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Enzyme Commission (EC) Numbers

Enzyme Commission (EC) Numbers

In the early 1950’s so many new enzymes were being discovered in the burgeoning field of biochemistry that enzyme nomenclature was in danger of getting out of hand. With no guidelines on how to name enzymes, researchers simply chose their own. Some enzymes were given names, like diaphorase or Zwischenferment, that conveyed nothing about the reaction they catalyzed and enzymes with similar function were given names that implied they were different. It was all very confusing and clearly if the situation continued it would be disastrous for our attempts to study enzymes.

In 1955 the International Union of Biochemistry stepped in to clear up the whole mess, by setting up an International Commission on Enzymes, with the mission ‘To consider the classification and nomenclature of enzymes and coenzymes….’.

Their solution was a system where enzymes are classified numerically, based on the chemical reactions they catalyze. Each enzyme has a set of four numbers, called an EC number that define the classes and sub-classes of enzyme it belongs to, each number giving a more specific definition than the last. For example, the restriction endonuclease EcoRI has the EC number EC, defining it as a member of the following groups of enzymes:

  • 3. Hydrolases (Enzymes that speed up hydrolysis – the breaking of a molecule using water)
  • 1. Hydrolases that act on ester bonds
  • 21. Endodeoxyribonucleases producing 5′-phosphomonoesters
  • 4. type II site-specific deoxyribonucleases

Note that although does not uniquely define EcoRI – all type II restriction enzymes have this EC number, but the EC number is an easy and systematic way to refer to an enzyme in a way that it’s function, and its functional relationship to other enzymes, is obvious.

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