Assertion (A) -Enzymes have active sites that hold substrate molecule for a chemical reaction.

Reason ( R) -Drugs compete with natural substrate by attaching covalently to the active site of enzyme.

  Enzyme hold the substrate for a chemical reaction. Active sites of enzymes hold the substrate molecule in a suitable position, so that it can be attacked by the reagent effectively. Substrates then bind to the active site of the enzyme though a variety of interactions such as ionic bond, hydrogen bonding, van der Waals’ interaction.