3.1.4.2 Many proteins are enzymes

Question 1

How do enzymes lower activation energy?

Answer 1

Enzymes lower activation energy by stabilising the transition state reducing repulsion between substrate molecules and providing an alternative reaction pathway.

Question 2

What is the induced-fit model of enzyme action?

Answer 2

The induced-fit model states that the enzyme’s active site is flexible and changes shape to fit the substrate more closely forming an enzyme-substrate complex which places strain on bonds and lowers activation energy.

Question 3

How does the tertiary structure of an enzyme determine its properties?

Answer 3

The tertiary structure determines the shape of the active site which must be complementary to the substrate allowing an enzyme-substrate complex to form. If the tertiary structure is altered the active site changes shape preventing substrate binding.

Question 4

Why are enzymes specific?

Answer 4

Enzymes are specific because their active site has a complementary shape to a particular substrate forming enzyme-substrate complexes due to specific interactions between R-groups in the active site and the substrate.

Question 5

How does enzyme concentration affect the rate of reaction?

Answer 5

Increasing enzyme concentration increases the rate as more active sites are available but if substrate is limited the rate plateaus as all substrate molecules are already bound.

Question 6

How does substrate concentration affect the rate of reaction?

Answer 6

Increasing substrate concentration increases the rate as more enzyme-substrate complexes form until all active sites are occupied causing the rate to plateau.

Question 7

How do competitive inhibitors affect enzyme activity?

Answer 7

Competitive inhibitors are similar in shape to the substrate and bind to the active site preventing substrate binding and reducing the rate but increasing substrate concentration reduces their effect.

Question 8

How do non-competitive inhibitors affect enzyme activity?

Answer 8

Non-competitive inhibitors bind to an allosteric site changing the enzyme’s tertiary structure and active site shape preventing enzyme-substrate complex formation and reducing the maximum rate regardless of substrate concentration.

Question 9

How does pH affect enzyme activity?

Answer 9

pH affects the ionic and hydrogen bonds in the enzyme’s tertiary structure altering the active site shape preventing enzyme-substrate complex formation. Extreme pH denatures the enzyme.

Question 10

How does temperature affect enzyme activity?

Answer 10

Increasing temperature increases kinetic energy causing more collisions and enzyme-substrate complexes but beyond the optimum temperature hydrogen bonds break altering the active site shape and denaturing the enzyme.

Question 11

How has the understanding of enzyme action changed over time?

Answer 11

The lock and key model proposed a fixed active site shape while the induced-fit model showed the active site changes shape to fit the substrate improving understanding of enzyme specificity and catalysis.

Question 12

What types of reactions do enzymes catalyse?

Answer 12

Enzymes catalyse intracellular reactions inside cells and extracellular reactions outside cells controlling metabolic pathways and processes across all biological levels.

Question 13

Enzymes – “Induced Fit” Model (3)

Answer 13

1. (before reaction) active site not complementary to/does not fit substrate;
2. Shape of active site changes as substrate binds/as enzyme-substrate complex forms;
3. Stressing/distorting/bending bonds (in substrate leading to reaction);

Question 14

Enzymes – Increased temperature and reaction rate (4)

Answer 14

1. particles have more kinetic energy
2. therefore they move more
3. so there are more collisions between substrates and active sites
4. so more ES complexes form

Question 15

Enzymes – Denaturation (5)

Answer 15

1. Heat above the optimum breaks hydrogen bonds
2. this causes the tertiary structure to unfold
3. so the active site changes shape
4. substrate can no longer bind to the active site, as it’s no longer complementary
5. so fewer ES complexes form

Question 16

Enzymes – Effect of Changes in pH (4)

Answer 16

1. Ionic bonds holding tertiary structure break
2. active site distorts and substrate no longer binds to active site
3. charges on amino acids in active site affected
4. fewer ES complexes form

Question 17

Enzymes – Concentration of Substrate (2)

Answer 17

1. (Rate of) increase in concentration of product slows as substrate is used up
   OR
   High initial rate as plenty of substrate/more E-S complexes;
2. No increase after 25 minutes/at end/levels off because no substrate left;

Reject ref. to enzyme being used up

Question 18

Enzymes – Describe and explain the temperature graph of enzyme rate (5)

Answer 18

1. Initial rate of reaction faster at 37 °C (than 25 °C);
2. Because more kinetic energy;
3. So more E–S collisions/more E–S complexes formed;
4. Graph reaches plateau /levels off at 37 °C;
5. Because all substrate used up;

Question 19

Enzymes – Comparison of Competitive and Non Competitive
Inhibition (4)

Answer 19

1. Competitive inhibitor binds to active sites of enzyme but non-competitive inhibitor binds at allosteric site/away from active site;
2. (Binding of) competitive inhibitor does not cause change in shape of active site but (binding of) non-competitive does (cause change in size of active site);
3. So with competitive inhibitor, at high substrate concentrations (active) enzyme still available but with noncompetitive inhibitor (active) enzymes no longer available;
4. At higher substrate concentrations likelihood of enzyme-substrate collisions increases with competitive inhibitor but this is not possible with non-competitive inhibitor;