3.1.3 General proteins and enzymes

Question 1

Describe and explain Protein Structure (7)

Answer 1

1. Polymer of amino acids;
2. Joined by peptide bonds;
3. Formed by condensation;
4. Primary structure is order of amino acids;
5. Secondary structure is folding of polypeptide
   chain due to hydrogen bonding; (into alpha
   helix or beta pleated sheet)
6. Tertiary structure is 3-D folding due to
   hydrogen bonding and ionic/disulphide bonds
   between R groups;
7. Quaternary structure is more than one
   polypeptide chains;

Question 2

Describe the Test for a protein

Answer 2

1. Add Biuret reagent to the sample (1);
2. colour change to lilac (1) (or lilac band
   appears

Question 3

Describe and explain the “Induced Fit” Model (3)

Answer 3

1. (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;
3. Stressing/distorting/bending bonds (in
   substrate leading to reaction);

Question 4

Describe the effect of Increased temperature
on reaction rate (4)

Answer 4

1. particles have more kinetic energy
2. therefore they move more
3. so there are more collisions between
   substrates and active sites
4. so more ES complexes form

Question 5

Describe and explain Denaturation (5)

Answer 5

1. Heat above the optimum breaks hydrogen
   bonds
2. this causes the tertiary structure to unfold
3. so the active site changes shape
4. substrate can no longer bind to the active
   site, as it’s no longer complementary
5. so fewer ES complexes form

Question 6

Describe the Effect of Changes in pH on enzymes
(4)

Answer 6

1. Ionic bonds holding tertiary structure break
2. active site distorts and substrate no longer
   binds to active site
3. charges on amino acids in active site affected
4. fewer ES complexes form

Question 7

Describe the effects of Concentration of
Substrate on enzymes (2)

Answer 7

1. (Rate of) increase in concentration of product
   slows as substrate is used up OR High initial rate
   as plenty of
   substrate/more E-S complexes;
2. No increase after 25 minutes/at end/levels
   off because no substrate left;
   Reject ref. to enzyme being used up

Question 8

Describe and explain
the temperature graph of enzyme
rate (5)

Answer 8

1. Initial rate of reaction faster at 37 °C (than 25
   °C);
2. Because more kinetic energy;
3. So more E–S collisions/more E–S complexes
   formed;
4. Graph reaches plateau /levels off at 37 °C;
5. Because all substrate used up;

Question 9

Compare Competitive and Non Competitive
Inhibition (4)

Answer 9

1. Competitive inhibitor binds to active sites of
   enzyme but non-competitive inhibitor binds at
   allosteric site/away
   from active site;
2. (Binding of) competitive inhibitor does not
   cause change in shape of active site but
   (binding of) non-competitive
   does (cause change in size of active site);
3. So with competitive inhibitor, at high
   substrate concentrations (active) enzyme still
   available but with noncompetitive inhibitor
   (active) enzymes no longer available;
4. At higher substrate concentrations likelihood
   of enzyme-substrate collisions increases with
   competitive inhibitor
   but this is not possible with non-competitive
   inhibitor;