The correct answer is this: Many of these drugs are designed to activate or inhibit ENZYMES.
Enzymes are the biological catalyst that increase the speed at which biochemical reactions take place in living things. Most enzymes are proteins. Rational drug design usually target enzymes because they take part in all biochemical reactions that occur in living organisms. By inhibiting or activating an enzyme, a drug can be designed to carry out a particular function or to have particular effects.