12.2 Proteins and Enzymes

Question 1

Structure of amino acid

Answer 1

• Amine group
• R group
• Carboxylic acid group
• Hydrogen

Question 2

What is a dipeptide?

Answer 2

• Two amino acids joined
• Via a condensation reaction
• Joined by a peptide bond

Question 3

Describe the structure of proteins

Answer 3

• 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 4

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

Answer 4

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

Question 5

Test for proteins

Answer 5

• Place the sample in a test tube
• Add equal volumes of Biuret solution
• Colour change to purple shows protein is present

Question 6

Enzyme

Answer 6

• Globular proteins which act as biological catalysts
• Increase the rate of reaction
• Lowering the activation energy
• Done by stressing AND distorting the bonds during the formation of the ESC

Question 7

Lock & Key model

Answer 7

• Active site is rigid and does not change shape
• Substrate binds to enzymes active site
• Substrate fits exactly to the complementary active site
• Products are formed so are released as no longer fit the active site
• Enzyme free to take part in another reaction

Question 8

Induced fit model

Answer 8

• Active site not complementary
• Active site changes shape as the substrate binds
• So stressed the substrate bonds to lower the Ea of the reaction
• Active site returns to its original shape

Question 9

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

Answer 9

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

Question 10

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

Answer 10

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 11

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

Answer 11

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 12

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

Answer 12

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 13

An enzyme catalyses only one reaction. Explain why.

Answer 13

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

Question 14

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 14

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

Question 15

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

Answer 15

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 16

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 16

• 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 17

Factors effecting enzyme action

Answer 17

Temperature
pH
Substrate conc
Enzyme conc

Question 18

Effect of temperature on ROR

Answer 18

• Increase in temp
• Increase in kinetic energy
• More successful collisions
• More ESC form per second
• Denaturation: past the optimum the hydrogen/ionic bonds will break causing a change in tertiary structure causing a change in the active site
• Active site is no longer complementary so less ESC

Question 19

Effect of pH on ROR

Answer 19

• Enzymes have an optimal pH
• If made more acidic or more basic
• Charge on R groups are changed
• Ionic bonds are broken
• Tertiary structure is changed
• Active site changes shape
• No longer complementary
• Less ESC formed

Question 20

Effect of substrate conc. on graph

Answer 20

• Increases
• Then plateaus
• As enzyme becomes the limiting factor

Question 21

Effect of enzyme conc. on graph

Answer 21

• Increases
• Then plateaus
• As substrate becomes the limiting factor

Question 22

Explain how a competitive inhibitor works

Answer 22

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 23

Describe how a non-competitive inhibitor works

Answer 23

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 24

What is the proteome of a cell?

Answer 24

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 25

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

Answer 25

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;