3.1.4 - PROTEINS & ENZYMES

Question 1

describe the structure of proteins (5)

Answer 1

1. polymer of amino acids
2. joined by peptide bonds
3. formed by a condensation reaction
4. primary structure is the sequence of amino acids
5. secondary structure is the folding of the polypeptide chain into an alpha helix or beta pleated sheet due to hydrogen bonding
6. tertiary structure is 3D folding due to hydrogen bonding and ionic bonds and disulphide bridges
7. quaternary structure involves 2 or more polypeptide chains

Question 2

the fruit fly is a species of small insect.

the fruit fly has a gene that codes for an enzyme called alcohol dehydrogenase (AD). AD catalyses the breakdown of alcohol when alcohol is in the insects’ food.

the gene coding for AD has two alleles ADF and ADS.

the enzyme encoded by the ADF allele catalyses the breakdown of alcohol faster than the enzyme encoded by the ADS allele. suggest why (3)

Answer 2

1. different primary structure/amino acid sequence
2. different tertiary structure/shape of active site
3. enzyme-substrate complexes are more likely with enzyme from ADF allele

Question 3

explain the main difference between the lock and key model and the induced fit model (2)

Answer 3

• in the lock and key model the enzyme has a fixed shale that is complementary to the substrate
• but, in the induced fit model the active site has to change shape slightly to allow the substrate to bind tightly

Question 4

in humans, the enzyme maltase breaks down maltose to glucose. this takes place at normal body temperature.

explain why maltase:
- only breaks down maltose
- allows this reaction to take place at normal body temperature (5)

Answer 4

1. tertiary structure/3D shape of enzyme means a specific, unique active site shape
2. active site is complementary to maltose (substrate) so they can fit together
3. induced fit = the active site changes shape slightly to allow for a tighter fit
4. enzyme lowers the activation energy required for the reaction (provides an alternative pathway for the reaction at a lower energy level)
5. by forming enzyme-substrate complex

Question 5

describe competitive and non-competitive inhibition of an enzyme (5)

Answer 5

1. inhibitors reduce binding of enzyme to substrate this prevents the formation of the enzyme-substrate-complex

competitive inhibition:
- inhibitor is a similar shape to the substrate
- it will bind to the active site of the enzyme
- it can be overcome by increasing the volume of substrate

non-competitive inhibition:
- inhibitor binds to a site on the enzyme other than the active site
- it changes the shape of the active site
- it cannot be overcome by increasing the volume of substrate

Question 6

describe the induced-fit model of enzyme action and how an enzyme acts as a catalyst (3)

Answer 6

1. substrate binds to the active site of the enzyme/enzyme-substrate-complex (ESC) forms
2. active site changes shape slightly so that is is complementary to the substrate
3. reduces activation energy

Question 7

a competitive inhibitor decreases the rate of an enzyme-controlled reaction. explain how (3)

Answer 7

1. inhibitor is similar shape to the substrate (complementary to active site)
2. fits/binds to active site
3. prevents enzyme-substrate complex (ESC) forming

Question 8

describe how the structure of a protein depends on the amino acids it contains (5)

Answer 8

1. structure is determined by relative position of amino acid/R group/interactions
2. primary structure is sequence/order of amino acids
3. secondary structure is formed by hydrogen bonding between amino acids (alpha-helix/beta pleated sheet)
4. tertiary structure formed by hydrogen, disulphide and ionic bonding
5. creates active site in enzymes
6. quaternary structure contains more than 1 polypeptide chain

Question 9

explain how the active site of an enzyme causes a high rate of reaction (3)

Answer 9

1. lowers activation energy
2. induced fit causes active site of enzyme to change shape
3. so enzyme-substrate complex strains bonds (or to break)

Question 10

describe a biochemical test to confirm the presence of protein in a solution (2)

Answer 10

1. add biuret reagent
2. positive result - purple

Question 11

a dipeptide consists of two amino acids joined by a peptide bond. dipeptides may differ in the type of amino acids they contain.

describe two other ways in which all dipeptides are similar and one way in which they may differ (3)

Answer 11

similarities:
1. amine group at the end
2. carboxyl group at the end
3. two R groups
4. all contain C and H and N and O

differences:
1. variable/different R groups

Question 12

describe how a non-competitive inhibitor can reduce the rate of an enzyme-controlled reaction (3)

Answer 12

1. attaches to enzyme at an allosteric site
2. changes shape of the active site OR changes tertiary structure of enzyme
3. so active site and substrate will no longer be complementary so they cannot fit/bind

Question 13

describe how a peptide bond is formed between two amino acids to form a dipeptide (2)

Answer 13

1. condensation reaction
2. between amine and carboxyl group

Question 14

the secondary structure of a polypeptide is produced by bonds between amino acids. describe how (2)

Answer 14

1. hydrogen bonds
2. forming alpha-helix or beta-pleated sheet

Question 15

two proteins have the same number and type of amino acids but different tertiary structures. explain why (2)

Answer 15

1. different sequence of amino acids/primary structure
2. forms ionic/hydrogen/disulphide bonds in different places

Question 16

formation of an enzyme-substrate complex increases the rate of reaction. explain how (2)

Answer 16

1. reduces activation energy
   2,. due to bending bonds OR without enzyme, very few substrates have sufficient energy for reaction

Question 17

suggest two variables that were controlled when investigating the effect of temperature on the rate of breakdown of a protein by protease (1)

Answer 17

any two:

1. initial starting substrate concentration
2. enzyme concentration
3. pH