Enzymes

Question 1

What are enzymes?

Answer 1

Globular proteins with a specific tertiary structure that act as biological catalysts and increase the rate of reaction by lowering the activation energy.

Question 2

What are anabolic reactions?

Answer 2

These reactions build up complex molecules from simpler ones.

Question 3

What are catabolic reactions?

Answer 3

These reactions break down complex molcules into smaller ones. Example: hydrolysis reactions.

Question 4

Why are enzymes soluble in water?

Answer 4

They have hydrophilic T-groups on the outside and hydrophobic R groups on the inside.

Question 5

Definition of activation energy

Answer 5

The minimum amount of energy required for a reaction to start.

Question 6

Explain the effect that enzymes have on the activation energy.

Answer 6

Enzymes lower the activation energy by bending bonds between molecules, so less energy is required for bonds to break when the reaction occurs.

Question 7

What is the lock and key model?

Answer 7

This model states that the active site does not change shape, the substrate and active site are fully complementary to one another.

Question 8

Explain how the lock and key model was supported (2 points).

Answer 8

1. The model was supported by the observation that enzymes are specific in the reactions that they catalyse.
2. The shape of the substrate (key) exactly fits the active site of the enzyme (lock).

Question 9

Explain a limitation to the lock and key model (5 points).

Answer 9

1. The enzyme is considered to be a rigid structure.
2. Scientists had observed that other molecules could bind to enzymes at sites other than the active site.
3. In doing so, they altered the activity of the enzyme.
4. This suggested that the enzyme’s shape was being altered by the binding of a molecule.
5. Therefore, its structure was not rigid but flexible.

Question 10

What is the induced fit hypothesis?

Answer 10

This hypothesis proposes that the active site forms as the enzyme and substrate interact.

Question 11

Explain the changes to the active site in the induced fit hypothesis (4 points).

Answer 11

1. Before the reaction, the active site is not complementary to / does not fit the substrate.
2. The shape of the active site changes as the substrate binds forming an enzyme-substrate complex.
3. The substrate forms temporary bonds with the R-groups of amino acids lining the active site which strains/bends the bond.
4. This strain distorts a particular bond(s) in the substrate and consequently lowers the activation energy needed to break the bond(s).

Question 12

Explain how to calculate the initial rate of reaction (4 points).

Answer 12

1. Draw a tangent to calculate the initial rate of reaction. The tangent must go through the intercept at 0.
2. Draw a triangle from this.
3. Read off the change in product (Y) and the change in time (X).
4. Calculate the gradient of this line using the equation: rate of reaction = (change in Y) / (change in X)

Question 13

Explain how to calculate rate from specific times (4 points).

Answer 13

1. Draw a tangent.
2. Draw a triangle.
3. Read off the change in product (Y) and the change in time (X).
4. Calculate the gradient of this line using the equation: rate = (change in Y) / (change in X).

Question 14

Explain the effect of temperature on enzymes (8 points).

Answer 14

1. At low temperatures there is less kinetic energy, so less collisions between the substrate and enzyme active site- rate is low.
2. As the temperature increases, the substrate and enzyme has more kinetic energy.
3. Frequency of collisions between the substrate and active sites increase so more enzyme-substrate complexes form, so more product forms and rate increases.
4. Optimum temperature is when collisions between the substrate and active sites are at their fastest.
5. Above the optimum temperature, the enzyme vibrates more until the hydrogen bonds maintaining the tertiary structure break.
6. The shape of the active site changes and the enzyme becomes denatured.
7. The active site and substrate are no longer complementary in shape.
8. Less enzyme-substrate complexes form- rate decreases rapidly.

Question 15

Definition of denatured

Answer 15

When the active site and substrate are no longer complementary in shape to one another.

Question 16

Explain the effect that pH has on enzymes (5 points).

Answer 16

1. At the optimum pH, the tertiary structure / shape of the active site is complementary to the substrate.
2. Enzyme-substrate complexes form- rate is at its highest.
3. A change in H+ concentration can disrupt/break hydrogen and ionic bonds, change charge of amino acids in the active site and change the tertiary structure and shape of the active site- enzyme is denatured.
4. The active site and substrate are no longer complementary in shape.
5. Less enzyme-substrate complexes form- rate decreases.

Question 17

Explain the effect that a change in H+ concentration has on enzymes (5 points).

Answer 17

1. Disrupt/break hydrogen and ionic bonds.
2. Change charge of amino acids in the active site.
3. Change tertiary structure and shape of the active site- enzyme is denatured.
4. The active site and substrate are no longer complementary in shape.
5. Less enzyme-substrate complexes form- rate decreases.

Question 18

Describe the method into the investigation into the effects of pH on amylase action (7 points).

Answer 18

1. Cut wells into the agar plate using a cork borer.
2. Place the same volume of a range of pH buffers in each well.
3. Place identical volumes of amylase solution into each well except one.
4. To the empty well, add an equal volume of distilled water as a control.
5. Incubate for 24 hrs in a dry oven at 35 degrees.
6. Flood the plate with iodine solution and rinse with water.
7. Measure the diameter of the cleared zone- this would give a measure of how much substrate had been turned into product.

Question 19

Control variables for enzyme reaction (3 points).

Answer 19

1. The concentration of the substrate.
2. The concentration of the enzyme.
3. The temperature as it can affect the rate of reaction.

Question 20

Explain the effect of substrate concentration on enzymes reactions (5 points).

Answer 20

1. As the concentration of substrate molecules increases, collisions between the substrates and active sites occur more often.
2. More enzyme-substrate complexes form so more products form which increases the rate of reaction.
3. Until a point is reached when all the active sites are occupied with substrates.
4. Further increases in substrate concentration has no effect on increasing the rate.
5. The enzymes are working at their fastest rate; Vmax.

Question 21

Explain the effect of having excess enzyme concentration on enzyme reactions (3 points).

Answer 21

1. When substrate is in excess. As the enzyme concentration increases the number of active sites increases.
2. More substrate molecules can bind to the active sites to form enzyme-substrate complexes.
3. So, more products form which increases the rate of reaction.

Question 22

What are enzyme inhibitors? (3 points).

Answer 22

1. These are substances which reduce the rate or stop enzyme catalysed reactions because they have some effect on the enzyme molecule.
   2.They can be temporary or permanent.
2. Inhibitors can be competitive or non-competitive.

Question 23

What are competitive inhibitors? (2 points).

Answer 23

1. These bind to the active site of the enzyme.
   2.They have a similar molecular shape to the substrate.

Question 24

Explain the role of competitve inhibitors (5 points).

Answer 24

1. These have a similar molecular shape to the substrate.
2. This allows them to occupy the active site of an enzyme.
3. Therefore, they compete with the substrate for the available active sites.
4. They occupy the active sites, which prevents formation of enzyme-substrate complexes.
5. This reduces the rate of reaction.

Question 25

What are non-competitve inhibitors?

Answer 25

These bind to the enzyme away from the active site at the allosteric enzyme.

Question 26

Explain how non-competetive inhibitors work (2 points).

Answer 26

1. Binding of the non-competetive inhibitor disrupts the tertiary structure of the enzyme and changes the 3D shape of the active site.
2. This means the substrate is no longer complementary and can’t bind to form enzyme-substrate complexes.

Question 27

Describe the effect that pH has on enzymes (2 points).

Answer 27

1. The pH where the rate of reaction is fastest is called the optimum pH.
2. Reducing or increasing the pH away from the optimum pH decreases the rate of reaction.

Question 28

Describe how temperature effects enzymes (3 points).

Answer 28

1. As the temperature increases, the rate of reaction increases slowly, then more quickly.
2. The temperature where the rate is the fastest called the optimum temperature.
3. Increasing above the optimum causes the rate of reaction to decrease rapidly.

Question 29

Describe the effect of substrate concentration on the rate of an enzyme reaction (2 points).

Answer 29

1. As the substrate concentration increases, the rate of reaction increases.
2. Until a certain point when increasing the substrate concentration will not further increase the rate.

Question 30

Describe the effect of enzyme concentration being in excess on enzyme reactions (3 points).

Answer 30

1. When substrate is in excess.
2. As the enzyme concentration increases the rate of reaction increases.
3. The rate of reaction is directly proportional to enzyme concentration.

Question 31

Describe the effect of enzyme concentration being limited on enzyme reactions. (2 points).

Answer 31

1. When substrate is limiting.
2. Until a certain point, when increasing the enzyme concentration has no further effect on the rate.

Question 32

Explain the effect of enzyme concentration being limiting on enzyme reactions.

Answer 32

There are more active sites than available substrate molecules to bind to them and the Vmax is reached.