What is an enzyme inhibitor?
An enzyme inhibitor is a molecule that binds to an enzyme's active site, or another site on the enzyme, and blocks the enzyme's catalysis of the reaction. Enzyme inhibitors are generally specific to one enzyme and control that enzyme's activity. They also control essential enzymes such as proteases or nucleases that, if left unchecked, may damage a cell. 

Many poisons produced by animals or plants are enzyme inhibitors, and many drug molecules are enzyme inhibitors that inhibit an aberrant human enzyme or an enzyme critical for the survival of a pathogen. Since anti-pathogen inhibitors generally target only one enzyme, such drugs are highly specific and generally produce few side effects in humans. 

Medicinal enzyme inhibitors often have low dissociation constants, meaning a minute amount of the inhibitor will inhibit the enzyme. The discovery of enzyme inhibitors and their refinement are researched in biochemistry and pharmacology.