Lesson 16 - Inhibitors

Question 1

What Is a Buffer?

Answer 1

An aqueous solution that has a highly stable pH. If you add acid or base to a buffered solution, its pH will not change significantly - it resists changes in pH.
If you are trying to control pH in an experiment you could use a buffer to do this

Question 2

Name 5 factors that affect the rate of enzyme controlled reactions

Answer 2

• enzyme concentration
• substrate concentration
• concentration of competitive/non-competitive inhibitors
• pH
• temperature

Question 3

What are enzyme inhibitors?

Answer 3

• Substances that directly/indirectly affect the functioning of the active site of a specific enzyme and so prevent the substrate binding, preventing the formation of enzyme substrate complexes and therefore decrease the rate of reaction
• Two types:
  Competitive
  Non-competitive

Question 4

Competitive inhibitors

Answer 4

• Have a similar shape/structure to the substrate, so they can bind to and occupy the active site of an enzyme
• Therefore, competitive inhibitors compete with the substrate for the active site
• Binding of a competitive inhibitor to the active site is not permanent (reversible), so when it leaves the active site, another molecule can enter (substrate or inhibitor).

Question 5

Non-competitive inhibitor

Answer 5

• Binds to an enzyme at a site that is NOT the active site (allosteric site)
• Binding of the inhibitor causes a change in the tertiary structure of the enzyme, therefore also changing the shape of the active site
• This means that the active site is no longer complementary to the substrate so the substrate can no longer bind (so no enzyme substrate complex formed and no reaction)

Question 6

What will happen to competitive inhibition when substrate concentration is increased?

Answer 6

It will overcome competitive inhibition because it is reversible

Question 7

Describe the difference between the graphs between a substrate concentration against RoR with and without a competitive inhibitor

Answer 7

The reaction will occur more slowly with competitive inhibitor but the max rate of reaction will be reached eventually when there is really high substrate concentration

Question 8

Describe the difference between the graphs between a substrate concentration against RoR with and without a non-competitive inhibitor

Answer 8

Increasing substrate concentration will NOT overcome non-competitive inhibition because it usually permanently changes the shape of the active site. So will not reach maximum rate of reaction.

Question 9

IN SUMMARY (if asked to compare inhibitors)…

Answer 9

In general, inhibitors:
- Directly/indirectly affect the functioning of the active site and so reduce binding of substrate to the enzymes active site
- Preventing/less successful collisions & preventing/less formation of enzyme-substrate complexes
Competitive:
- Similar shape/structure to the substrate
- Bind to/compete for the active site
- Can be overcome by increasing substrate concentration
Non-competitive:
- Bind to the enzyme away from the active site (allosteric site)
- Change the shape of the active site
- Cannot be overcome by increasing substrate concentration

Question 10

Markscheme comparing competitive and non-competitive inhibitors:

Answer 10

1. Inhibitors reduce binding of enzyme to substrate / prevent formation of ES complex;
   (Competitive inhibition),
2. Inhibitor similar shape (idea) to substrate;
3. (Binds) in to active site (of enzyme);
4. (Inhibition) can be overcome by more substrate;
   (Non-competitive inhibition),
5. Inhibitor binds to site on enzyme other than active site;
6. Prevents formation of active site / changes (shape of) active site;
7. Cannot be overcome by adding more substrate;

Question 11

Feedback inhibition/negative feedback
(with metabolic pathways)

Answer 11

• Metabolic pathways are a sequence of chemical reactions in which there are multiple steps and each step is catalysed by a different enzyme
• Hundreds of metabolic pathways occur in every cell that are necessary to keep organisms alive
• The sequence occurs in a very precise order because the product of one reaction is the substrate for the next reaction

Question 12

Why are inhibitors needed in cells?

Answer 12

Cells don’t want to make more product than required, so negative feedback is used to speed up/slow down/stop enzymes to control the metabolic pathway based on the cells demands

Question 13

Briefly explain how inhibitors are used in the human body

Answer 13

• Cell constantly making more resources for growth and repair
• Eg. don’t want more cellulose to be made than needed for a cell wall
• So …at high concentrations of end product, enzymes at early stages of the process are controlled or stopped by inhibition - stops process
• So the end product in high concs can act like a competitive or non-competitive inhibitor to early enzymes