An enzyme inhibitor is a molecule that binds to an enzyme and decrease its activity, thus decreasing the reaction rate. The binding of an inhibitor can stop a substrate from entering the active site of the enzyme, hindering the enzyme from catalyzing the reaction. Consequently, the amount of product produced by the reaction is decreased, which is inversely proportional to the concentration of inhibitor molecules.

Inhibitors are classified into two categories, reversible and irreversible inhibitors, based on the nature of binding with enzyme.

• Irreversible inhibitors react with enzyme and can chemically change it by forming new covalent bonds. The key amino acid residues needed for enzymatic activity are often modified by these inhibitors, thereby the inhibition cannot by reversed.
• Reversible inhibitors, in contrast, attach to enzymes with non-covalent interaction, such as hydrogen bonds, hydrophobic interaction and ionic bonds. These inhibitors generally do not undergo chemical reactions and can be easily removed by dilution or dialysis.