3.1.4 Proteins and Enzymes

Question 1

Draw the structure of an amino acid

Answer 1

See notes

Question 2

A polymer on amino acids

Answer 2

Polypeptide

Question 3

Describe how a dipeptide forms

Answer 3

1. Condensation reactions
2. Between carboxyl and amino group
3. Forms a peptide bond

Question 4

Primary sequence of protein

Answer 4

Number, sequence, order

Question 5

Secondary structure of protein

Answer 5

• Hydrogen bonds between carbons and amino group
• Causes a long polypeptide chain to twist in a 3D shape (coil)
• Forms alpha helix or beta pleated sheet

Question 6

Tertiary structure of proteins

Answer 6

• R groups in polypeptide chain determine how it folds into specific 3D shape
• Forms ionic, hydrogen or disulfide bonds

Question 7

Quaternary structure of proteins

Answer 7

Joining of 2 polypeptide chains

Question 8

Describe how to test for proteins

Answer 8

• Biurets reagent
• Blue to purple

Question 9

Lyxose is a sugar which binds to enzymes. It is not involved in inhibition.

Explain why lyxose increases the rate of reaction of enzymes

(4 marks)

Answer 9

1. Lyxose changes the tertiary structure
2. This induces a change in the shape of the active site
3. Making it more complementary
4. Allowing more enzyme substrate complexes to form.

Question 10

Explain how enzymes increase the rate of reaction

Answer 10

They lower the activation energy by putting pressure and bending bonds allowing reactions to take place at a lower temperature

Question 11

3 conditions needed for enzyme action

Answer 11

1. Enzymes and substrates must collide with sufficient energy
2. Requires activation energy
3. Free energy of products must be lower than reactants

Question 12

Describe the structure of enzymes (2)

Answer 12

Any 2 from
- Globular proteins with a specific 3D shape
- Contain an active site
- The active site is complementary allowing only one substrate to bind
- Enzyme substrate complexes can form

Question 13

Describe the difference between the lock and key model and induced fit model of enzyme action (2)

Answer 13

1. Lock and key model does not take into account that the active site is not rigid/fixed/only complementary to one substrate
2. The induced fit model causes the active site to induce a change in shape (making it more complementary)

Question 14

Describe the induced fit model of enzyme action (4)

Answer 14

1. Substrate is not complementary (to the active site)
2. Substrate enters the active site and induces a change in shape of the active site, so, an enzyme substrate complexes forms
3. This stresses the bonds lowering activation energy
4. When products leave, the active site returns to it’s original shape

Question 15

How can enzyme catalysed reactions be measured?

Answer 15

• Formation of products
• Disappearance of substrate

Question 16

Explain what would be seen in an enzyme catalysed reaction (5)

Answer 16

1. At first, lots of substrate but little product, substrate can easily make contact with empty active sites
2. Active sites are filled and substrate is rapidly broken down into products
3. Amount of substrate decreases, formation of product increases
4. Becomes difficult for substrate molecules to to contact the active site as products begin to obstruct it
5. Rate of reaction decreases, causing the graph to tail off

If no other mark awarded, allow:
- Graph flattens as all the substrate has been used up

Question 17

The effect of temperature on enzyme action

Answer 17

• Temperature increase: increased kinetic energy, higher frequency of collisions, more enzyme substrate complexes form, rate of reaction increases
• However, too high and it could causes hydrogen and ionic bonds between R groups of amino acids to break, so tertiary structure changes causing the active site to change shape, this denatures the enzyme

Question 18

State the definition of ‘denatured’

Answer 18

Permanent change, so the enzyme will never function again

Question 19

The effect of pH on enzyme action

Answer 19

• Alters charge of amino acids that form active site, substrate can’t bind, and enzyme substrate complexes cannot form
• Depending on how significant the change is, may cause bonds maintaining tertiary structure to break

Question 20

What does a buffer allow?

Answer 20

pH to remain constant

Question 21

How might isolation change an enzymes activity

Answer 21

Outside the optimum conditions of the host cell

Question 22

If substrate is limiting, how will this affect enzyme activity?

Answer 22

Increasing enzyme concentration will have no effect

Question 23

Explain how substrate concentration affects rate of enzyme action (enzyme has a fixed concentration)

Answer 23

1. As substrate concentration increases, the rate of reaction increases in proportion to concentration of substrate, because at low substrate concentrations, enzyme molecules have limited number of substrate molecules to collide with
2. As more substrate is added, active sites gradually fill, until they work at fastest rate, after this, adding more substrate has no effect

Question 24

Explain how a competitive inhibitor decreases the rate of reaction (3)

Answer 24

1. Have a similar shape to the substrate
2. (So) bind to the active site
3. Preventing enzyme substrate complexes forming

Question 25

How can a competitive inhibitor be overcome?

Answer 25

Increasing the substrate concentration

Question 26

Explain how a non competitive inhibitor decreases the rate of enzyme action (3)

Answer 26

1. Bind away from the active site
   allow allosteric site
2. Alter/change the shape of the active site so it is no longer complementary
3. Prevents enzyme substrate complexes forming

Question 27

Explain how enzymes with different amino acid sequences can catalyse the same reaction (2)

Answer 27

1. Both have similar tertiary structures
2. So can form enzyme substrate complexes

Question 28

List the four factors affecting enzyme controlled reactions (4)

Answer 28

1. Temperature (ignore heat)
2. pH
3. Substrate concentration
4. Enzyme concentration
   Accept in any order
   Ignore any reference to volume/amount/mass of substrate/enzyme