Enzyme X has kcat = 100 per second and Km = 1 mM. Enzyme Y has kcat = 200 per second and Km = 4 mM. Acting on their respective substrates at low concentrations, which statement is correct?
- A Enzyme X is twice as efficient because its kcat/Km ratio is higher ✓
- B Both enzymes have identical catalytic efficiency
- C Enzyme Y is twice as efficient because its kcat is higher
- D Efficiency cannot be compared without knowing Vmax of each enzyme
Correct answer: A. Enzyme X is twice as efficient because its kcat/Km ratio is higher
Explanation
At substrate concentrations well below Km, velocity equals (kcat/Km)[E][S], so the ratio kcat/Km is the measure of catalytic efficiency. Enzyme X gives 100/0.001 = 10 to the power 5 per M per second, while Enzyme Y gives 200/0.004 = 5 times 10 to the power 4, exactly half. Option C ignores Km entirely, and Vmax is not needed once kcat and Km are known, eliminating D.
Reference: Lehninger Principles of Biochemistry, 7th ed.
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