In Michaelis-Menten kinetics, Km is defined as what?

• A. Substrate concentration at half Vmax
• B. Maximum rate of reaction
• C. Substrate concentration at which velocity equals Vmax
• D. The enzyme concentration at half Vmax

Answer: (A)

Km is the substrate concentration that yields half of the maximum rate. In the Michaelis–Menten equation V0 = Vmax [S] / (Km + [S]), setting [S] = Km gives V0 = Vmax/2, which is why Km is defined this way. This value reflects the enzyme’s affinity for the substrate: a small Km means the enzyme reaches half-maximum velocity at a low substrate concentration (higher affinity), while a large Km means you need more substrate to get there (lower affinity). Km is intrinsic to the enzyme–substrate pair and does not depend on how much enzyme is present; Vmax, on the other hand, scales with enzyme concentration.

Why the other statements don’t fit: the maximum rate is Vmax, not Km. To reach the maximum rate, [S] would have to be effectively infinite; V0 approaches Vmax only at very high [S]. The enzyme concentration at half Vmax would change Vmax, but Km remains unchanged; Km is about affinity, not how much enzyme you have.