Proteins and Enzymes

Question 1

Describe the structure of proteins (6)

Answer 1

Polymer of amino acids
Joined by peptide bonds
Formed by condensation reactions
Primary structure is sequence and number of amino acids
Secondary structure is folding of polypeptide chain into alpha helixes and beta pleated sheets due to hydrogen bonding (C=O:H-N)
Tertiary structure is 3D folding due to hydrogen bonds, ionic bonds and disulphide 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 (2)

Answer 2

Condensation reaction

Between amine group and carboxyl group

Question 3

Describe how an enzyme-substrate complex increases the rate of reaction (2)

Answer 3

Reduces activation energy

Due to bending bonds

Question 4

Describe how a change in the base sequence of the DNA coding for an enzyme may result in a non-functional protein. (3)

Answer 4

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

Question 5

What is the proteome of a cell? (1)

Answer 5

number of different proteins that a cell is able to produce at a given time

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 structure
Antibody binds to antigen because it is complementary
Antigens (on pathogen) bind to antibody to form antibody-antigen complex

Question 7

Describe how to carry out a test for proteins. (2)

Answer 7

Add equal volume of Biuret reagent

Purple/Violet colour change indicates protein is presen

Question 8

Sucrase doesn’t hydrolyse lactose. Use your knowledge of the way in which enzymes work to explain why. (2)

Answer 8

Lactose has a different structure

Doesn’t bind/fit to active site of enzyme/sucrase

Question 9

Describe the induced fit model of enzyme action (2)

Answer 9

Active site not complementary
Active site is flexible
(Change in enzyme’s shape of active site) allows substrate to bind and form enzyme-substrate complex

Question 10

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

Answer 10

Active site is rigid/fixed

So substrate fits and is complementary before binding

Question 11

An enzyme catalyses only one reaction at a time. Explain why. (1)

Answer 11

Enzyme’s active site has a specific shape

So only one substrate binds

Question 12

Suggest why a protein can be the substrate for two different enzymes. (2)

Answer 12

Different parts of the protein have different amino acid sequences so are a different shape
Each enzyme active site is a specific shape and complementary to a different part of the protein

Question 13

What is the effect of temperature on the rate of an enzyme controlled reaction? (3 marks)

Answer 13

As temperature increases, there is more kinetic energy so more frequent successful collisions take place
More enzyme substrate complexes formed per second
If temperature increases beyond optimum temperature, the weak Hydrogen bonds, ionic bonds and disulphide bridges break and changes the tertiary structure which alters the shape of the active site and the enzyme can become denatured
Therefore, less enzyme substrate complexes are formed per second

Question 14

What is the effect of pH on the rate of an enzyme controlled reaction? (3 marks)

Answer 14

a slight change in the pH can denature the enzyme and alter the active site
so less enzyme substrate complexes form per second so rate of reaction decreases
enzyme becomes denatured

Question 15

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

Answer 15

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 16

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

Answer 16

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 17

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

Answer 17

Increases then plateaus
It plateaus as all active sites occupied / Saturated
maximum number of Enzyme-Substrate complexes per second;

Question 18

Explain how a competitive inhibitor works (3 marks)

Answer 18

Inhibitor has similar shape to substrate
Inhibitor can bind to enzyme’s active site
Less enzyme-substrate complexes are formed per second

Question 19

Describe how a non-competitive inhibitor works. (3 marks)

Answer 19

Attaches to the allosteric site on the enzyme
Causes conformational change to the active site
Less enzyme substrate complexes can form per second

Question 20

The secondary structure of a polypeptide is produced by bonds between amino acids. Describe how. (2 marks)

Answer 20

Hydrogen bonds

Between N-H and C=0 OR forming beta-pleated sheets and alpha helixes

Question 21

Two proteins have the same number and type of amino acids but different tertiary structures. Explain why (2 marks)

Answer 21

Different sequence of amino acids

Forms ionic bonds, hydrogen bonds and disulphide bridges in different places

Question 22

Explain how the shape of an enzyme is related to its function (3 marks)

Answer 22

Enzyme has specific 3D tertiary structure
Substrate complementary to enzyme’s active site
Substrate binds to active site