Proteins and enzymes

Question 1

Describe the structure of proteins.

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 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.

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

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 5

• 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 6

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

Answer 6

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 7

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

Answer 7

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 8

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

Answer 8

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 9

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

Answer 9

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 10

An enzyme catalyses only one reaction. Explain why.

Answer 10

1. (Enzyme has) active site is a specific shape;

2. Only one substrate fits / binds (the active site);

Question 11

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 11

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

Question 12

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

Answer 12

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 13

Explain how a competitive inhibitor works

Answer 13

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 14

Describe how a non-competitive inhibitor works

Answer 14

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

Question 15

What is the proteome of a cell ?

Answer 15

(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;