Enzyme_Exam_AS_2021

• A

• B

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• D

• A

• B

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• D

• A

• B

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• D

• A

• B

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• D

In an investigation, the same concentration of the enzyme phosphorylase was added to different concentrations of glucose phosphate and incubated at 30 °C. At 1 minute intervals, one drop of

the reaction mixture was removed and added to a drop of iodine solution on a white tile. The diagram shows the results of this investigation.

What explains the trend in the results of this investigation?

• A. Phosphorylase catalyses a reaction converting glucose phosphate to starch.

• B. The maximum rate of reaction is reached at 20 mg dm–3 of glucose phosphate.

• C. Substrate concentration is limiting at concentrations of glucose phosphate 25 mg dm–3 or

  less.

• D. Enzyme concentration is limiting at concentrations of glucose phosphate 25 mg dm–3 or less.

What is the definition of the Michaelis-Menten constant, Km, for an enzyme?

• A. Vmax

• B. half Vmax

• C. the substrate concentration that gives Vmax

• D. the substrate concentration that gives half Vmax

A fixed volume and concentration of substrate and enzyme were mixed. All other variables were kept constant. The enzyme-catalysed reaction was left until it was complete.

Which graph shows how the rate of reaction changes with time?

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The enzyme β-galactosidase can catalyse the hydrolysis of four substrates, A, B, C and D, with similar structures.

Each substrate has a different Km value.

For which substrate does β-galactosidase have the lowest affinity?

• Km = 4 × 10–3 mol dm–3

• Km = 1 × 10–3 mol dm–3

• Km = 2 × 10–4 mol dm–3

• Km = 1 × 10–4 mol dm–3

Which is correct for competitive inhibitors of enzymes?

1 They occupy the active site of an enzyme.

2 They have exactly the same shape as the substrate.

3 They can be used to control the rate of enzyme activity.

4 They can bind to a site on an enzyme other than the active site.

• 1, 2 and 3

• 1 and 3 only

• 1 only

• 2, 3 and 4

A mutation occurred within the DNA sequence coding for an enzyme, causing a decrease in the rate of a reaction catalysed by this enzyme.

Which statements could explain the decrease in the rate of reaction?

1. An inhibitor for this enzyme has an increased affinity for the enzyme and forms an enzyme–inhibitor complex more easily.

2. The active site of the enzyme might have changed shape and so is no longer complementary.

3. The activation energy for the reaction with the mutated enzyme is greater than with the non-mutated enzyme.

• A 1, 2 and 3

• B 1 and 2 only

• C 1 and 3 only

• D 2 and 3 only