Enzymes

Question 1

What are enzymes?

Answer 1

They are biological catalysts that increase the rate of reaction without being used up in the process

Question 2

How do they increase the rate of reaction?

Answer 2

They provide an alternative reaction pathway with a lower activation energy

Question 3

What type of proteins are enzymes?

Answer 3

They are globular protein which means they are soluble in water

Question 4

How do enzymes work in a reaction?

Answer 4

They have a special region called the active site, and this is complementary to the shape of the substrate

Question 5

What is the lock and key model?

Answer 5

It describes the enzyme as rigid like a lock and the substrate like a key which has a complementary shape that fits perfectly to the active site

Question 6

What is the induced-fit model?

Answer 6

It is a model that describes how the enzyme active site changes its tertiary structure shape as it interacts with the substrate

Question 7

Why is the induced-fit model better?

Answer 7

1. It accounts for the specificity of the enzyme
2. It explains how the activation energy is lowered- the enzyme puts a strain on the substrate, lowering the activation energy

Question 8

What is the role of enzymes in reactions?

Answer 8

Anabolic reaction-the building up of molecules
Catabolic reaction- the breaking down of molecules

Question 9

Types of enzymes

Answer 9

Intracellular and
Extracellular enzymes

Question 10

What are intracellular enzymes?

Answer 10

They are enzymes that are produced and function inside the cell (being produced from)

Question 11

An example of an intracellular enzyme and reaction it catalyses

Answer 11

Catalase
Hydrogen peroxide ➡ water + oxygen

Question 12

What are extracellular enzymes?

Answer 12

They are enzymes that are secreted by cells and function outside the cell (being secreted from)

Question 13

Example of an extracellular enzyme and reaction it catalyses

Answer 13

Amylase
Starch ➡ maltose

Question 14

Describe how the structure of proteins determine enzyme activity (4 marks)

Answer 14

Proteins have a specific tertiary structure (1) which determines the shape of the active site (1), which binds to the substrate(1) and catalyses the reaction (1)

Question 15

Explain how catabolism and anabolism are related to metabolism (3 marks)

Answer 15

Catabolism is the breaking down of molecules (1), anabolism is the building up of molecules (1) and metabolism is the sum of all reactions in the body (1)

Question 16

Explain what is meant by the term model with reference to enzyme theories (lock and key and induced fit model)
(2 marks)

Answer 16

Models are simple/easy to understand (1) representations(1)

Question 17

Explain how the following terms are relevant to each of the models: complementary, flexibility, R group interactions, bond strain (4 marks)

Answer 17

Both models substrate interact with R groups in active sites (1) leading to bond strain in substrate molecules (1). Lock and key substrate is complementary to the active site of the enzyme (1). Induced fit active site is flexible (1) active site changes shape as substrate binds, closer fit between active site and substrate

Question 18

What is meant by the term “activation energy” (2 marks)

Answer 18

It is the energy required(1) to start a reaction (1)

Question 19

Discuss why the models of enzyme action have changed over time (4 marks)

Answer 19

Idea of improved technology (1), idea of continually investigated (1), more evidence (1), more accurate representation (1)

Question 20

What are factors affecting enzyme activity?

Answer 20

Temperature
pH
Substrate and enzyme concentration

Question 21

How does temperature affect enzyme activity?

Answer 21

As temperature ⬆, the kinetic energy of both the enzyme and substrate ⬆. This means there will be more successful collisions between the enzyme and substrate and so the rate of reaction ⬆

Question 22

What is optimum temperature?

Answer 22

It is the maximum temperature at which enzyme activity is at its highest

Question 23

What happens when the temperature increases past the optimum temperature?

Answer 23

If the temperature continues to increases past the optimum temperature, the rate of reaction ⬇. This is because the molecules in the enzyme move rapidly and this breaks the bonds between the R groups of the enzyme and substrate which changes the tertiary structure of the enzyme as well as shape of the the active site making it no longer complementary to the substrate

Question 24

What is the temperature coefficient?

Answer 24

It is a measure of how much the rate of reaction increases when the temperature is increased by 10 ° C

Question 25

Equation for temperature coefficient (Q10)

Answer 25

Temperature coefficient= rate of reaction at temperature X+10°C ➗ rate of reaction at X

Question 26

What does pH depend on?

Answer 26

pH depends on the concentration of hydrogen ions

Question 27

How does pH affect enzyme activity?

Answer 27

If the pH changes from the optimum pH, the H+ ions and the OH- disrupt the bonds between the R groups of the amino acid between the enzyme and substrate causing the 3D shape to change and so the active site is no longer complementary to the substrate decreasing the rate of reaction

Question 28

What is renaturation?

Answer 28

It is when the protein returns to its normal shape because of the pH returning to its optimum

Question 29

How does enzyme concentration affect enzyme activity?

Answer 29

As enzyme concentration ⬆, the rate of reaction ⬆. This is because there are more active sites for the substrate molecule to collide with which forms more enzyme-substrate complex which increases the rate of reaction

Question 30

What happens if enzyme concentration continues to increase?

Answer 30

If it continues to increase, the rate of reaction doesn't increase any further (Vmax). This is because the concentration of substrate becomes the limiting factor as there isn't enough substrate molecule to collide with the extra active sites

Question 31

How does substrate concentration affect enzyme activity?

Answer 31

As substrate concentration ⬆, the rate of reaction ⬆. This is because there will be a higher collision rate between the enzyme and substrate forming more enzyme-substrate complex

Question 32

What happens when substrate concentration continues to increase?

Answer 32

It reaches Vmax. This is the point where the rate of reaction doesn't increase any further. This is because all of the active sites have already been occupied by substrates so no more enzyme-substrate complexes can be formed.

Question 33

Explain the term "denatured" with reference to enzymes (3 marks)

Answer 33

It refers to when the R group interactions are disrupted (1) and this changes the tertiary structure(1) of the active site preventing binding with substrate (1)

Question 34

Bacteria that colonise hydrothermal vents, where temperatures are very high, have enzymes with very high optimum temperatures. Suggest why these bacteria are unlikely to cause infections in humans (3 marks)

Answer 34

Bacterial enzymes have high optimum temperatures, human body temperature is lower (1). Enzymes will have low activity (1) and bacteria will not thrive (1).

Question 35

Enzymes with very optimum temperatures tend to have quite flexible structures. Using your knowledge of collision theory, explain why this flexibility is necessary (6 marks)

Answer 35

At low temperatures kinetic energy is low (1); substrates / enzymes, move slowly (1); (so) fewer collisions (1); collisions have less energy (1); increased flexibility of active site (1); increases chances of successful collision (1).

Question 36

What are inhibitors?

Answer 36

They are molecules that prevent enzymes from carrying out their normal function of catalyses

Question 37

What are the types of inhibitors?

Answer 37

Competitive inhibitors and; Non-competitive inhibitors

Question 38

What are competitive inhibitors?

Answer 38

They are molecules that have a similar shape to the substrate and bind to the active site of the enzyme blocking the substrate from binding to the active site. This reduced the frequency of successful collisions between the active site and the substrate reducing the rate of reaction

Question 39

Effect of competitive inhibitors on the rate of reaction

Answer 39

It reduces the rate of reaction for a given concentration of substrate. This is because it blocks the active site and prevents it from forming enzyme-substrate complex

Question 40

How can the effect of competitive inhibitor be reduced?

Answer 40

The effects can be reduced by increasing the concentration of substrates. This increases the chance of a substrate molecule occupying the active site rather than a competitive inhibitor

Question 41

What are non-competitive inhibitors?

Answer 41

They are molecules that bind to the allosteric site of the enzyme and this causes the tertiary structure of the enzyme to change which means the active site is no longer complementary to the substrate

Question 42

What effect do they have on rate of reaction

Answer 42

They reduce the rate of reaction as they change the shape of the active site which means the substrate cannot bind to the active site any more reducing the rate of reaction

Question 43

What is the role of end-product inhibition?

Answer 43

1. Prevents excess product from being made 2. Prevents resources from being wasted

Question 44

What is an example of end-product inhibition?

Answer 44

Production of ATP through respiration

Question 45

Explain why a non-competitive inhibitor does not need to have a similar shape to a substrate molecule (3 marks)

Answer 45

A non-competitive inhibitor binds to an enzyme away from the active site (1) at an allosteric site (1), which has a different shape than the active site (1)

Question 46

Explain why increasing the concentration of substrate will never produce the Vmax of a reaction after the addition of a non-competitive inhibitor. (2 marks)

Answer 46

Inhibitor will always be present (1); some enzymes always inhibited (1).

Question 47

End product inhibition is likely to be competitive rather than non-competitive. Suggest reasons for this, and give an example of end product inhibition (4 marks)

Answer 47

End-product inhibition regulates rate of reaction (1); concentrations of substrate and product determine reaction rate (1); (so must be) competitive (1); substrate concentration has no effect in noncompetitive inhibition (1); e.g. ATP and PFK in respiration (1).

Question 48

Ethylene glycol present in antifreeze is poisonous when ingested. Ethylene glycol is oxidised using the same enzymes used to oxidise ethanol (alcohol). The products made during the breakdown of ethylene glycol, rather than ethylene glycol itself,are responsible for the toxic effects. Ethylene glycol is able to leave the body unchanged in urine. Suggest why ethanol is often used in emergency departments as an antidote to antifreeze poisoning (6 marks)

Answer 48

Ethanol has a similar shape to ethylene glycol (1) ethanol binds to the active set of the enzyme which breaks down ethylene glycol (1). This is competitive inhibition (1) Less ethylene glycol is broken down (1), more ethylene glycol leaves the body unchanged (1) fewer toxic effects (1)

Question 49

What are cofactors?

Answer 49

They are chemicals that are needed by enzymes so that they can perform their function of catalyzing reactions

Question 50

Example of cofactors

Answer 50

Chloride ion which aids amylase in the hydrolyses of starch into maltose

Question 51

What are coenzymes?

Answer 51

They are large organic molecules that also aid enzymes to perform their functions. They do not bind permanently to the structure of the enzymes

Question 52

Example of coenzymes

Answer 52

NAD which is involved in transferring hydrogen ions between molecules in respiration

Question 53

What are prosthetic groups?

Answer 53

They are cofactors that bind permanently to the enzymes structure

Question 54

Examples of prosthetic groups?

Answer 54

Iron ion in haemoglobin Zn ion in carbonic anhydrase, which is involved in the formation of carbonic acid from carbon dioxide and water

Question 55

Describe two ways in which cofactors are necessary for the catalytic role for some enzymes (2 marks)

Answer 55

They transfer atoms between reactions (1) which forms part of the active site (1)

Question 56

Explain, using an appropriate example of each, how prosthetic groups are different from coenzymes (4 marks)

Answer 56

Coenzymes bind loosely to enzymes (1); e.g. NAD (1); prosthetic groups are a permanent feature of enzymes (1); e.g. iron ion in haemoglobin (1).

Question 57

Explain what is meant by the term Vmax with respect to enzymes? (3 marks)

Answer 57

Vmax is when reaction is at maximum rate (1); all active sites occupied(1); enzyme activity is maximum(1)

Question 57

Explain why end product inhibitors have to be both reversible and competitive (3 marks)

Answer 57

It needs to be reversible so more products can be made when needed(1); with competitive inhibitors,more substrates would overcome inhibition(1); (leading to) excess product (1)

Question 58

Explain why end product enzyme inhibition is essential for the control of cellular activity (3 marks)

Answer 58

It prevents build-up of excess products(1) prevent waste of resources /energy(1); ensure sufficient levels of required products(1)