Which statement best describes competitive inhibition?

• A. Binds to the active site; Km increases; Vmax remains unchanged.
• B. Binds to an allosteric site; Km decreases; Vmax increases.
• C. Binds to the enzyme–substrate complex; Km decreases; Vmax decreases.
• D. Binds to the active site; Vmax decreases; Km increases.

Answer: (A)

In competitive inhibition, the inhibitor and substrate compete for the enzyme’s active site. Because they vie for the same spot, more substrate is needed to outcompete the inhibitor, so the apparent affinity for the substrate decreases, which raises Km. However, if you flood the system with substrate, the enzyme can still reach its maximum rate, so Vmax stays the same.

That’s why the statement describing binding at the active site with an increased Km and unchanged Vmax best fits competitive inhibition. Other descriptions don’t match this mechanism: binding at an allosteric site would alter activity differently and typically change Vmax; interacting with the enzyme–substrate complex or decreasing Vmax points to uncompetitive or noncompetitive forms of inhibition, not competitive.