Enzymes

Question 1

What are the four factors that affect the rate of an enzyme catalysed reaction?

Answer 1

• Enzyme concentration
• Substrate concentration
• pH
• Temperature

Question 2

What is pH?

Answer 2

A measure of the hydrogen ion (H+) concentration.

Question 3

What does a buffer solution do?

Answer 3

Resists changes in pH to keep the pH of a solution stable.

Question 4

How do you calculate pH?

Answer 4

-log10 [H+]

Question 5

Why does an enzyme catalyse one specific reaction?

Answer 5

The active site of the enzyme has a specific tertiary structure and is only complementary to the specific substrate. Only the substrate can bind to the active site to form an enzyme substrate complex.

Question 6

Why are enzymes classed as biological catalysts?

Answer 6

They lower the activation energy of a reaction so the reaction happens at a faster rate.

Question 7

What is an enzyme?

Answer 7

A protein that is a catalyst for a reaction involving a specific substrate as it lowers the activation energy.

Question 8

Describe the induced fit model.

Answer 8

• The enzyme’s active site is not completely complementary to the substrate.
• Instead the active site undergoes a conformational change when exposed to the substrate (changes shape slightly) to become complementary to the substrate as it binds.
• This causes bond strain is the substrate which decreases the activation energy.

Question 9

What is the effect of enzyme concentration on the rate of enzyme-controlled reactions?

Answer 9

• A high enzyme concentration increases the frequency of successful collision.
• This leased to an increase in the number of enzyme substrate complexes being formed.
• As a result there is an increase in the rate of reaction.
• However, at very high enzyme concentrations the rate will slow and plateau since the number of enzyme molecules starts to exceed the number of substrate molecules.

Question 10

What is the effect of substrate concentration on the rate of enzyme-controlled reactions?

Answer 10

• As substrate concentration increases the frequency of successful collisions also increases.
• This leads to an increase in the number of enzyme substrate complexes being formed.
• As a result there is an increase in the rate of reaction.
• However, when the substrate concentration becomes very high the enzyme becomes saturated with substrate as all the active sits are occupied therefore a further increase in substrate concentration will not result in any more enzyme substrate complexes being formed as the reaction is at a maximum rate.

Question 11

What is the effect of pH on the rate of enzyme-controlled reactions?

Answer 11

• Changes in pH changes the charges on the R groups in the active site which affects the binding of the substrate to the active site.
• Changes in pH changes the charges on the R groups in the protein and affects the hydrogen bonds and ionic bonds that stabilise the tertiary structure leading to denaturation.

Question 12

What is the effect of temperature on the rate of enzyme-controlled reactions?

Answer 12

• As temperature increases, the rate of reaction increases due to increased kinetic energy of enzyme and substrate molecules, so there are more collisions and more enzyme substrate complexes are formed.
• However, above the optimal temperature, the rate of reaction decreases due to the increased kinetic energy breaking the hydrogen / ionic bonds between the amino acids’ R groups which changes the tertiary structure of the active site so that is is no longer complementary to the substrate.

Question 13

What are inhibitors?

Answer 13

Substances that directly or indirectly interfere with the functioning of the active site of a specific enzyme and so decrease the rate of reaction.

Question 14

What are the two types of inhibitors?

Answer 14

• Competitive
• Non-competitive

Question 15

Describe how a competitive inhibitor works.

Answer 15

• The inhibitor is a similar shape to the substrate (complementary to the active site).
• The inhibitor binds to the active site.
• This prevents / reduces the enzyme substrate complexes forming.

Question 16

How is the rate of reaction affected by a competitive inhibitor?

Answer 16

• The rate of reaction is reduced as the inhibitor competes with the substrate.
• The maximum rate can be achieved if the substrate levels are high enough.

Question 17

Describe how a non-competitive inhibitor works.

Answer 17

• The non-competitive inhibitor attaches to the enzyme at the allosteric site.
• This changes the shape of the active site / the tertiary structure of the enzyme.
• The active site and substrate are no longer complementary so fewer / no substrate can bind and so fewer / no enzyme-substrate complexes form.

Question 18

How is the rate of reaction affected by a non-competitive inhibitor?

Answer 18

• The rate of reaction is reduced as the enzyme active site is altered.
• The maximum rate can’t be achieved by raising the substrate levels.