PROTEINS AND ENZYMES

Question 1

Describe the structure of
proteins (5)

Answer 1

· Polymer of amino acids; · Joined by peptide bonds; · Formed by condensation reactions; · Primary structure is number AND order of amino acids; · Secondary structure is folding of polypeptide chain into Alpha-helix and Beta-pleated sheets due to hydrogen bonding; · Tertiary structure is 3-D folding due to hydrogen bonding and ionic bonding and disulfide bridges; · Quaternary structure is two or more polypeptide chains joined together;

Question 2

Describe how a peptide bond is formed between two amino acids to form a dipeptide

Answer 2

1. Condensation (reaction) / loss of water;
2. Between amine / NH2 and carboxyl / COOH;

Question 3

Describe how an enzyme-substrate complex increases the rate of reaction

Answer 3

1. Reduces activation energy
2. Due to bending/distorting bonds OR Without the enzyme, very few substrates have sufficient energy for the reaction.

Question 4

Describe how a change in the base sequence of the DNA coding for an enzyme may result in a non-functional protein. (4)

Answer 4

1. Change in primary structure changes
   sequence of amino acids;
2. Hydrogen bonds and Ionic bonds and Disulphide bonds form in different positions;
3. Alters the tertiary structure of the enzyme / alters shape of active site;
4. No Enzyme-Substrate complexes can be formed;

Question 5

What is the proteome of a cell?

Answer 5

(The proteome is the full) range of / number of different proteins that a cell is able to produce (at a given time);
OR
(The proteome is the full) range of / number of different proteins the genome / DNA is able to code for;

Question 6

When a pathogen causes an infection, plasma cells secrete antibodies which destroy this pathogen.
Explain why these antibodies are only effective against a specific pathogen.(2)

Answer 6

• Antigens (on pathogen) are a specific shape/ have specific tertiary / 3D structure;
• Antibody fits/binds / is complementary to antigen/ antibody-antigen complex forms; OR Antibodies are a specific shape / have specific tertiary/ 3D structure;
• Antigens (on pathogen) fit/ bind/ are complementary to antibody / antibody-antigen complex forms;

Question 7

Describe & explain how you could use the biuret test to distinguish a solution of enzyme, lactase, from a solution of lactose(2)

Answer 7

1. Add Biuret reagent to both solutions) – no
   mark;
2. Lactase / enzyme will give purple / lilac / mauve;
   OR
3. Lactose / reducing sugar will not give purple /
   lilac /mauve / will remain blue;
4. Because Lactase is a protein;

Question 8

Sucrase does not hydrolyse lactose. Use your knowledge of the way in which enzymes work to explain why.(3)

Answer 8

1. Lactose has a different shape/structure;
2. Does not fit/bind to active site of enzyme/sucrase;
   OR
3. Active site of enzyme/sucrase has a specific shape/structure;
4. Does not fit/bind to lactose so no Enzyme-Substrate Complexes formed.

Question 9

Describe the induced fit model of enzyme action.(2)

Answer 9

1. Active site not complementary;
2. Active site changes (shape) / is flexible;
3. (Change in enzyme allows) substrate to able to fit / Enzyme-Substrate complex to form;

Question 10

Describe one way that the lock and key model is different from the induced fit model(2)

Answer 10

1. Active site does not change (shape) / is fixed (shape) / is rigid / does not wrap around
2. substrate / (already) fits the substrate / is
3. complementary (before binding);

Question 11

An enzyme catalyses only one reaction. Explain why. 1.

Answer 11

(Enzyme has) active site is a specific shape;
2. Only one substrate fits / binds (the active site);

Question 12

Diabetes mellitus is a disease that can lead to an increase in blood glucose concentration. Some diabetics need insulin injections. Insulin is a protein so it cannot be taken orally. Suggest why insulin cannot be taken orally.(2)

Answer 12

1. Broken down by enzymes / digested / denatured (by pH) /too large to be absorbed;
2. Insulin no longer functional

Question 13

What is the effect of substrate concentration on the rate of an enzyme controlled reaction(3)

Answer 13

1. Increases then plateaus / constant / steady / rate does not change;
2. It plateaus as all active sites occupied / Saturated;
3. (rate of reaction) / maximum number of Enzyme-Substrate complexes per second;

Question 14

Explain how a competitive inhibitor works

Answer 14

1. Inhibitor is a similar shape to substrate;
2. Inhibitor enters active site / competes with substrate;
3. Less substrate binds/fewer enzyme-substrate complexes form per second.

Question 15

Describe how a non-competitive inhibitor works (3)

Answer 15

1. Attaches to the enzyme at at allosteric site
2. Changes (shape of) the active site by Changing tertiary structure.
3. (So active site and substrate) no longer complementary so less/no substrate can bind so less/no enzyme-substrate complexes form’);