Chemicals that block enzyme activity are called enzyme inhibitors. They can take many forms, including small molecules, peptides and metal ions. Many drugs are enzyme inhibitors, including penicillin and aspirin. Some poisons also act by inhibiting enzyme activity. All enzyme inhibitors ultimately work by rendering enzymes unable to bind to their substrates, which are the molecules whose reactions they catalyze.
Enzymes are proteins that catalyze reactions in cells, meaning they speed up biochemical reactions. At the active site of an enzyme, which is a crevice that geometrically and chemically fits reactants, substrates of the right size and shape bind and undergo chemical reactions. Enzyme inhibitors work by directly or indirectly blocking an enzyme's active site so that substrates can no longer access it.
Reversible and Irreversible Inhibition
Enzyme inhibitors can be classified as either reversible or irreversible inhibitors. Reversible inhibitors bind to active sites transiently, and often compete with natural reactants for access to an enzyme's active site. A competitive inhibitor will reversibly inhibit enzyme activity in a concentration-dependent way, meaning the higher the proportion of inhibitor, the less the natural substrates will have access to the enzyme's active site, and the lower the enzyme activity will be. An example of a reversible inhibitor is the HIV drug ritonavir, which mimics the shape of the substrates of HIV proteases. Another kind of reversible inhibitor is a noncompetitive inhibitor, which binds to the enzyme at a location other than the active site. This causes a change in the enzyme's shape, thereby indirectly blocking the active site. Irreversible enzyme inhibitors bind to the active site and block enzyme activity in a long-lasting way. Heavy metals like mercury are noncompetitive inhibitors.
Inhibitors as Medicines
Antibiotics are a class of chemicals that inhibit enzyme activity. For example penicillin irreversibly inhibits an enzyme called transpeptidase, which is important for synthesizing bacterial cell walls. Without active transpeptidase bacteria cannot divide. Amoxycilin also prevents synthesis of bacterial cell walls, but by inhibiting the enzyme beta lactamase. Aspirin modifies the enzyme cyclooxygenase and blocks its activity in the inflammatory pathway, thus stopping both pain and the inflammatory healing process.
Inhibitors as Poisons
Tthe enzymes that the body needs to function are also susceptible to inhibition. Heavy metals are poisonous, often because of their ability to noncompetitively inhibit enzymes. Mercury, copper, lead, and silver can all react with a particular amino acid present in proteins, and by doing so, they irreversibly inhibit enzyme activity. Natural products like glycoalkaloids from nightshade plants can exhibit toxic effects through their ability to inhibit enzymes like aceytlcholinesterase, an enzyme required for muscle movement. In addition, pesticides like organophosphates and carbamates inhibit acetylcholinesterase, and are toxic to all animals, humans and insects.
- Chemguide: Enzyme Inhibitors
- Chemguide: Proteins as Enzymes
- Biochemistry: Enzymes Can Be Inhibited by Specific Molecules
- Immunopaedia: Mode of Action: Protease Inhibitors
- Phytochemistry: Membrane Disruption and Enzyme Inhibition by Glycoalkaloids
- Current Pharmaceutical Design: Antiviral Drug Discovery
- Extoxnet: Cholinesterase Inhibition
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