Km vs. kcat vs. catalytic efficiency
Enzyme X has Km = 1 μM and kcat = 100 s⁻¹. Enzyme Y has Km = 10 μM and kcat = 500 s⁻¹. Which statement is correct?
Answer B: explanation for every choice
A
A larger Km means lower apparent affinity: more substrate is needed to reach half of Vmax.
B
Correct. X has the lower Km, so higher affinity. Catalytic efficiency is kcat/Km: X = 100 / (1 × 10⁻⁶ M) = 1 × 10⁸ M⁻¹s⁻¹, and Y = 500 / (1 × 10⁻⁵ M) = 5 × 10⁷ M⁻¹s⁻¹.
C
kcat alone is turnover at saturating substrate. Efficiency at low substrate concentration is kcat/Km, which is lower for Y.
D
The ratios differ by a factor of 2: 1 × 10⁸ versus 5 × 10⁷ M⁻¹s⁻¹.
TakeawayLow Km = high affinity. kcat = turnover. kcat/Km = efficiency.
Equation usedTurnover number and catalytic efficiency
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