Competitive inhibitors are defined as which of the following?

• A. A substance that reduces the activity of an enzyme by entering the active site in place of the substrate whose structure it mimics.
• B. A molecule that binds to the enzyme away from the active site altering the conformation of the enzyme so that its active site no longer functions.
• C. A substance that degrades the enzyme.
• D. A molecule that enhances the enzyme's activity.

Answer: (A)

Competitive inhibition occurs when a molecule binds to the enzyme’s active site, competing directly with the substrate. Because the inhibitor mimics the substrate and fits into the same pocket, it blocks substrate binding. The enzyme’s maximum velocity can still be reached, but only if the substrate concentration is high enough to outcompete the inhibitor, so Vmax stays the same while the apparent Km increases. This description matches a substance that reduces activity by entering the active site in place of the substrate whose structure it mimics. Other mechanisms involve inhibitors binding elsewhere to change shape (noncompetitive), degrading the enzyme, or enhancing activity, which is not what’s described here.