What term describes the substrate concentration at which an enzyme-catalyzed reaction reaches half of its maximum velocity (Vmax)?

• A. Affinity
• B. Concentration
• C. Substrate Power
• D. Michaelis-Menten Constant

Answer: (D)

The main idea here is how enzyme speed relates to substrate amount in Michaelis-Menten kinetics. The substrate concentration at which the reaction proceeds at half of its maximum rate is defined as the Michaelis-Menten constant, Km.

This comes from the equation v = (Vmax [S])/(Km + [S]). When the velocity is half of Vmax, you can substitute and simplify to find [S] = Km. So Km is literally the [S] needed to reach half of the enzyme’s full catalytic rate.

Interpreting Km, a smaller Km means the enzyme achieves half-maximal speed with less substrate, indicating higher affinity for the substrate. A larger Km means lower affinity. At very high substrate levels, the reaction runs near Vmax because the enzyme becomes saturated with substrate. In practical contexts, certain factors like inhibitors can alter the apparent Km (without changing Vmax in competitive cases), shifting how readily the enzyme reaches half-max speed.

So the term that matches the description is the Michaelis-Menten constant, Km.