Enzymes

Question 1

What is an enzyme?

Answer 1

A biological catalyst, that speeds up the rate of reaction by lowering the activation energy needed for the reaction to occur and without being used up.

Question 2

Explain the lock and key model?

Answer 2

The shape of the active site is exactly complementary to the specific substrate and the substrate fits in exactly as it binds to the active site forming an enzyme-substrate complex. The enzyme substrate complex places stress on the bonds causing them to form or break, leading to the reaction and products being formed. The products are released and the enzyme is unchanged.

Question 3

Explain the induced fit model?

Answer 3

Before the reaction the shape of the substrate is not complementary to the active site. The substrate binds to the active site causing the shape of the active site to change so it becomes complementary to the substrate. An enzyme-substrate complex then forms causing the bonds to form or break due to the bonds being stressed, leading to the reaction and products being formed. The products are released and the enzyme returns to its original position.

Question 4

What are the similarities between the lock and key and induced fit model?

Answer 4

In both the substrate binds to the active site and an enzyme-substrate complex is formed.

Question 5

What are the differences between the lock and key and induced fit model?

Answer 5

In the induced fit model the active site isn’t complementary at the start, but in the lock and key model the active site is complementary.
The active site changes shape when the substrate binds in the induced fit model but does not in the lock and key model.

Question 6

How do enzymes speed up the rate of reaction?

Answer 6

All reactions require a certain amount of energy before they will occur. Enzyme work by lowering the activation energy needed. This makes the reaction occur faster and also allows the reaction to occur at lower temperatures.

Question 7

How do enzymes lower the activation energy?

Answer 7

By forming an enzyme-substate complex. When the substrate binds to the active site, the active site changes to become a complementary shape to the substate to form an enzyme-substate complex. This stresses the bonds in the substrate allowing them to break or form more easily meaning less energy is needed.

Question 8

How does increasing the temperature up to the enzymes optimum effect the enzymes activity?

Answer 8

The enzymes and substrates have more kinetic energy, so move faster, so an increased frequency of successful collisions, so an increase in enzyme-substrate complexes forming, so an increased rate of reaction.

Question 9

How does carrying out the reaction at the enzymes optimum temperature effect the rate of reaction?

Answer 9

The maximum number of successful collisions occurring, so the maximum number of enzyme-substrate complexes forming.

Question 10

How does increasing the temperature past the enzymes optimum effect enzyme activity?

Answer 10

The enzymes gain even more kinetic energy, causing the hydrogen bonds that hold the tertiary structure together to break, this changes the tertiary structure, which causes changes in the active site. At first the substrate fits less easily into the active site so fewer enzyme-substrate complexes form so the rate of reaction decreases. At even higher temperatures the enzymes denature because so many hydrogen bonds are broken, the active site is not complementary shaped, so no enzyme-substrate complexes form and the rate of reactions decreases further.

Question 11

How does carrying out the reaction at the enzymes optimum pH effect the rate of reaction?

Answer 11

The maximum number of successful collisions so the maximum number of enzyme-substrate complexes form because the active site is most complementary to the substrate.

Question 12

How does increasing the pH above or below the optimum effect the enzyme activity?

Answer 12

Below the optimum H+ increases and above the optimum H+ decreases. H+ ions are attracted to amino acids that form the active site which alters their charge. This causes the hydrogen and ionic bonds holding the tertiary structure together to break , so the tertiary structure changes, so the active site changes shape, so it is no longer complementary to the substrate, so it can’t bind, which means less or no enzyme-substrate complexes form and the rate of reaction decreases.

Question 13

Explain the rate of reaction at low enzyme concentrations?

Answer 13

When there is an excess of substrate the enzyme concentration limits the rate of reaction. There are not enough active sites available for all the substrates to bind to at one time so less enzyme-substrate complexes form at one time. The enzymes active sites are saturated.

Question 14

Explain the rate of reaction as you increase the enzyme concentration?

Answer 14

Increasing the enzyme concentration means there are more active sites available for substrates to bind to. This means more enzyme-substrate complexes form at one time.

Question 15

Explain the rate of reaction as you increase the enzyme concentration further?

Answer 15

Enzyme concentration is no longer the limiting factor and substrate concentration is. There are now empty active sites as there is not enough substrate available to bind to all active sites at one time.

Question 16

Explain the rate of reaction at low substrate concentrations?

Answer 16

Fewer collisions occur between enzyme and substrate so less enzyme-substrate complexes form because there are too few substrates to bind to all the available active sites at one time.

Question 17

Explain the rate of reaction as you increase the substrate concentration?

Answer 17

Increasing the substrate concentration increases the number of collisions between enzymes meaning more enzyme-substrate complexes form.

Question 18

Explain the rate of reaction as you increase the substrate concentration further?

Answer 18

Substrate concentration is no longer the limiting factor and enzyme concentration is. All active sites are occupied at any one time meaning the maximum number of enzyme-substrate complexes are being formed. The active sites are saturated.

Question 19

What are enzyme inhibitors?

Answer 19

Molecules that reduce the rate of reaction of enzyme catalysed reactions. They do this by reducing the number of enzyme-substrate complexes that can be made at any one time.

Question 20

Describe how competitive inhibitors work?

Answer 20

They are a similar shape to the substrate, so they can bind to the active site instead of the substrate. Less substate can now bind to the active site so less enzyme-substate complexes form and the rate of reaction decreases.

Question 21

What would happen if you increase the concentration of the competitive inhibitor but the substrate concentration remains fixed?

Answer 21

The rate of reaction would decrease because as the inhibitor concentration increases, the inhibitors are more likely to bind to the active site than the substrate so less enzyme-substrate complexes form.

Question 22

What would happen to the rate of reaction if you increased the concentration of substrate but the concentration of inhibitor remained fixed?

Answer 22

The rate of reaction increases because there is a higher chance of the substrate binding to the active site than an inhibitor so more enzyme-substrate complexes form.

Question 23

Describe how non-competitive inhibitors work?

Answer 23

They do not have a similar shape to the substrate and therefore bind to anywhere on the enzyme but not the active site. This causes the tertiary structure of the enzyme to change which changes the shape of the active site. The active site is no longer a complementary shape to the substrate so the substrate can’t bind. This means that less enzyme-substrate complexes will be formed and the rate of reaction is reduced.

Question 24

What would happen to the rate of reaction if you increased the concentration of non-competitive inhibitor but the substrate concentration remained fixed?

Answer 24

The rate of reaction would decrease because there are more non-competitive inhibitor which will bind to the enzyme and cause a change in the tertiary structure, which changes the shape of the active site so it is no longer a complementary shape so the substrate can’t bind and less enzyme-substrate complexes will form.

Question 25

What would happen to the rate of reaction if you increased the concentration of substrate but the concentration of non-competitive inhibitor remained fixed?

Answer 25

The rate of reaction will stay the same as you can still only form the same number of enzyme-substrate complexes at one time because there will be the same number of incorrectly shaped active site.

Question 26

Compare and contrast competitive and non-competitive inhibitors?

Answer 26

Competitive- similar shape to the substrate, bind to the active site. They decrease the rate of reaction by binding to the active site instead of the substrate so less substrates can bind so less enzyme-substrate complexes form. When substrate concentration increases the rate of reaction increases as there is a higher chance of a substrate binding than an inhibitor so more enzyme-substrate-complexes will form.
Non-competitive- Different shape to the substrate, bind to anywhere but the active site (allosteric site). They decrease the rate of reaction by changing the shape of the active site when they bind, by altering the tertiary structure so it is no longer a complementary shape to the substrate so enzyme substrate complexes can’t form. When substrate concentration increases the rate of reaction will remain constant, the same number of enzyme-substrate complexes will form at any one time as there will be the same number of incorrectly shaped active sites.