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

1. in the lock and key model the enzyme has a fixed shape that is complementary to the substrate
2. 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

explain how a competitive inhibitor decreases the rate of an enzyme-controlled reaction (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. quaternary structure contains more than 1 polypeptide chain
6. creates active site in enzymes

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. 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

Question 18

describe how monomers join to form the primary structure of a protein (3)

Answer 18

1. condensation reaction between amino acids
2. forming peptide bonds
3. creating specific sequence/order of amino acids

Question 19

describe one similarity and one difference between the induced fit model of enzyme action and the lock and key model of enzyme action (2)

Answer 19

similarity
1. substrate binds/fits active site
2. enzyme substrate complex formed

difference
1. active site changes shape in induced fit, in lock and key it doesn’t
2. initially the active site is not complementary to substrate with induced fit, but it is in lock and key model

Question 20

state how enzymes help reactions to proceed quickly at lower temperatures (1)

Answer 20

lower activation energy

Question 21

which test could be used to identify that there are amino acids in white wine (1)

Answer 21

biuret

Question 22

name the chemical element found in all amino acids that is not found in triglycerides (1)

Answer 22

nitrogen

Question 23

describe how amino acids join to form a polypeptide so there is always NH2 at one end and COOH at the other end (2)

Answer 23

1. one amine group joins to a carboxyl group to form peptide bond
2. so in chain there is free amine group at one end and a free carboxyl at the other

Question 23

explain how two enzymes with different amino acid sequences can catalyse the same reaction (2)

Answer 23

1. both active sites have similar tertiary structures
2. so form ESCs with same substrate

Question 24

describe the induced-fit model of enzyme action (2)

Answer 24

1. before reaction active site not complementary to/does not fit substrate
2. shape of active site changes as substrate binds/as ESC forms
3. bending bonds in the substrate leading to reaction

Question 25

describe the primary structure of all proteins (2)

Answer 25

1. sequence/order of amino acids
2. joined by peptide bonds

Question 26

suggest how amyloid-precursor protein can be the substrate of two different enzymes (2)

Answer 26

1. different parts/amino acids of the protein
2. each enzyme is complementary to a different part of the protein

Question 27

describe what happens in the hydrolysis reaction that produces the smaller protein from amyloid-precursor protein (2)

Answer 27

1. peptide bond is broken
2. using water

Question 28

describe how a quaternary protein is formed from its monomers

do not include the process of translation in your answer (5)

Answer 28

1. amino acids joined by peptide bonds
2. by condensation reactions
3. secondary structure is formed by hydrogen bonding
4. tertiary structure formed by disulphide bridges, ionic bonds between R groups
5. quaternary structure contains more than one polypeptide

Question 29

MiTMAB acts as a non-competitive inhibitor of an enzyme called dynamin.

suggest how MiTMAB causes dynamin to become inactive (3)

Answer 29

1. MiTMAB binds to dynamin on an allosteric site
2. changes the shape of dynamin active site OR changes the tertiary structure of dynamin
3. so it is no longer complementary so substrate dos not bind to active site OR not complementary so no/fewer enzyme-substrate complexes form