Enzymes with the same catalytic activity but different structures are called what?

• A. Isoenzyme
• B. Isotope
• C. Isoform
• D. Protease

Answer: (A)

Enzymes that catalyze the same chemical reaction but have different amino acid sequences and subunit structures are called isoenzymes. This difference in structure allows each isoenzyme to have its own regulatory properties, kinetic behavior, and tissue distribution, so the same reaction can be controlled differently in various parts of the body. A classic example is lactate dehydrogenase, which has several isoenzymes (often labeled LDH-1 to LDH-5) that function similarly but are optimized for the metabolic needs of different tissues. Creatine kinase and alkaline phosphatase also have tissue-specific isoenzymes. While sometimes the term isoform is used for protein variants from the same gene, isoenzyme specifically refers to enzymes that share the same catalytic activity but differ in structure.