Enzymes

Question 1

What is a catalyst? (2)

Answer 1

• A catalyst speeds up reactions without using itself up (without any permanent change to itself)
• by lowering the activation energy

Question 2

What is activation energy? (1)

Answer 2

The minimum energy required to start a reaction

Question 3

What are enzymes and what type of protein are they? (2)

Answer 3

• Enzymes are biological catalysts
• They are globular proteins

Question 4

Give an example of enzymes catalysing metabolic reactions at a cellular level and extracellularly (2)

Answer 4

• Cellular - Respiration
• Extracellular - During digestion in mammals

Question 5

Describe the structure of an enzyme (2) and how it finds a substrate to bind with (1)

Answer 5

• Enzymes have a tertiary structure
• with an active site which has a specific shape
• which is complementary to a particular substrate molecule

Question 6

What is formed when an enzyme and substrate collide (1)

Answer 6

An enzyme-substrate complex (ES-complex)

Question 7

What happens to the products made from the reaction being catalysed? (1)

Answer 7

• They leave the active site and the enzyme goes on to form more ES-complexes

Question 8

Enzymes allow metabolic reactions to happen at a lower temperature because … (2)

Answer 8

• They hold substrates closer together, reducing repulsion so they bond more easily
• They put more strain on the bonds of a substrate so the bonds break more easily

Question 9

State and describe the two models for enzymes (4)

Answer 9

• Lock and key model (original)
  Stated that the active site of an enzyme is exactly complementary to the substrate so when they collide an ES-complex is formed
• Induced-fit model (updated)
  States that the active site is not exactly complementary to the substrate and when it collides with an enzyme, the active site slightly changes shape to fit around the substrate and an ES-complex is formed

Question 10

State the 4 factors affecting enzyme concentration (4)

Answer 10

• Temperature
• Enzyme concentration
• Substrate concentration
• pH

Question 11

Explain how enzyme concentration can affect the rate of a reaction (3)

Sketch a graph to show this and check in your notes (1)

Answer 11

• Increasing the enzyme concentration increases the rate of reaction
• because there are more enzymes for the substrates to collide with and form an ES-complex
• but this is only true until the substrates become a limiting factors so the rate of reaction stops

Question 12

Explain how substrate concentration can affect the rate of a reaction (3)

Sketch a graph to show this and check in your notes (1)

Answer 12

• Increasing the substrate concentration increases the rate of reaction
• because there are more substrates for the enzymes to collide with and form an ES-complex
• but this is only true until the enzymes become a limiting factors so the rate of reaction slows down

Question 13

Explain how temperature can affect the rate of a reaction (4)

Sketch a graph to show this and check in your notes (1)

Answer 13

• Increasing the temperature increases the rate of reaction
• because there is more kinetic energy –> molecules move faster –> more collisions –> more ES-complexes formed
• but this is only true until the optimum temperature of the enzyme has been passed
• as the molecules will vibrate too much and break bonds –> no tertiary structure –> active site changes shape –> enzyme denatures –> rate of reaction stops

Question 14

Explain how the pH can affect the rate of a reaction (3)

Sketch a graph to show this and check in your notes (1)

Answer 14

• The rate of reaction increases as the pH increases or decreases closer to the optimum pH
• Most enzymes work best at pH7 but not all (e.g. pepsin works best at pH2)
• Above and below the optimum pH for enzymes cause the H+ and OH- ions to disrupt the hydrogen and ionic bonds –> tertiary shape changes –> active site shape changes –> no ES-complex can be formed –> enzyme denatures

Question 15

State the two types of enzyme inhibition (2)

Answer 15

• Competitive inhibition
• Non-competitive inhibition

Question 16

Describe competitive inhibition (2)
Describe the effect of increasing substrate concentration on the rate of reaction (1)

Answer 16

• The inhibitor has a similar shape to the substrate and it competes with the substrate to bind to the active site
• The inhibitor blocks the substrate from forming an ES-complex with the active site
• Increasing the substrate concentration still increases the rate of reaction but it reverses the effect of inhibition as the substrate will out-compete the inhibitor for the active site

Question 17

Describe non-competitive inhibition (2)
Describe the effect of increasing substrate concentration on the rate of reaction (1)

Answer 17

• The inhibitor does not have a similar shape to the substrate so they don’t compete
• Instead it binds to the allosteric site of the enzyme, causing the active site to change shape so it isn’t complementary to the substrate anymore and no ES-complexes can be formed
• Increasing the substrate concentration has no effect on the rate of reaction as they can’t bind to the active site since it has changed shape