4.3- Enzyme Inhibitors Flashcards
How is metabolic activity in cells controlled?
The steps in reaction pathways are controlled by different enzymes.
Enzymes can be activated with cofactors
And inactivated with inhibitors (molecules that prevent enzymes carrying out their normal function).
Explain competitive inhibition.
A molecule that has a similar shape to the substrate of an enzyme can fit into the active site of that enzyme.
- this blocks the actual substrate from entering the active site, preventing the enzyme from catalysing the reaction.
- the enzyme cannot carry out its function and is said to be inhibited.
- inhibitor = the non-substrate molecule binding to active site.
- effect is usually reversible (some exceptions eg, aspirin).
- degree of inhibition depends on the concentration of the substrate, inhibitor and enzyme.
How does competitive inhibition affect rate of reaction?
Inhibitors and substrate molecules compete for the active site. This reduces the number of substrates binding to active sites in a given time, therefore decreasing rate.
The competitive inhibitor doesn’t change the Vmax of the enzyme it exhibits.
Examples of competitive inhibition.
- Statins are competitive inhibitors of an enzyme that synthesises cholesterol. They are prescribed to help reduce blood cholesterol concentration as high cholesterol levels can result in heart disease.
- Aspirin irreversibly inhibits the active site of COX enzymes, preventing the synthesis of chemicals like thromboxane which are responsible for producing pain and fever.
Explain non-competitive inhibition.
The inhibitor binds to the enzyme at a location other than the active site. This location is called the allosteric site.
- binding of inhibitor causes tertiary structure of enzyme to change, therefore active site changes shape.
-substrate is no longer able to bind to the active site as it doesn’t have a complimentary shape.
-enzyme cannot carry out its function so is inhibited. Usually irreversible.
Inhibitor does not compete with a substrate so hence non-competitive.
How does non-competitive inhibition affect rate?
Decreases rate.
Increasing the concentration of the inhibitor will decrease the rate further as more active sites become unavailable.
Examples of non-competitive inhibitors?
Irreversible inhibitors cannot be removed and are often toxic.
1. organophosphates (used as insecticides) irreversibly inhibit an enzyme needed for nerve impulse transmission. This can lead to muscle cramps, paralysis and even death if ingested.
- Proton pump inhibitors are used to treat long term indigestion. They block an enzyme system responsible for secreting H ions into the stomach. This reduces the production of excess acids and treats long-term indigestion.
What is end-product inhibition?
Enzyme inhibition that occurs when the product of a reaction acts as an inhibitor to the enzyme that produced it.
- negative feedback mechanism for the reaction.
- is an example of non-competitive reversible inhibition.
- excess products not produced and resources not wasted.
Examples of metabolic poisons that are enzyme inhibitors.
Cyanide is an irreversible inhibitor of cytochrome oxidase, an enzyme that catalyses respiration reactions. Cells die if they are unable to respire.
Malonate inhibits succinate dehydrogenase which also catalyses respiratory reactions.
Arsenic inhibits action of pyruvate dehydrogenase. Also catalyses respiration reactions.
