Proteins and enzymes A1

Question 1

Give examples of different proteins and state their function. (6)

Answer 1

1. Haemoglobin - transports oxygen
2. Antibodies - defend body against pathogens
3. Enzymes - biological catalysts
4. Actin and Myosin - involved in muscle contraction
5. Keratin - found in nails and hooves (structural)
6. Collagen - found in tendons (structural)

Question 2

Name 5 elements that make up amino acids. (5)

Answer 2

1. Nitrogen (N)
2. Carbon (C)
3. Hydrogen (H)
4. Oxygen (O)
5. some contain Sulphur (S)

Question 3

Draw the general structure of an amino acid.

Answer 3

.. H H O
H - N - C - C - OH
R

Question 4

What bond do 2 amino acids form during which reaction? (2)

Answer 4

1. peptide bond
2. condensation reaction

Question 5

Describe the structure of a protein (7)

Answer 5

1. Polymer of amino acids;
2. Joined by peptide bonds;
3. Formed by condensation reactions;
4. Primary structure is order of amino acids;
5. Secondary structure is folding of polypeptide
   chain into A helix and B pleated sheets due to
   hydrogen bonding;
6. Tertiary structure is 3-D folding due to
   hydrogen bonding and ionic bonding and
   disulfide bridges;
7. Quaternary structure is two or more
   polypeptide chains;

Question 6

Describe the effect of temperature and PH on a protein structure (4 temp) (1 PH)

Answer 6

1. protein denatures at high temperatures (NOT LOW)
2. increasing temp increases kinetic energy of molecules
3. higher frequency vibrations break weak bonds holding structure together
4. tertiary shape of molecule is lost (denatured)
5. change in PH can also disrupt ionic bonds in tertiary structure, denaturing protein.

Question 7

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.

Answer 7

1. Antigens (on pathogen) are a specific shape/
   have specific tertiary / 3D structure;
2. Antibody fits/binds / is complementary to
   antigen/ antibody-antigen complex forms;
   OR
3. Antibodies are a specific shape / have specific
   tertiary/ 3D structure;
4. Antigens (on pathogen) fit/ bind/ are
   complementary to antibody / antibody-antigen
   complex forms;

Question 8

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

Answer 8

1. Add Biuret reagent to both solutions) – no
   mark;
2. Lactase / enzyme will give purple / lilac;
   OR
3. Lactose / reducing sugar will not give purple /
   lilac / will remain blue;
4. Because Lactase is a protein;

Question 9

Describe how you could make a biuret test. (3)

Answer 9

1. add sodium hydroxide
2. add a few drops of copper sulphate
3. to a small amount of the extract in a labelled test tube

Question 10

What are enzymes?

Answer 10

1. proteins
2. biological catalyst

Question 11

Define the term catalyst.

Answer 11

A catalyst increases the rate at which chemical reactions occur but remain unchanged or are unaffected by the reaction.

Question 12

Explain how enzymes lower activation energy. (5)

Answer 12

1. they stress and bend the bonds in the substrate
2. during the formation of enzyme substrate complexes
3. collisions will occur between specific parts of the molecules involved in the reaction
4. so bonds can break and reform more efficiently
5. therefore less activation energy required

Question 13

Describe the structure of an enzyme.

Answer 13

1. globular proteins (with a specific 3D shape)
2. with an active site (specific and complementary to substrates

Question 14

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

Answer 14

1. lactose has a different shape/structure
2. does not fit/bind to active site of enzyme/sucrase
3. active site of enzyme/sucrase has a specific shape/structure
4. does not fit/bind to lactose

Question 15

Describe the induced fit model of enzyme action.

Answer 15

1. active site/enzyme not complementary
2. active site changes shape as the substrate binds/as enzyme substrate complexes form
3. stressing/distorting/bending bonds

Question 16

Describe the way that the lock and key model is different from the induced fit model.

Answer 16

Active site does not change shape/is fixed/does not wrap around substrate/is complementary before binding.

Question 17

One enzyme will catalyse only one reaction. Explain why.

Answer 17

1. enzyme has active site
2. only specific substrate fit to the active site

Question 18

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

Answer 18

1. different parts of the protein have different amino acid sequences so have a different shape
2. each enzyme active site is a specific shape and complementary to a different part of the protein

Question 19

Describe how temperature effects the rate of an enzyme controlled reaction.

Answer 19

1. increases rate of reaction
2. until reaches past optimum, then hydrogen and ionic bonds begin to break between R group in tertiary structure (denatured) - no longer fits substrate AS

Question 20

Describe how PH effects the rate of an enzyme controlled reaction.

Answer 20

1. change in PH can alter the charge on the R group of amino acid
2. Hydrogen and ionic bonds in tertiary structure are broken
3. changing active site (denatured)

Question 21

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 it cannot be taken orally.

Answer 21

1. broken down by enzymes/digested/denatured(by PH0/ too large to be absorbed

Question 22

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

Answer 22

1. increases then plateaus/constant steady/rate does not change
2. it plateaus as all active sites occupied/saturated/enzyme limiting rate of reaction/maximum of E-S complexes formed

Question 23

Figure 6 shows that the maximum initial rate of a reaction when a competitive inhibitor is present (curve B) is different from that when a non-competitive inhibitor is present (curve C).
Explain this difference (4)

Answer 23

1. competitive inhibitor binds to active site of enzyme but non-competitive inhibitor binds to allosteric site/away from active site.
2. binding of competitive inhibitor does not cause change in shape of active site but binding of non-competitive does
3. so with competitive inhibitor, at high substrate concentrations (active) enzyme still available, but with non-competitive inhibitor (active) enzymes no longer available
4. at higher substrate concentrations likelihood of enzyme-substrate collisions increases with competitive inhibitor, but this is not possible with non-competitive inhibitor

Question 24

Explain how a competitive inhibitor works (3)

Answer 24

1. inhibitor is a similar shape to substrate
2. inhibitor enters active site/is a competitive inhibitor
3. less substrate binds/fewer E-S complexes

Question 25

Explain how a non-competitive inhibitor works.

Answer 25

1. inhibitor is not a similar shape to substrate
2. inhibitor enters away from active site/at allosteric site/is a non-competitive inhibitor
3. changes the shape of active site
4. less substrate binds/fewer E-S complexes
5. less (named product) formed

Question 26

Why can sulphanilamide prevent bacteria producing folic acid (application question) (4)

Answer 26

1. sulphanilamide is similar shape to substrate
2. is competitive
3. inhibitor binds to active site so there are less substrates to bind
4. fewer complexes so less folic acid produced

Question 27

Explain why a faulty form of TK (active site already present for binding which increases cell division) leads to cancer.

Answer 27

1. doesn’t need a phosphate group to function
2. so the enzyme cannot be controlled so cell division is uncontrollable.

Question 28

Other than ethical reasons, suggest two reasons why they chose to use kittens as model organisms?

Answer 28

1. kittens are mammals so have same reaction as humans
2. kittens are easy to keep alive/large numbers

Question 29

Explain why monitoring the PH of the mixture could show whether the cats milk contains lipase.

Answer 29

1. hydrolysis of lipids produces fatty acids
2. which lowers PH of mixture

Question 30

Describe how scientists could use data to produce a calibration curve and how they would use the calibration curve to find the concentration of a protein in a sample of blood plasma. (4)

Answer 30

1. produce known concentration of protein
2. measure absorbance of each concentration
3. measure concentration with colorimeter
4. plot graph and use it to find concentration from curve