3.1.4.2.2 Factors affecting enzyme-controlled reactions Quiz: AQA Biology, Unit 1

The 20 questions

1. Where does a competitive inhibitor bind to an enzyme?

   • To an allosteric site away from the active site, changing its shape
   • To the phosphate groups on the enzyme surface
   • To the substrate molecule, making it unable to enter the enzyme
   • To the active site, blocking the substrate from binding

2. Where does a non-competitive inhibitor bind to an enzyme?

   • To the active site, where it competes with the substrate for the same position on the enzyme
   • To the end of the enzyme's polypeptide chain only, where it blocks the entry of the substrate
   • To a site other than the active site, changing the shape of the active site
   • To the substrate molecule itself, so that it cannot be converted into the product of the reaction

3. What effect does increasing substrate concentration have on the rate of a reaction inhibited by a competitive inhibitor?

   • The inhibition is reduced, because the substrate competes more successfully for the active site
   • The inhibition is increased, because more substrate blocks the active site
   • The inhibition stops completely, because the enzyme is denatured by the substrate
   • The inhibition is unchanged, because the inhibitor binds only to the substrate

4. Why does the rate of reaction reach a plateau as substrate concentration increases, with enzyme concentration fixed?

   • The substrate is used up, so there are no remaining molecules available to form any new product
   • All the active sites are occupied, so enzyme concentration becomes the limiting factor
   • The substrate begins to inhibit the enzyme by forming strong covalent bonds with the active site
   • The enzyme is denatured by the high concentration of substrate, so the active sites are destroyed

5. What happens to an enzyme when the temperature rises above its optimum?

   • The enzyme is hydrolysed, releasing free amino acids into the solution
   • The tertiary structure is disrupted, so the active site changes shape and the rate falls sharply
   • The enzyme changes its primary structure, forming new peptide bonds
   • The enzyme becomes more active because the kinetic energy of the substrate increases

6. Why does a change in pH affect an enzyme's rate of reaction?

   • It converts the enzyme into a substrate, which then reacts with the product to form a new compound
   • It alters the ionisation of R groups, which can change the shape of the active site and its binding ability
   • It changes the number of substrate molecules that are available to take part in the reaction itself
   • It breaks the peptide bonds in the primary structure of the enzyme at all pH values that are tested

7. Which factor is a limiting factor for the rate of reaction when substrate concentration is high and the enzyme is present in a fixed amount?

   • Enzyme concentration
   • Presence of a competitive inhibitor
   • Substrate concentration
   • Temperature below the optimum

8. Why does the rate of an enzyme-controlled reaction increase with temperature from a low value up to the optimum?

   • The enzyme forms additional peptide bonds that increase its stability
   • The substrate converts into product at a faster rate at all temperatures
   • Molecules have more kinetic energy, so more enzyme-substrate collisions are successful
   • The enzyme's active site expands, allowing more substrate to bind

9. A non-competitive inhibitor is added to an enzyme. Increasing the substrate concentration does not restore the maximum rate. Why?

   • The inhibitor has altered the active site shape, so fewer active sites are available whatever the substrate concentration
   • The substrate is now too large to bind to any part of the enzyme molecule, so binding is prevented altogether
   • The inhibitor has bound to the substrate and removed it from solution, so less substrate is available for reaction in every case
   • The inhibitor has increased the activation energy of the substrate, so fewer molecules can react at any time

10. In an iodine clock experiment, a colour change takes 40 s. What is the rate of reaction in s^-1?

    • 2.5 s^-1
    • 40 s^-1
    • 0.4 s^-1
    • 0.025 s^-1

11. A student measures the rate of an enzyme reaction at pH 5 and pH 7, and finds a much lower rate at pH 5. Which statement best explains the result if pH 7 is the optimum?

    • At pH 5 the enzyme is hydrolysed, releasing its R groups into solution
    • At pH 5 the substrate is converted into an inhibitor, which blocks the enzyme
    • At pH 5 the enzyme has more active sites, but the substrate is less soluble
    • At pH 5 the ionisation of R groups alters the active site, so fewer substrate molecules bind

12. Adding a competitive inhibitor to an enzyme reaction gives a rate of 3 units. Doubling the inhibitor concentration gives a rate of 1.5 units with the same substrate concentration. What does this suggest?

    • The inhibitor is being converted into the product, which then slows the reaction as it accumulates
    • The inhibitor competes with the substrate for the active site, so higher inhibitor concentration reduces the rate further
    • The inhibitor is denaturing the enzyme, so the active site is destroyed by the increasing concentration
    • The inhibitor is non-competitive, because its effect on the rate increases as its concentration is raised

13. A student measures the rate of an enzyme-controlled reaction at 30 C and at 40 C. The rate doubles between these temperatures. What is the Q10 value for this range?

    • 1
    • 2
    • 10
    • 4

14. Which change would increase the maximum rate (Vmax) of an enzyme-catalysed reaction?

    • Adding more of a competitive inhibitor to the mixture
    • Lowering the temperature to below the optimum for the enzyme
    • Increasing the pH well above the optimum for the enzyme
    • Increasing the enzyme concentration, so more active sites are present

15. A student says that doubling the substrate concentration always doubles the rate of an enzyme-catalysed reaction. Which evaluation is correct?

    • Incorrect, because doubling the substrate always halves the rate of an enzyme reaction
    • Correct, because substrate binding always increases the number of active sites
    • Incorrect, because the rate reaches a plateau once all active sites are saturated
    • Correct, because the rate of an enzyme reaction is always proportional to substrate concentration

16. Explain why a competitive inhibitor's effect can be overcome by raising substrate concentration, while a non-competitive inhibitor's effect cannot.

    • Competitive inhibitors change the primary structure of the enzyme, so more substrate restores the active site
    • Both types of inhibitor act on the same site of the enzyme, so the substrate can always displace them easily
    • Competitive inhibition is reversed by competing for the active site, whereas non-competitive inhibition reduces functional active sites
    • Non-competitive inhibitors bind to the substrate itself, so raising the substrate concentration can overcome them

17. Explain why changing pH affects the rate even when enzyme and substrate concentrations are constant.

    • Changes in pH increase the activation energy of the enzyme reaction without changing the shape of the enzyme
    • Changes in pH break the peptide bonds in the enzyme, so it becomes a set of separate amino acid molecules
    • Changes in pH alter the ionisation of R groups, disrupting the bonds that maintain the active site shape and substrate binding
    • Changes in pH alter the number of substrate molecules present in the mixture, so fewer collisions occur

18. In a required practical on a named variable, which is the best way to ensure that the measured rate reflects only the variable being tested?

    • Change two variables at the same time so that you can see which of them has the larger effect on the rate
    • Measure only a single replicate, because the chosen variable will then be much easier to control
    • Use a different enzyme for each repeat of the experiment so that the results are more varied and useful
    • Keep all other variables, such as temperature, pH and enzyme and substrate concentration, constant

19. What is meant by the optimum temperature of an enzyme?

    • The temperature at which the enzyme is first denatured by the substrate
    • The temperature at which the enzyme-controlled reaction proceeds at its greatest rate
    • The temperature at which the enzyme has the lowest number of active sites
    • The temperature at which the substrate concentration becomes zero

20. Which statement best explains why enzymes in the stomach work well at a low pH, while enzymes in the small intestine work at a higher pH?

    • The stomach contains more substrate, so the enzymes need lower pH to bind it
    • The intestine contains more enzymes, so a higher pH is needed to keep them active
    • Each enzyme has an optimum pH that matches the conditions in the part of the gut where it works
    • Enzymes in the stomach are denatured at high pH, so they must work at low pH

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