enzymes

Question 1

Scientists have investigated the effects of competitive and non-competitive inhibitors of the enzyme maltase.

Describe and non-competitive inhibition of an enzyme.

Answer 1

Inhibitor binds to site on enzyme other than active site;

6. Prevents formation of active site / changes (shape of) active site;

Accept does not allow max rate of reaction to be reached / max product will not be formed

7. Cannot be overcome by adding more substrate

Question 2

Scientists have investigated the effects of competitive and non-competitive inhibitors of the enzyme maltase.

Describe and -competitive inhibition of an enzyme.

Answer 2

Inhibitors reduce binding of enzyme to substrate / prevent formation of ES complex;

Max 3 if only one type of inhibition dealt with. Accept maltase and maltose as examples of enzyme and substrate (and others)

Only once, for either inhibitor

(Competitive inhibition),

2. Inhibitor similar shape (idea) to substrate;
3. (Binds) in to active site (of enzyme);

Accept allows max rate of reaction to be reached / max product will eventually be formed

Accept complementary to active site

4. (Inhibition) can be overcome by more substrate;

Question 3

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 3

Tertiary structure / 3D shape of enzyme (means);

Accept references to active site

2. Active site complementary to maltose / substrate / maltose fits into active site / active site and substrate fit like a lock and key;

Idea of shapes fitting together

3. Description of induced fit;
4. Enzyme is a catalyst / lowers activation energy / energy required for reaction;

Accept “provides alternative pathway for the reaction at a lower energy level”

5. By forming enzyme-substrate complex;

Question 4

Describe the induced-fit model of enzyme action.

Answer 4

(before reaction) active site not complementary to/does
not fit substrate;

2. Shape of active site changes as substrate binds/as
   enzyme-substrate complex forms;

Question 5

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

Answer 5

specific 3D tertiary structure / shape;
substrate complementary shape;

(reject same shape)

substrate (can bind) to active site / can fit into each active site;

Question 6

Cyclin D stimulates the phosphorylation of DNA polymerase, which activates the DNA polymerase.

Describe how an enzyme can be phosphorylated.

Answer 6

Attachment/association of (inorganic) phosphate (to the enzyme);

For ‘phosphate/Pi’ accept PO43– and P in a circle.

Accept ‘phosphate goes to the enzyme’.

Ignore named bonding or position of phosphate attaching to enzyme but reject formation of E-S complex.

2. (Released from) hydrolysis of ATP

Question 7

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

Answer 7

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

2 folds in a particular way / has particular tertiary structure giving an active site with a unique structure;

3 shape of active site complementary to / will only fit that of substrate;
maximum of three marks for inhibition, points 5 – 8

4 inhibitor fits at site on the enzyme other than active site;

5 distorts active site;

6 so substrate will no longer fit / form enzyme-substrate complex

Question 8

Explain how inhibitors affect the rate of enzyme-controlled reactions.

Answer 8

Competitive
2 Similarity of shape of inhibitor and substrate;
3 Inhibitor can enter / bind with active site (of enzyme);

Non-competitive
4 Affect / bind to enzyme other than at active site;
5 Distorts shape of active site;

Inhibitors
6 Prevent entry of / binding of substrate to active site;
7 Therefore fewer / no enzyme-substrate complexes formed

Question 9

Explain how heating an enzyme leads to it being denatured

Answer 9

More (kinetic) energy;
Bonds / specified bonds (holding tertiary structure) break;

Question 10

Describe how the sequence of amino acids in part of the protein from Job’s Tears could enable this protein to act as an enzyme inhibitor.

Answer 10

Sequence of amino acids gives shape;

2. This is tertiary structure;
3. Has similar shape to substrate;
4. Fits / competes for active site;
5. Fits at site other than active site;
6. Distorting active site;
7. Therefore substrate will not fit (active site);

Question 11

Explain why xanthine oxidase is able to catalyse this reaction but it is not able to catalyse other reactions.

Answer 11

Substrate has specific shape;

Allows binding / fitting / forms ES complex with active site;

Or

Active site has specific shape;

Allows binding / fitting / forms ES complex with substrate;

structure = shape

Question 12

Gangliosides are lipids found in the cell surface membranes of nerve cells. Hexosaminidase is an enzyme present in blood that breaks down gangliosides. If gangliosides are not broken down, they damage nerve cells.

(a) Hexosaminidase only breaks down gangliosides. It does not break down other lipids.

Explain why this enzyme only breaks down gangliosides.

Answer 12

Active site (complementary / specific) structure / shape;

(Only) fits / binds to gangliosides;

Forms enzyme-substrate complexes;