Biochemistry Module 6 Practice Exam

What does Km represent in Michaelis-Menten kinetics?

Km is the substrate concentration at half-maximal velocity; lower Km indicates higher apparent affinity. Km is the maximum rate. Km is the turnover number. Km is the substrate concentration at half-minimal velocity.

Correct Answer:

Km is the maximum rate.

Explanation:

Km tells you the substrate amount needed to push the reaction to half of its maximum speed. In Michaelis-Menten terms, v = Vmax[S] / (Km + [S]); when [S] equals Km, the velocity is Vmax/2. This makes Km a practical measure of how tightly the enzyme binds the substrate: a smaller Km means the enzyme reaches half-max at a lower substrate concentration, i.e., higher apparent affinity. It is not the maximum rate (that’s Vmax) nor the turnover number (kcat). The standard description is the substrate concentration at half-maximal velocity.

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