4.3 - Enzyme Inhibitors

Question 1

Define inhibitors.

Answer 1

Molecules that prevent enzymes (or slow down) their catalysis

2 types: Competitive and non-competitive

Question 2

Describe the process of competitive inhibition.

Answer 2

1) Molecule or part of molecule with similar shape to substrate fits active site
2) Blocks substrate from binding - enzyme can no longer catalyse reaction
3) Non-substrate molecule is inhibitor

Question 3

How do competitive inhibitors slow the rate of reaction?

Answer 3

Substrate and competitive inhibitors compete for active sites in solution.
- Reduces number of substrates binding to active sites
Degree of inhibition depends on enzyme-substrate-inhibitor conc.

Question 4

What is the effect of competitive inhibitors on reaction rate?

Answer 4

Inhibitors slow reaction rate but do not change Vmax, substrate conc. can be increased to reach Vmax

Question 5

Are the effect of competitive inhibitors reversible?

Answer 5

Most bind temporarily so effect is reversible, some exceptions (e.g. aspirin)

Question 6

Give examples of competitive inhibitors and their roles.

Answer 6

Medicinal Drugs
Statins - competitive inhibitors involved in reducing cholesterol synthesis

Aspirin - irreversibly inhibits cox enzyme - prevents syntehsis of prostaglandins and thromboxane (chemicals responsible for pain and fever)

Question 7

Describe the process of non-competitive inhibition.

Answer 7

1. Inhibitor binds to alternative site called allosteric site.
2. Binding causes proteins tertiary structure at active site to change
3. No longer complementary to subsstrate
4. Enzyme cannot carry out function.

Question 8

What is the effect of non competitive inhibitors on reaction rate?

Answer 8

• Increasing enzyme/substrate conc. doesn’t overcome the effect of the NCI
• Adding more NCI decreases reaction rate further.
• Lowers Vmax

Question 9

Give examples of irreversible non competitive inhibitors.

Answer 9

• Cannot be removed from allosteric site (often toxic not always)

Organophosphates - used in insecticides and herbicides - irreversibly inhibits acetyl cholinesterase (enzyme used in nerve impulse transmission)

PPI (protein pump inhibitor) - used to treat long term indigestion.

• Blocks enzyme responsibly for releasing H+ into stomach.
• PPI reduces excess acid addition to stomach which could otherwise cause indigestion.

Question 10

What is end product inhibition?

Answer 10

When the product of an enzyme reaction acts as an inhibitor (negative feedback)
- Resources won’t be wasted as products are formed
This is an example of non competitive reversible inhibition.

Question 11

Give an example of end product inhibition.

Answer 11

Respiration produces ATP,

Glucose broken down by 1) adding 2 phosphate groups, 2) when adding the 2nd phosphate group, it is catalysed by enzyme PFK.
- PFK is inhibited by ATP - ATP regulates its own production.

• When ATP levels rise, it binds to the allosteric site of PFK preventing addition of second phosphate, vice versa.