What is a competitive inhibitor?

• A. A molecule that binds to the enzyme-substrate complex after binding has occurred.
• B. A molecule that binds to the enzyme's allosteric site, changing shape and reducing activity.
• C. A molecule that increases enzyme turnover by stabilizing the transition state.
• D. A molecule that binds to the enzyme's active site, blocking substrate binding and reducing reaction rate.

Answer: (D)

Competitive inhibition happens when a molecule blocks the enzyme’s active site by competing with the substrate for binding. It often looks like the substrate and fits into the same pocket, so it can prevent substrate binding. This type of inhibition is reversible and can be overcome by increasing substrate concentration, which allows the enzyme to bind substrate again and reach its catalytic potential. Because the inhibitor does not affect the catalytic steps once the substrate is bound, the maximum rate (Vmax) stays the same, but the apparent affinity for the substrate decreases: you need a higher substrate concentration to reach half of Vmax, so Km appears increased. The other scenarios involve inhibitors binding elsewhere on the enzyme or stabilizing the transition state to enhance activity, which are not competitive inhibition.