An inhibitor is a substance that binds to an enzyme and reduces its activity. Inhibitors can be classed by how strongly and how permanently they bind.Reversible inhibitors bind to the enzyme by weak bonds, such as hydrogen bonds and ionic bonds. They bind and leave again, so the enzyme is only inactive while the inhibitor is attached. If the inhibitor is removed (for example by dialysis or by being broken down), full enzyme activity returns. Competitive and non-competitive inhibitors can both be reversible. Many drugs and the cell's own control molecules are reversible inhibitors.Irreversible inhibitors bind very tightly, usually by forming covalent bonds with the enzyme, or they change the tertiary structure permanently. The enzyme cannot recover, so it is permanently inactivated, and the cell must make new enzyme molecules. Examples:
• cyanide irreversibly inhibits cytochrome c oxidase, an enzyme of respiration, so it is very poisonous
• heavy metal ions, such as mercury and lead, bind to sulfur-containing groups in the enzyme and change its shape
• some nerve agents and insecticides irreversibly inhibit acetylcholinesteraseWorked example: an enzyme is mixed with an inhibitor and its activity falls. The inhibitor is then removed by dialysis and the activity returns to its original value. The inhibition was therefore reversible.
• cyanide irreversibly inhibits cytochrome c oxidase, an enzyme of respiration, so it is very poisonous
• heavy metal ions, such as mercury and lead, bind to sulfur-containing groups in the enzyme and change its shape
• some nerve agents and insecticides irreversibly inhibit acetylcholinesteraseWorked example: an enzyme is mixed with an inhibitor and its activity falls. The inhibitor is then removed by dialysis and the activity returns to its original value. The inhibition was therefore reversible.