Enzyme molecules normally interact with substrate molecules. Some medicines work by blocking enzyme activity in pathogens. These medicines are effective because:

A. They are the same size as the enzyme
B. They are the same size as the substrate molecules
C. Have a shape that fits in the enzyme
D. Have a shape that fits into all cell receptors

Every enzyme has a special part that is called active site. This active site is responsible for all the enzymatic activity and its shape is such that only some substrates can bind with it (Please see figure).

This exactly is the mechanism through which some medicines work. They have a shape that matches with the active site of the enzyme and thus can bind to it. When these medicines bind with the active site of the pathogen’s enzyme, they decrease enzyme’s activity instead of increasing. The medicines in this case are called as inhibitors, because they inhibit the enzyme’s function.

The quality of a medicine is often assessed by its specificity for the enzyme and its quantity needed to inhibit the enzyme. If its shape is perfectly matching with active site and very small quantity is required to inhibit the enzyme activity, it means that medicine has low side effects and more positive effects.

Many herbicides and pesticides used for crop improvement work on this mechanism.