Enzymes I

Question 1

What are some functions of enzymes? [5 marks]

Answer 1

• Digestion
• Blood clotting
• Defence
• Movement
• Nerve conduction

Question 2

What causes phenylketonuria? [1 mark]

Answer 2

Reduced levels of the enzyme phenylalanine hydroxylase.

Question 3

What processes can be affected in glycogen storage diseases? [3 marks]

Answer 3

• Glycogen synthesis
• Glycogen breakdown
• Glycolysis

Question 4

What happens in Tay-Sachs disease and what causes it? [2 marks]

Answer 4

• Cerebrosides build up in the brain.

- This is caused by insufficient activity in hexosaminidase A.

Question 5

What does penicillin inhibit? [1 mark]

Answer 5

Cell wall synthesis

Question 6

What does aspirin block? [2 marks]

Answer 6

• Cyclooxygenase (COX)

- This is involved in the synthesis of prostagladins.

Question 7

How does methotrexate help treat cancer? [2 marks]

Answer 7

• Inhibits dihydrofolate reductase (which is needed to produce a folic acid precursor)
• Prevents thymine from being made and prevents DNA synthesis in cancer cells.

Question 8

What are the key properties of enzymes? [5 marks]

Answer 8

• Increases reaction rate
• Specific
• Unchanged at the end of reactions
• Doesn’t alter reaction equilibrium
• Decrease the free energy of activation

Question 9

When is free energy at its highesr? [1 mark]

Answer 9

At the transition state.

Question 10

What does the lock and key model explain and what does it fail to explain? [2 marks]

Answer 10

• Explains specificity

- Doesn’t explain stabilisation of the transition state.

Question 11

What does the induced fit model explain? [2 marks]

Answer 11

• Specificity

- Stabilisation of the induced fit (via confirmational change of the active site)

Question 12

What is the binding energy? [1 mark]

Answer 12

The free energy that is released by the formation of weak interactions between a substrate and enzyme.

Question 13

What is the binding energy used for? [5 marks]

Answer 13

• Freeze the substrate in place
• Stabilise charges in the transition state
• Provide a reaction pathway with a lower energy
• Strain specific bonds in the substrate
• Use cofactors

Question 14

What is V-max? [2 marks]

Answer 14

• Maximum initial velocity

- Unit: μmol/min

Question 15

What is Kcat/K3? [2 marks]

Answer 15

• The turnover number

- Divide V-max by the number of enzymes to get this

Question 16

What is Km? [2 marks]

Answer 16

• Michaelis constant

- Substrate concentration at half V-max

Question 17

How is the V-max and Km affected with a competitive inhibitor? [2 marks]

Answer 17

• V-MAX: Unaffected

- Km: Decreased

Question 18

How is the V-max and Km affected with a non-competitive inhibitor? [2 marks]

Answer 18

• V-max: Decreased

- Km: Unaffected

Question 19

How is enezyme activity regulated? [5 marks]

Answer 19

• Control of gene expression
• Control of amount of enzyme
• Compartmentalisation
• Covalent modification to change enzyme shape and activity
• Allosteric modification

Question 20

Do allosteric enzymes demostrate Michaelis-Menten kinetics and why/why not? [2 marks]

Answer 20

• No (V v. S plots are sigmodal)

- Effectors binding to allosteric regions affect enzyme activity

Question 21

What is the definition of feedback inhibition? [1 marks]

Answer 21

When the product of a pathway inhibits enzymes involved in its synthesis.

Question 22

What is ATCase? [1 mark]

Answer 22

Aspartate Carbamoyl Transferase is involved in the synthesis of pyrimidine.

Question 23

What is the structure of ATCase? [4 marks]

Answer 23

• 12 catalytic subunits with active sites
• 6 catalytic subunits form a hemisphere (2 in total)
• 6 regulatory subunits
• regulatory dimers hold catalytic hemispheres together

Question 24

What happens when cytidine triphosphate binds to ATCase? [1 mark]

Answer 24

Catalytic hemispheres come together