Enzyme inhibitors (e)

Question 1

What is an inhibitor?

Answer 1

A factor that prevents or reduces the rate of an enzyme-catalysed reaction.

Question 2

Describe competitive inhibition

Answer 2

• a molecule that has a similar shape to the substrate can fit into the enzymes active site
• this blocks the substrate from entering the active site, preventing the enzyme from catalysing the reaction - cannot function = enzyme inhibited
• the substrate and inhibitor molecules will compete to bind to the active sites of the enzymes catalysing the reaction, reducing the number of substrates binding in a given time = slowing the rate of reaction
• depends on relative concentrations of enzyme and substrate and inhibitor
• most bind temporarily - so effect is reversible
• decreased rate of reaction but does not change Vₘₐₓ - if the substrate concentration is high enough, substrate > inhibitor so Vₘₐₓ can still be reached.

Question 3

Medicinal competitive inhibitors

Answer 3

• statins are competitive inhibitors of an enzyme used in the synthesis of cholesterol - to help reduce blood cholesterol concentration
• aspirin irreversibly inhibits the active site of COX enzymes, preventing the synthesis of prostaglandins + thromboxane - chemicals responsible for producing pain and fever

Question 4

Metabolic poisons and medicinal drugs

Answer 4

• drugs: antiviral drugs, antibiotics

- poisons: cyanide, malonate, arsenic

Question 5

Describe non-competitive inhibition

Answer 5

• the inhibitor binds to the enzyme at an alternative location to the active site
• causes the tertiary structure of the enzyme to change, altering the active site shape
• no longer complementary so substrate cannot bind and enzyme cannot carry out function = inhibited
• changes in concentration of substrate or enzyme will not overcome the effects of the inhibitor - increased inhibitor means decreased rate of reaction
• irreversible inhibitors cannot be removed from the part of the enzyme they are attached to - often toxic

Question 6

Medicinal non-competitive inhibitors

Answer 6

• organophosphates used as insecticides + herbicides irreversibly inhibit enzyme for nerve impulse transmission
• proton pump inhibitors (PPIs) are used to treat long-term indigestion by irreversibly blocking an enzyme system responsible for secreting hydrogen ions into the stomach

Question 7

What is end-product inhibition?

Answer 7

• the product of a reaction inhibits the enzyme required for the reaction - negative feedback control mechanism
• no excess products made + no resources wasted
• non-competitive reversible
• e.g. respiration