According to the International Union of Biochemistry and Molecular Biology (IUBMB) classification, which enzyme class (EC 5) comprises isomerases?

Introduction / Context:
Enzymes are grouped into six major EC classes by the type of reaction catalyzed. Knowing these classes is useful for pathway analysis, annotation, and literature search.

Given Data / Assumptions:

• Using IUBMB EC numbering: EC 1 through EC 6.
• Focus on the reaction class names.

Concept / Approach:
EC 1: oxidoreductases; EC 2: transferases; EC 3: hydrolases; EC 4: lyases; EC 5: isomerases; EC 6: ligases (synthetases). Therefore, EC 5 corresponds to isomerases.

Step-by-Step Solution:

Verification / Alternative check:
Database entries (e.g., BRENDA, ExplorEnz) confirm EC 5 = isomerases; subclasses include racemases, epimerases, intramolecular transferases, and cis–trans isomerases.

Why Other Options Are Wrong:

• Hydrolases: EC 3.
• Oxidoreductases: EC 1.
• Lyases: EC 4.
• Ligases: EC 6.

Common Pitfalls:
Swapping EC numbers for adjacent classes (EC 4 vs EC 5) or misremembering “ligase” numbering.

Final Answer:
Isomerases (EC 5).