3.1.4.2 Enzymes

Question 1

Describe what is meant by an enzyme

Answer 1

• Enzymes are biological catalysts, which lower the activation energy of a reaction, enabling it to occur more quickly
• Enzymes are not used up

Question 2

Describe what an enzyme controlled reaction is determined by

Answer 2

• The number of enzyme substrate complexes that form
• The more enzyme substrate complexes that form, the lower the
  activation energy of the substrate, and therefore the faster the rate of reaction.

Question 3

What are the 4 factors that affect the rate of an enzyme controlled reaction

Answer 3

• Temperature
• PH
• Enzyme concentration
• Substrate concentration

Question 4

Explain the induced fit model of enzyme action

Answer 4

• Initially, the enzyme’s active site and substrate are not exactly complementary.
• When the substrate binds to the active site, the active site changes shape, so that it’s complementary to the substrate.
• As the active site changes shape, stress is placed on the bonds of the substrate, lowering the activation energy of the reaction.

Question 5

Describe and explain the graph showing the relationship between enzyme activity and temperature

Answer 5

• As the temperature increases, the rate of reaction increases because particles gain more KE, and so collisions between the substrate and active site are more likely to occur. This means that more enzyme substrate complexes form.
• As the temperature increases above the optimum, the hydrogen bonds in the tertiary structure are overcome, causing the shape of the tertiary structure, and therefore the active site to change. This means that the active site is no longer complimentary to the substrate, and so less enzyme substrate complexes form.

Question 6

Describe and explain the graph showing the relationship between enzyme activity and pH

Answer 6

• Changes in hydrogen ion concentration in the external
  solution, can change the charges of the amino acids present, which affects the hydrogen and ionic bonds in the tertiary structure. This changes the shape of the tertiary structure, as well as the shape of the active site, so it’s no longer complimentary to the active site, therefore less enzyme substrate complexes form.

Question 7

Describe and explain the graph showing the relationship between enzyme activity and enzyme concentration

Answer 7

• As the enzyme concentration increases, the rate of reaction also increases. This is because there are more active sites, and therefore more enzyme substrate complexes form
• However, as the enzyme concentration increases further, the rate of reaction plateaus. This is because the substrate concentration
  becomes the limiting factor, and so no additional enzyme substrate complexes can form.

Question 8

Describe and explain the graph showing the relationship between enzyme activity and substrate concentration

Answer 8

As the substrate concentration increases, the rate of reaction
also increases because more enzyme substrate complexes form. However, as the substrate concentration increases further, the rate of reaction plateaus because the enzyme
concentration becomes the limiting factor, and so no additional enzyme substrate complexes form.

Question 9

Describe what enzyme inhibitors do

Answer 9

Inhibitors reduce enzyme activity, either permanently or temporarily

Question 10

What are the 2 types of inhibitors

Answer 10

• Competitive
• Non-competitive

Question 11

Explain competitive inhibition of enzymes

Answer 11

In competitive inhibition, the inhibitor has a similar shape to the substrate, so it’s able to bind to the active site in place of the substrate. Therefore, fewer enzyme substrate complexes form because the active site is blocked by the inhibitor.

Question 12

Describe how the effects of competitive inhibition can be reduced

Answer 12

• Increasing the substrate concentration

Question 13

Explain non-competitive inhibition of enzymes

Answer 13

In non-competitive inhibition, the inhibitor doesn’t bind to the
active site. Instead, it binds to another part of the enzyme called the allosteric site, which causes the shape of the tertiary structure to change, which changes the shape of the active
site. Therefore, fewer enzyme substrate complexes form since
the active site and substrate are no longer complimentary

Question 14

Explain why increasing the substrate concentration does not reduce the effect of non-competitive inhibition

Answer 14

Non-competitive inhibitors do not compete for the active site

Question 15

Sucrase does not hydrolyse lactose. Use your knowledge of
the way in which enzymes work to explain why.

Answer 15

• Enzymes have a specific shape which is complementary to their substrates.
• Lactose has a different structure/shape to sucrose and therefore lactose will not bind to the active site of sucrase.

Question 16

A covering of lignin protects cellulose from enzyme attack.
Use your understanding of the way in which enzymes work to explain why cellulose-digesting enzymes do not digest lignin.

Answer 16

Enzymes are specific; the shape of lignin molecules will not fit the active site of the enzyme

Question 17

Explain how increasing substrate concentration affects
rate of reaction when a competitive inhibitor is
affecting the enzymes

Answer 17

As substrate concentration increases, so does the rate of reaction. This is because as more substrates are present, and so the chance of substrates colliding with the active site, rather than the competitive inhibitor increases. Therefore, more enzyme substrate complexes form.

Question 18

Explain how increasing substrate concentration affects
rate of reaction when a noncompetitive inhibitor is
affecting the enzymes

Answer 18

Increasing substrate concentration will have no effect on the rate of reaction if a non-competitive inhibitor is present. Non-competitive inhibition changes the shape of the active site so that it is no longer complementary to the shape of the substrate, therefore no additional enzyme substrate complexes can form.

Question 19

Biological washing powders contain enzymes which
hydrolyse substances that cause stains on clothes. Many
of the substances causing the food stains are large, insoluble proteins. Suggest how a biological
washing powder removes this type of stain.

Answer 19

• Enzyme hydrolyses / breaks down protein to amino acids;
• Products are soluble / can be washed away;

Question 20

Biological washing powders contain enzymes which
hydrolyse substances that cause stains on clothes. They
contain several types of enzymes. This enables them to
remove a wider range of stains from clothes.
Explain why a number of enzymes are required to
remove a wider range of stains.

Answer 20

• Stains caused by different substances;
• Enzymes are specific;
• Active site specific to substrate / other substrates cannot fit active site;

Question 21

Methotrexate is a drug used in the treatment of cancer. It is a competitive inhibitor and affects the enzyme folate
reductase. Explain how the drug lowers the rate of reaction
controlled by folate reductase and Explain why this drug does not affect other enzymes.

Answer 21

Methotrexate / drug is a similar shape / structure to substrate so binds to / fits / is
complementary to active site, therefore Less substrate binds / less enzyme-substrate
complexes formed. Methotrexate / drug is only similar shape to specific substrate /
only fits this active site so will not affect other enzymes

Question 22

Alzheimer’s disease leads to
dementia. This involves small βamyloid proteins binding together to form structures called plaques in the brain. Nerve cells in the brain produce a large protein called amyloidprecursor protein that has a complex shape. This protein is the substrate of two different enzymes, α-secretase and β-secretase. These enzymes are
normally produced in the brain. One product of the reaction catalysed by β-secretase is a smaller protein
that can lead to β-amyloid protein formation. Many people with Alzheimer’s disease have mutations that decrease
α-secretase production, or increase β-secretase
production.

Suggest how amyloid-precursor protein can be the substrate of two
different enzymes, α-secretase and β-secretase

Answer 22

• The amyloid precursor protein is a large molecule made from several different parts/amino acid sequences.
• Each enzyme is specific/fits/binds/complementary to a different part of the amyloid precursor protein;

Question 23

Alzheimer’s disease leads to
dementia. This involves small βamyloid proteins binding together to form structures called plaques in the brain. Nerve cells in the brain produce a large protein called amyloidprecursor protein that has a complex shape. This protein is the substrate of two different enzymes, α-secretase and β-secretase. These enzymes are normally produced in the brain. One product of the reaction catalysed by β-secretase is a smaller protein that can lead to β-amyloid protein formation. Many people with Alzheimer’s disease have mutations that decrease α-secretase production, or increase β-secretase production.
Explain how this type of drug could
prevent Alzheimer’s disease
becoming worse.

Answer 23

• Inhibitor binds to/blocks active site of β-secretase/enzyme;
• Stops/reduces production of β-amyloid/plaque;