Proteins and enzymesAQA A-Level Biology: Topic test

20 questions, 54 marks

AQA A-Level Biology

Proteins and enzymes topic test

Total 54 marks

Name

Class

Date

1
Collagen is a fibrous protein found in tendons. Each collagen molecule is made of three polypeptide chains wound around one another and held together by many hydrogen bonds.
(a)
Which level of protein structure is shown by three polypeptide chains wound around one another?
[1 mark]
APrimary
BSecondary
CTertiary
DQuaternary
(b)
Which feature of collagen gives tendons high tensile strength?
[1 mark]
AMany hydrogen bonds between the polypeptide chains
BA specific active site
CSolubility in water
DA single polypeptide chain that is not folded
(c)
Name the two groups that react when a peptide bond forms between two amino acids, and name the other product of the reaction.
[2 marks]
Total for question 1: 4 marks
2
Lysozyme is an enzyme found in tears. It is a globular protein with a cleft-shaped active site, and it hydrolyses a bond in a polysaccharide found in the cell walls of some bacteria.
(a)
Which statement describes the induced-fit model of enzyme action?
[1 mark]
AThe substrate fits the active site exactly, like a key in a lock, with no change of shape
BThe active site changes shape slightly as the substrate binds, moulding around the substrate
CThe substrate changes shape so that it fits a rigid active site
DThe enzyme is used up as the substrate binds to it
(b)
A mutation replaces one amino acid in the active site of lysozyme, and the enzyme stops working. What is the most likely reason?
[1 mark]
AThe peptide bonds of the enzyme are hydrolysed
BThe primary structure of the substrate changes
CThe tertiary structure of the active site changes, so the substrate no longer fits
DThe enzyme concentration falls
(c)
Lysozyme does not hydrolyse starch, although starch is also a polysaccharide. Explain why.
[2 marks]
Total for question 2: 4 marks
3
Urease is an enzyme that catalyses the hydrolysis of urea to ammonia and carbon dioxide. A student measured the rate of the reaction at 20 °C and at 40 °C in a solution at pH 7, and found the rate was higher at 40 °C. At 75 °C, and also in a solution at pH 2, no reaction took place.
(a)
Use pH = −log₁₀[H⁺] to calculate the hydrogen ion concentration at pH 2 and at pH 7, and the number of times greater the concentration is at pH 2.
[3 marks]
(b)
Explain why the rate of reaction was higher at 40 °C than at 20 °C, and why no reaction took place at 75 °C.
[4 marks]
Total for question 3: 7 marks
4
A company makes a protease for use in washing powders. The enzyme is a single polypeptide of 274 amino acids that contains two disulfide bridges and works best at 50 °C.
(a)
Explain how the primary structure of this enzyme determines its tertiary structure, and why the enzyme acts only on protein stains.
[6 marks]
(b)
Explain how increasing the temperature from 20 °C to 70 °C, and a large change in pH away from the optimum, would each affect the rate of reaction.
[6 marks]
Total for question 4: 12 marks
5
A student investigates the effect of starch concentration on the rate of a reaction catalysed by amylase at 35 °C. She mixes amylase solution with starch solution and tests samples with iodine solution every 30 seconds. With one starch concentration the colour stayed blue-black until 3 minutes 30 seconds, when it first stayed orange-brown.
(a)
The rate of reaction can be calculated as 1 ÷ time. What is the rate for this starch concentration?
[1 mark]
A4.8 × 10⁻³ s⁻¹
B2.1 × 10² s⁻¹
C2.9 × 10⁻¹ s⁻¹
D3.3 × 10⁻² s⁻¹
(b)
The student repeats the investigation with a competitive inhibitor added. Why does the inhibitor reduce the rate?
[1 mark]
AIt changes the pH of the mixture
BIt binds to a site other than the active site
CIt increases the activation energy of the substrate
DIt has a shape similar to the substrate and binds to the active site, so fewer enzyme-substrate complexes form
(c)
State two variables, other than temperature, that the student should keep constant in the investigation.
[2 marks]
Total for question 5: 4 marks
6
Sucrase is an enzyme that catalyses the hydrolysis of sucrose to glucose and fructose. It has no effect on maltose, which is also a disaccharide.
(a)
What is the effect of sucrase on the activation energy of the hydrolysis of sucrose?
[1 mark]
AIt increases the activation energy
BIt has no effect on the activation energy
CIt lowers the activation energy
DIt reduces the activation energy to zero
(b)
Why does sucrase not hydrolyse maltose?
[1 mark]
AMaltose is not a disaccharide
BMaltose is not complementary to the shape of the active site
CMaltose has no glycosidic bond
DMaltose denatures sucrase
(c)
Explain how the formation of an enzyme-substrate complex lowers the activation energy of the hydrolysis of sucrose.
[2 marks]
Total for question 6: 4 marks
7
Insulin is a protein made of two polypeptide chains held together by disulfide bridges. Chain A has 21 amino acids and chain B has 30 amino acids.
(a)
Calculate the number of peptide bonds in one insulin molecule. State the type of reaction by which each bond forms and the other product of the reaction.
[3 marks]
(b)
Describe the roles of hydrogen bonds, ionic bonds and disulfide bridges in the structure of a protein such as insulin.
[4 marks]
Total for question 7: 7 marks
8
A drug company tests a drug that inhibits an enzyme that a bacterium needs to make an essential compound. With the substrate at 2.0 mmol dm⁻³, the rate of reaction is 8.0 μmol min⁻¹ without the drug and 3.0 μmol min⁻¹ with the drug. With the substrate at 50 mmol dm⁻³, the rate is 20.0 μmol min⁻¹ without the drug and 19.2 μmol min⁻¹ with the drug.
(a)
Deduce whether the drug is a competitive or a non-competitive inhibitor. Explain your answer.
[6 marks]
(b)
Describe how you could investigate, in the laboratory, the effect of the concentration of the drug on the rate of the reaction catalysed by the enzyme.
[6 marks]
Total for question 8: 12 marks

End of questions

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).