What does Km represent in Michaelis-Menten kinetics?

What does Km represent in Michaelis-Menten kinetics?

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

Answer: (B)

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.

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.