- An enzyme inhibitor is any substance or molecule that slows down the rate of an enzyme-controlled reaction by affecting the enzyme molecule is some way.
(1 ) Competitive inhibitor
- Competitive inhibitor molecules have a similar shape to that of the substrate molecules, so they compete for the active site.
- This means they can occupy the active site, forming enzyme-inhibitor complexes, but do not lead to the formation of the products because the inhibitor is not identical to the substrate.
- Enzyme inhibition occurs because whenever an inhibitor molecule is occupying an enzyme's active site, a substrate molecule cannot enter.
- So the number of enzyme-substrate complexes is reduced and the reaction rate slows down.
- The level of inhibition depends on the concentration of inhibitor and substrate.
- Where the number of substrate molecules are increased, the level of inhibition decreases because a substrate molecule is more likely than an inhibitor to collide with an active site.
(2) Non-competitive inhibitor
- Non-competitive inhibitor molecules do not compete with substrate molecules for a place in the active site.
- Instead, they attach to the enzyme molecule in a region away from the active site.
- The attachment of non-competitive inhibitor molecule to an enzyme molecule distorts the tertiary structure of the enzyme molecule.
- This leads to a change in shape of the active site, meaning the substrate no longer fits into the active site.
- Therefore enzyme-substrate complexes cannot form and the reaction rate decreases.
- The level of inhibition depends on the number of inhibitor molecules present.
- If there are enough inhibitor molecules to bind to all the enzyme molecules present, then the enzyme-controlled reaction will stop.
- Changing the substrate concentration will have no effect on this form of inhibition.
(3) Reversible inhibitors
- Reversible inhibitors are inhibitors that bind to the active site for a short period and then leave.
- The removal of the inhibitor from the reacting mixture leaves the enzyme molecules unaffected.
(4) Irreversible inhibitors
- Irreversible inhibitors are inhibitors that bind permanently to the enzyme molecule.
- Any enzyme molecules bound by inhibitor molecules are effectively denatured.
- A cofactor is any substance that must be present to ensure enzyme-controlled reactions can take place at the appropriate rate.
- Some cofactors are part of the enzymes (prosthetic groups); others affect the enzyme on a temporary basis (coenzymes and inorganic ion cofactors).
Coenzymes are small, organic, non-protein molecules that bind for a short period of time to the active site.
Their role is to carry chemical groups between enzymes so they link together enzyme-controlled reactions that needs to take place in sequence.
Like substrates, they are changed in some way but they are recycled back to take part in the reaction again.
E.g. Vitamin B3 (nicotinamide) plays an important role in helping the body break down carbohydrates and fat to release energy.
The vitamin is used in the body to make a coenzyme that is required for the enzyme pyruvate dehydrogenase to function properly.
The enzyme catalyses one of the reactions involved in respiration so is necessary for life.
- A prosthetic group is a coenzyme that is a permanent part of an enzyme molecule.
- They are vital to the function of the enzyme (or protein) molecule.
- They contribute to the final 3D shape, and to other properties of the molecule, including its charges.
- E.g. The enzyme carbonic anhydrase contains a zinc-based prosthetic group.
- This enzymes is a vital component red blood cells, where it is involved in catalysing the combining of carbon dioxide and water to produce carbonic acid.
- This is an important reaction that enables carbon dioxide to be transported in the blood.
Inorganic Ion Cofactors
- In some enzyme-controlled reactions, the presence of certain ions can increase the reaction rate.
- Ions may combine with either the enzyme or the substrate.
- The binding of the ions makes the enzyme-substrate complex form more easily, because it affects the charge distribution and in some cases, the shape of the enzyme-substrate complex.
Metabolic Poisons (Enzyme inhibitors)
- Potassium cyanide inhibits cell respiration.
- It is a non-competitive inhibitor for a vital respiratory enzyme called cytochrome oxidase, found in mitochondria.
- Inhibition of this enzyme decreases the use of oxygen, so ATP cannot be made and so the organism must respire anaerobically, which leads to a build up of lactic acid in the blood.
- Only 100-200mg of cyanide must be absorbed in order to make an adult lose consciousness in as little as 10 seconds.
- After 45 minutes the body goes into a coma and after 2 hours, it results in death.
Ethylene Glycol (Antifreeze)
- Ethylene glycol is broken down in the liver by the enzyme alcohol dehydrogenase
- The breakdown product, oxalic acid, is extremely toxic and ethylene glycol poisoning can lead to death.
- Treatment: Fomepizol - a strong inhibitor of alcohol dehydrogenase reducing the rate of oxalic acid being produced.
- Treatment: Ethanol - a natural substrate for alcohol dehydrogenase so ethanol is metabolised in preference to ethylene glycol - alcohol intoxication.
- Snake venom is a mixture of toxins and different enzymes.
- Phosphodiesterases interfere with the working of the prey's heart causing a fall in blood pressure.
- The enzyme acetylcholinesterase inhibits nerve transmission, resulting in paralysis.
- The enzyme hyaluronidase is a disgestive enzyme that breaks down connective tissue and so helps toxins penetrate tissues quickly.
- ATP-ases are used to break down ATP to disrupt the prey's use of energy.
Medicinal Drugs Work
- Antibiotics - kill or inhibit the growth of microorganisms and are used to treat diseases caused by bacterial infections.
- Penicillin - inhibitor of a bacterial enzyme that forms cross-links in the bacterial cell wall of some bacteria, meaning the walls of growing bacteria is not made, so bacterial reproduction is halted.
- Cystic fibrosis sufferers often have a blocked passage so digestive enzymes cannot pass. Doctors prescribe an enzyme in tablet form which are packaged in an acid-resistant coat so they are not destroyed by the acid and the protein-digesting enzymes in the stomach.
That's the end of the topic!
Drafted by Bonnie (Biology)