Which of the following is an example of a competitive inhibitor?

• A. Antineoplastic drug methotrexate
• B. Alanine and ATP
• C. Methane
• D. Water

Answer: (A)

Competitive inhibition occurs when an inhibitor binds to the enzyme’s active site, competing with the normal substrate for binding. This means more substrate is needed to reach the same rate (Km increases) while the maximum rate (Vmax) stays the same. Methotrexate is a folate analog that mimics the natural substrate (dihydrofolate) and binds the active site of dihydrofolate reductase more tightly, blocking the real substrate from binding. That blockage prevents the enzyme from converting dihydrofolate to tetrahydrofolate, slowing nucleotide synthesis necessary for DNA replication. The other options don’t illustrate this mechanism: alanine and ATP are substrates or cofactors, while methane and water are not acting as competitive inhibitors of this enzyme.