Enzyme Classification (EC Numbers)
Start with the big picture
An EC number has four parts: class, subclass, sub-subclass, and a serial identifier. Its first digit places an enzyme in one of seven major classes: oxidoreductases, transferases, hydrolases, lyases, isomerases, ligases, or translocases. The remaining digits progressively specify the affected group or bond, reaction details such as donor or acceptor, and the individual enzyme within its sub-subclass. Classification is based on enzyme activity rather than protein sequence, and EC numbers are species-independent. Because new mechanistic evidence can lead to reclassification, older numbers may remain as references. Understanding the classes helps learners interpret reaction types and use standardized enzyme annotations in research and pathway resources.
What you'll learn
- Explain the structure and purpose of a four-part EC number.
- Identify the reaction types represented by the seven major EC classes.
- Describe what each digit contributes to an EC number.
- Explain why EC classifications may be revised and how numbers support annotation.
Continue your study
Work through the complete notes and reinforce the topic with the study tools available in the full lesson.
