In competitive inhibition, what happens to the apparent Km?

• A. It increases
• B. It decreases
• C. It remains unchanged
• D. It becomes zero

Answer: (A)

In competitive inhibition, the inhibitor and the substrate compete for the same active site on the enzyme. This competition makes the substrate seem less able to bind, so the enzyme appears to have a lower affinity for the substrate. That shows up as an increase in the apparent Michaelis constant: Km,app becomes larger (Km,app = Km × [1 + (I/Ki)]). However, the maximum rate the enzyme can achieve, Vmax, doesn't change because if you flood the system with substrate, the substrate outcompetes the inhibitor and the enzyme can reach the same maximal velocity as in the absence of inhibitor. On a plot of reaction rate versus substrate concentration, lines with inhibitor show the same Vmax but require higher substrate concentration to reach half-max velocity, reflecting the increased Km.