Michaelis constant (Km) is defined as which of the following?

Michaelis constant (Km) is defined as which of the following?

• A. Substrate concentration at half max velocity
• B. Maximum velocity
• C. Rate with no substrate
• D. Temperature coefficient

Answer: (A)

Km tells you the amount of substrate required for the enzyme to work at half of its maximum possible rate. In the Michaelis-Menten relationship, velocity v equals Vmax[S] divided by (Km plus [S]); when the substrate concentration [S] equals Km, the rate is exactly half of Vmax. This makes Km a reflection of how tightly the enzyme binds its substrate: a lower Km means the enzyme reaches half-max at a lower substrate level, indicating higher affinity, while a higher Km means you need more substrate to get to half-max. The other ideas don’t describe Km: the maximum velocity is Vmax, not Km; the rate with no substrate is typically zero under standard Michaelis-Menten assumptions; and the temperature coefficient refers to how reaction rate changes with temperature, not the substrate-binding properties captured by Km.

Km tells you the amount of substrate required for the enzyme to work at half of its maximum possible rate. In the Michaelis-Menten relationship, velocity v equals Vmax[S] divided by (Km plus [S]); when the substrate concentration [S] equals Km, the rate is exactly half of Vmax. This makes Km a reflection of how tightly the enzyme binds its substrate: a lower Km means the enzyme reaches half-max at a lower substrate level, indicating higher affinity, while a higher Km means you need more substrate to get to half-max.

The other ideas don’t describe Km: the maximum velocity is Vmax, not Km; the rate with no substrate is typically zero under standard Michaelis-Menten assumptions; and the temperature coefficient refers to how reaction rate changes with temperature, not the substrate-binding properties captured by Km.