molecular Biology (IUBMB) continuously maintains and updates this classification system through its ExplorEnz database, which serves as the primary resource for enzyme nomenclature. As new enzymes are characterized and our understanding of biochemical reactions deepens, the system evolves while maintaining its foundational principles.

What do you think? How has the systematic classification of enzymes improved your understanding of their functions? Can you identify which enzyme class would be most relevant to your area of food science research?

References

1. https://iubmb.qmul.ac.uk/enzyme/history.html
2. https://iubmb.qmul.ac.uk/enzyme/rules.html
3. https://www.ncbi.nlm.nih.gov/books/NBK554481/
4. https://pubmed.ncbi.nlm.nih.gov/34773359/
5. https://www.creative-enzymes.com/resource/types-of-enzymes-and-their-biological-functions_187.html
6. https://pmc.ncbi.nlm.nih.gov/articles/PMC2686581/