Enzymes

Question 1

Describe the induced fit model of enzyme action

Answer 1

(before the reaction) Active site not
complementary to substrate;

Active site changes shape as the substrate binds, so the active site becomes complementary to the substrate

So an E-S complex can form;

This stresses/distorts bonds in substrate, leading to a reaction

Question 2

Explain how the shape of an enzyme molecule is related to its function.

Answer 2

Enzymes have a specific 3D tertiary structure/shape;

The active site is a complementary shape to the substrate

Therefore the substrate can bind to the active site

Question 3

Explain why enzymes only act on one substrate

Answer 3

Active site (complementary / specific) structure / shape;

(Only) fits / binds to one substrate;

Forms enzyme-substrate complexes;

OR

Active site (complementary / specific) structure / shape;

(Does not) fit / bind with other substrates;

Does not form enzyme-substrate complexes;

Question 4

Explain how temperature affects rate of reaction

Answer 4

As the temperature increases, the rate of
reaction increases

Enzymes and substrates have more kinetic
energy,

So there are more collisions

More enzyme-substrate complexes are formed

If temperature exceeds the optimum, rate of reaction decreases

Hydrogen bonds break

Tertiary structure changes

Shape of active site changes

No enzyme-substrate complexes can form

Question 5

Explain how decreasing temperature can affect an enzyme reaction

Answer 5

Lower temperature means less kinetic energy

fewer collisions between enzyme and substrate

fewer E–S complexes formed;

Question 6

Explain how increasing temperature above the optimum affects an
enzyme’s activity.

Answer 6

1.Enzyme denatured / hydrogen bonds / bonds holding tertiary structure
broken / tertiary structure changed;

2. Change in shape of active site (of enzymes);
3. Substrate / protein no longer fits / binds (into active site) / few or no ES
   complexes;

Question 7

Explain how dramatically decreasing the pH affects enzyme activity

Answer 7

A decrease in pH increases the concentration of H+ ions;

This changes the charges of the amino acids

So hydrogen and ionic bonds are disrupted;

So the tertiary structure of the enzyme changes / the enzyme is denatured

So the shape and/or charge of active site is
changed;

Therefore enzyme-substrate complexes can no longer form

So rate of enzyme activity decreases

Question 8

Explain how substrate concentration affects the rate of reaction

Answer 8

As substrate concentration increases, the
rate of reaction increases

Because substrate concentration is the
limiting factor

As substrate concentration increases
further, rate of reaction stays constant

Because all the active sites are occupied the number of enzymes is the limiting
factor

Question 9

Explain how enzyme concentration affects the rate of reaction

Answer 9

As enzyme concentration increases, the rate of reaction increases

Because enzyme concentration is the limiting factor

As enzyme concentration increases further,
rate of reaction stays constant

Because the amount of substrate is the limiting factor

Or the temperature is not high enough for the reaction to happen any faster

Question 10

Use your knowledge of protein structure to explain why enzymes are specific
and may be affected by non-competitive inhibitors.

Answer 10

1 each enzyme has specific primary structure / amino acid sequence;

2 so it folds in a particular way/ has particular tertiary structure;

3 this forms an active site with unique shape/structure;

4 the shape of active site is only complementary to one specific substrate;

5 a non-competitive-inhibitor binds to site on the enzyme other than active site ;

6 this changes the same of the active site

7 So no more enzyme-substrate complexes can form

Question 11

Describe how competitive and non-competitive inhibitors can limit
enzyme activity

Answer 11

Competitive

Inhibitor is a SIMILAR shape to the substrate

Inhibitor is complementary to the shape of the ACTIVE SITE

So inhibitor binds to the active site

So fewer E-S complexes are formed

Non-competitive

Inhibitor is complementary to the ALLOSTERIC SITE on the enzyme

So inhibitor binds to the allosteric site

So the shape of the active site changes

So the active site is no longer complementary to the substrate

So fewer E-S complexes are formed

Question 12

In humans, the enzyme maltase breaks down maltose to glucose. This takes place at normal body
temperature. Explain why maltase:

• only breaks down maltose
• allows this reaction to take place at normal body temperature

Answer 12

1. Tertiary structure / 3D shape of enzyme (means);
2. Active site complementary to maltose / substrate / maltose fits into active site / active site and substrate fit like a lock and key;
3. Description of induced fit;
4. Enzyme is a catalyst / lowers activation energy / energy required for reaction;
5. By forming enzyme-substrate complex;

Question 13

Enzymes can be trapped within gel beads. Suggest two advantages of
trapping enzymes within gel beads

Answer 13

The enzymes do not get mixed with the product

The enzymes can be reused