In Michaelis-Menten kinetics, Km is defined as

• A. The substrate concentration at which the reaction rate is half of Vmax.
• B. The rate constant at high substrate.
• C. The maximum rate at saturating substrate.
• D. The energy barrier for the reaction.

Answer: (A)

Km is the substrate concentration that yields half of the maximum reaction rate. In the Michaelis-Menten equation v = Vmax[S]/(Km + [S]), setting v = Vmax/2 and solving gives [S] = Km. This makes Km a practical measure of the enzyme’s affinity for the substrate: a smaller Km means the enzyme reaches half-maximal speed at a lower substrate level, indicating tighter binding. It also explains how the rate behaves: at very low [S], the rate is roughly proportional to [S], and at very high [S], the rate plateaus at Vmax because the enzyme becomes saturated. The other descriptions don’t fit this concept: Km is not a rate constant at high substrate, not the maximum rate itself, and not an activation energy barrier.