During competitive inhibition, what happens to Km and Vmax?

• A. Km increases and Vmax remains the same
• B. Vmax decreases
• C. Km stays the same; Vmax increases
• D. Km increases; Vmax increases

Answer: (A)

In competitive inhibition, the inhibitor binds to the enzyme’s active site, blocking substrate binding, but this blocking is reversible and can be overcome by high levels of substrate. Because you can outcompete the inhibitor by increasing substrate concentration, the maximum rate (Vmax) you can achieve at saturating substrate is unchanged. However, the presence of the competitor means you need more substrate to reach half of that maximum rate, so the apparent Km increases. In other words, the inhibitor lowers apparent affinity (needs more substrate for the same velocity) without altering the maximum speed the enzyme can achieve when substrate is abundant.