2.3.3 Enzyme mechanism, specificity and activation energy Quiz: Pearson Edexcel Biology A (Salters-Nuffield), Unit 2

The 20 questions

1. What is the role of an enzyme in a chemical reaction?

   • It becomes part of the product, which is why it is consumed during the reaction
   • It lowers the activation energy, so the reaction proceeds faster, without being used up
   • It supplies the energy needed for the reaction, so the reactants gain energy from its breakdown
   • It increases the temperature of the reaction mixture so that particles collide more often

2. What does activation energy mean?

   • The minimum energy that colliding particles must have for a reaction to occur
   • The energy needed to break the peptide bonds in a polypeptide during hydrolysis
   • The total energy released when a reaction is complete, measured per mole of product
   • The energy stored in the enzyme's active site, which is released when the substrate binds

3. Which part of an enzyme binds the substrate?

   • The allosteric site only
   • The peptide bond that links the amino acids
   • The cofactor site on the outer surface
   • The active site

4. Which model describes the enzyme-substrate interaction in which the active site changes shape slightly to fit the substrate?

   • Competitive inhibition model
   • Induced fit model
   • Fibrous model
   • Lock and key model only

5. Why is an enzyme specific to a particular substrate?

   • The shape and chemical properties of its active site are complementary to that substrate
   • The enzyme has a fixed number of hydrogen bonds that bind any molecule in the cell
   • The enzyme binds any substrate with the same molecular mass as itself
   • The enzyme is made of the same amino acids as the substrate, which allows binding

6. What happens to an enzyme when its three-dimensional structure is disrupted by high temperature?

   • It becomes more active, because higher temperature increases the rate of substrate binding indefinitely
   • It forms covalent bonds with water, which increases its size and activity
   • It is converted into a substrate, which is then broken down by another enzyme in the cell
   • It denatures, its active site changes shape, and the substrate can no longer bind effectively

7. Which of these is an example of an intracellular enzyme?

   • Amylase, which is secreted into the mouth to digest starch
   • Pepsin, which is secreted into the stomach to digest proteins
   • Catalase, which breaks down hydrogen peroxide inside cells
   • Trypsin, which is secreted into the small intestine to digest proteins

8. Which of these is an example of an extracellular enzyme?

   • DNA polymerase, which works inside the nucleus to copy DNA
   • Catalase, which is found in the cytoplasm and breaks down hydrogen peroxide
   • Hexokinase, which phosphorylates glucose inside the cytoplasm during respiration
   • Amylase, which is secreted into the mouth and small intestine to digest starch

9. In an experiment, the initial rate of an enzyme reaction is measured as the gradient of the tangent at time zero. Why is the initial rate used?

   • It measures the rate after the enzyme has been denatured, which shows the maximum possible rate
   • It measures the rate at the point where all the substrate has been used up and the reaction stops
   • It measures the average rate over the whole reaction, including the period when the substrate runs out
   • It measures the rate before substrate concentration falls significantly, so the rate is not affected by product build-up

10. An enzyme catalyses a reaction and produces 20 cm^3 of gas in 40 seconds. What is the average rate of gas production?

    • 0.5 cm^3 per second
    • 2 cm^3 per second
    • 800 cm^3 per second
    • 0.05 cm^3 per second

11. When substrate concentration is increased while enzyme concentration is kept constant, what happens to the rate of reaction at low substrate concentrations?

    • The rate increases approximately in proportion to the substrate concentration
    • The rate becomes zero, because the substrate binds the enzyme and blocks its active site completely
    • The rate is unchanged, because the enzyme is saturated even at low substrate concentrations
    • The rate falls, because the enzyme is inhibited by the presence of more substrate

12. Why does the rate of reaction plateau at high substrate concentrations, with the enzyme concentration constant?

    • The substrate is destroyed by the enzyme at high concentrations, which reduces the number of molecules
    • All the active sites are occupied, so the enzyme is saturated and extra substrate cannot increase the rate
    • The activation energy rises as substrate concentration increases, which slows the reaction
    • The enzyme is denatured by high substrate concentrations, which stops the reaction at a fixed rate

13. In an investigation, the rate of reaction is increased by doubling the enzyme concentration while substrate is in excess. What is the expected result?

    • The rate halves, because the extra enzyme blocks the active sites of the existing enzyme molecules
    • The rate is unchanged, because the enzyme concentration has no effect when substrate is in excess
    • The rate falls to zero, because the enzyme denatures when its concentration is doubled
    • The rate increases, because more enzyme molecules are available to catalyse the reaction

14. Why does a rise in temperature from 20 to 37 degrees Celsius usually increase the rate of an enzyme-catalysed reaction?

    • The substrate becomes less soluble in the cytoplasm at higher temperature, which forces more molecules into the active sites
    • The enzyme becomes more specific at higher temperature, so it binds more substrate molecules at each active site
    • The activation energy is increased by the rise in temperature, which speeds up the reaction by providing more energy
    • Molecules have more kinetic energy, so substrate collides with the active site more frequently and more often with enough energy

15. Why is there an optimum temperature for an enzyme-catalysed reaction?

    • Above the optimum, the enzyme denatures, so the active site is lost and the rate falls
    • Below the optimum, the enzyme becomes more active, so the rate falls at higher temperatures
    • The optimum is set by the cell's supply of ATP, which is fixed regardless of temperature
    • The optimum is the temperature at which the enzyme is converted into a substrate, which ends the reaction

16. Which of these is a potential effect of a change in pH on an enzyme?

    • The substrate is removed from solution, which means the enzyme has no effect on the reaction
    • Changes in the charges on the amino acids in the active site, which can alter the shape and reduce binding
    • The enzyme is converted into a carbohydrate, which then binds the substrate in the active site
    • The activation energy of the reaction is increased to a fixed value at every pH

17. A student uses catalase from liver in a series of tubes with different enzyme concentrations. Which variable should be controlled to ensure a fair test?

    • The number of tubes, which should vary to show the effect of the enzyme
    • The colour of the liver used, which should be different for each tube
    • Temperature, pH and the volume and concentration of the hydrogen peroxide solution
    • The time of the experiment, which should be different for each tube to show the trend

18. Which statement describes a competitive inhibitor?

    • It binds to the active site and blocks the substrate, and its effect can be overcome by increasing substrate concentration
    • It is a substrate that is converted into product faster than the normal substrate in every reaction
    • It binds to a site away from the active site and permanently denatures the enzyme in all cases
    • It increases the activation energy of the reaction by binding to the substrate and making it more stable

19. Why are enzymes described as globular proteins?

    • They are folded into a compact, roughly spherical shape that is soluble in water
    • They are made of glucose units joined by glycosidic bonds into a compact polysaccharide
    • They are long, insoluble fibres that form the structure of the cell membrane
    • They are made of fatty acids linked to glycerol and are insoluble in water

20. Which of these explains why enzymes lower activation energy?

    • They are used up in the reaction, which releases energy that drives the reaction forward
    • They supply extra energy to the substrate, so the molecules start the reaction with more energy
    • They bind the substrate and hold it in an orientation that makes the bond-breaking or bond-making step easier
    • They raise the temperature of the active site so that the substrate reacts more quickly

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