AS/A-level Biology - Enzyme (2)

• 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.

• 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.

Potassium Cyanide

• 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

• 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.

• 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!

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