In Michaelis-Menten kinetics, what does Km represent?

• A. The maximum velocity of the reaction
• B. The substrate concentration at which velocity is half of Vmax
• C. The enzyme concentration
• D. The rate constant for the slow step

Answer: (B)

The key idea is what Km tells you about how much substrate is needed to push the enzyme toward its fastest rate. In Michaelis–Menten kinetics, the velocity is given by v = Vmax[S] / (Km + [S]). If you plug in a substrate concentration equal to Km, the equation becomes v = Vmax/2. So Km is defined as the substrate concentration at which the reaction runs at half of its maximum rate.

Km also relates to how tightly the enzyme binds the substrate. A smaller Km means you only need a lower [S] to reach half-max velocity, which implies stronger binding or greater affinity in the sense of forming the ES complex efficiently. But Km is a composite kinetic parameter, not just a simple binding constant; it combines the rates of ES formation and breakdown with the catalytic step.

This differs from the maximum velocity, which is the fastest rate the system can achieve when the enzyme is saturated with substrate; it’s also different from the enzyme concentration, which scales how much enzyme is present. It’s not the rate constant for the slow step either—that would be a specific step’s rate constant (for example, kcat for turnover), whereas Km summarizes the overall balance that determines half-max performance.