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5. Enzymes — Practice Questions 3
This chapter introduces enzyme structure, environmental influences, activation energy, and metabolic coupling mechanisms.
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(Multiple Choice — Click to Reveal Answer)
1. Which property allows enzymes to act as biological catalysts?
(A) Their ability to permanently bind substrates
(B) Their ability to be used up in reactions
(C) Their specific shape and chemical reactivity
(D) Their high free energy content
Answer
(C) — Enzymes have a specific shape that matches their substrates and facilitates reactions.
2. Which statement about enzyme active sites is correct?
(A) Active sites are non-specific.
(B) Active sites have shapes complementary to their substrates.
(C) Active sites are destroyed after each reaction.
(D) Active sites bind products, not reactants.
Answer
(B) — Active sites are highly specific and fit the shape of their substrates.
3. What would most likely happen to an enzyme in a highly acidic environment far from its optimum pH?
(A) Enzyme activity would improve.
(B) Enzyme shape would remain unchanged.
(C) The enzyme would denature.
(D) The reaction would become exergonic.
Answer
(C) — Extreme pH changes can disrupt bonds and denature enzymes.
4. Which of the following best defines a cofactor?
(A) A protein helper for enzymes
(B) An inorganic ion that assists enzyme function
(C) A product formed after enzymatic reaction
(D) An enzyme that works independently of substrates
Answer
(B) — Cofactors are inorganic ions that assist enzymes in catalysis.
5. Which outcome would result from the binding of a noncompetitive inhibitor?
(A) Increased substrate binding
(B) Decreased enzyme activity
(C) Increased Vmax
(D) A higher optimum temperature
Answer
(B) — Noncompetitive inhibitors reduce enzyme efficiency by altering enzyme shape.
6. What effect does an optimal temperature have on enzyme-catalyzed reactions?
(A) It denatures the enzyme.
(B) It maximizes reaction rate.
(C) It prevents substrate binding.
(D) It increases activation energy.
Answer
(B) — At optimal temperature, enzyme activity is at its maximum.
7. Which of the following is a characteristic of a competitive inhibitor?
(A) Binds to a different site and changes enzyme shape
(B) Binds to the active site and blocks substrate binding
(C) Denatures the enzyme at high concentration
(D) Lowers the activation energy further
Answer
(B) — Competitive inhibitors bind directly to the active site.
8. Which process is an example of a coupled reaction in biology?
(A) Diffusion of water
(B) Passive transport of glucose
(C) Using ATP hydrolysis to power ion transport
(D) Denaturation of an enzyme by heat
Answer
(C) — ATP hydrolysis drives many endergonic cellular processes.
9. How does a coenzyme assist in enzyme function?
(A) By breaking the enzyme apart
(B) By lowering the substrate's free energy
(C) By binding temporarily and facilitating substrate interaction
(D) By changing the substrate into an inhibitor
Answer
(C) — Coenzymes help stabilize the enzyme-substrate interaction.
10. Which statement best explains the enzyme-substrate complex?
(A) It is a random collision of enzyme and substrate.
(B) It involves temporary binding that positions reactants favorably.
(C) It creates a permanent bond between enzyme and product.
(D) It increases the free energy of the products.
Answer
(B) — The enzyme temporarily binds the substrate to lower activation energy.
11. What happens when an enzyme exceeds its optimum temperature?
(A) The reaction rate doubles.
(B) The enzyme becomes more stable.
(C) The enzyme denatures, losing activity.
(D) The activation energy sharply decreases.
Answer
(C) — High temperatures disrupt the enzyme’s structure and function.
12. Why are enzymes not consumed during reactions they catalyze?
(A) They form covalent bonds with products.
(B) They only lower activation energy without being permanently changed.
(C) They break apart after each use.
(D) They become incorporated into the substrate.
Answer
(B) — Enzymes facilitate reactions but are not used up.
13. If the shape of an enzyme's active site is altered, the most likely consequence is:
(A) Faster reaction rate
(B) Irreversible product formation
(C) Decreased substrate binding
(D) Enhanced coenzyme binding
Answer
(C) — Altered active sites reduce or prevent substrate binding.
14. Which best describes activation energy?
(A) Energy released during a reaction
(B) Energy needed to initiate a chemical reaction
(C) Energy trapped in enzyme-substrate complexes
(D) Energy used up by cofactors
Answer
(B) — Activation energy is required to start a reaction.
15. Which would be the immediate result of adding a large amount of substrate to an enzyme-catalyzed reaction at saturation?
(A) Dramatic increase in reaction rate
(B) No significant change in reaction rate
(C) Decrease in enzyme efficiency
(D) Permanent denaturation of the enzyme
Answer
(B) — At saturation, adding more substrate doesn't increase the rate further.
16. In an enzymatic reaction, what term describes the molecule an enzyme acts upon?
(A) Product
(B) Substrate
(C) Catalyst
(D) Inhibitor
Answer
(B) — The substrate is the molecule an enzyme binds to and modifies.
17. Which of the following occurs when enzyme concentration increases while substrate concentration remains constant?
(A) Reaction rate initially increases
(B) Reaction rate decreases
(C) Activation energy increases
(D) Enzyme denaturation occurs immediately
Answer
(A) — More enzymes allow more substrates to be processed initially.
18. Which is the best description of a transition state in an enzymatic reaction?
(A) The most stable configuration
(B) A temporary, high-energy structure
(C) A final, low-energy product
(D) An inhibitor-bound enzyme complex
Answer
(B) — The transition state is a high-energy, unstable intermediate during a reaction.
19. Which scenario would increase the reaction rate of an enzyme-catalyzed reaction at suboptimal low temperatures?
(A) Adding a noncompetitive inhibitor
(B) Adding more substrate
(C) Slightly increasing the temperature toward optimum
(D) Decreasing enzyme concentration
Answer
(C) — Raising the temperature closer to the optimum will increase kinetic energy and collisions.
20. If an enzyme is subjected to a pH significantly higher than its optimum, the most likely outcome is:
(A) Increased substrate binding
(B) Enzyme denaturation
(C) Lower activation energy
(D) More efficient catalysis
Answer
(B) — Extreme pH changes disrupt ionic bonds in enzymes, causing denaturation.
21. What would likely happen if the active site of an enzyme became blocked by another molecule?
(A) Increased product formation
(B) Reaction rate would decrease
(C) Activation energy would decrease further
(D) Substrate concentration would fall to zero
Answer
(B) — Blocking the active site prevents substrate binding, slowing the reaction.
22. Which of the following correctly describes allosteric regulation of an enzyme?
(A) The substrate binds permanently to the enzyme.
(B) A molecule binds to a site other than the active site, altering enzyme activity.
(C) The enzyme is consumed during the reaction.
(D) The enzyme unfolds completely before binding the substrate.
Answer
(B) — Allosteric regulators bind elsewhere and modify the enzyme's function.
23. How do enzymes affect the speed of biological reactions compared to uncatalyzed reactions?
(A) They slow reactions by stabilizing substrates.
(B) They eliminate the need for energy input.
(C) They increase reaction speed by lowering activation energy.
(D) They transform products into new reactants instantly.
Answer
(C) — Lowering activation energy allows reactions to proceed much faster.
24. A graph comparing catalyzed and uncatalyzed reactions would show:
(A) Higher activation energy for catalyzed reactions
(B) Lower activation energy for catalyzed reactions
(C) Higher overall free energy for catalyzed reactions
(D) No change in activation energy
Answer
(B) — Enzymes lower the activation energy required to start reactions.
25. What is the function of the enzyme catalase in biological systems?
(A) Synthesizes hydrogen peroxide
(B) Breaks down hydrogen peroxide into water and oxygen
(C) Converts water into hydrogen peroxide
(D) Produces ATP from ADP and phosphate
Answer
(B) — Catalase breaks hydrogen peroxide into harmless water and oxygen gas.
26. In an enzyme-catalyzed reaction, which change would most likely result if a noncompetitive inhibitor is introduced?
(A) Vmax decreases while Km stays the same.
(B) Both Vmax and Km increase.
(C) Km decreases while Vmax stays the same.
(D) Both Vmax and Km decrease.
Answer
(A) — Noncompetitive inhibitors lower Vmax without changing Km.
27. In a reaction profile, what does the height difference between the reactants and the transition state represent?
(A) The total energy released
(B) The activation energy
(C) The energy of the products
(D) The free energy difference
Answer
(B) — Activation energy is the energy needed to reach the transition state from reactants.
28. Which of the following best explains how enzymes lower activation energy?
(A) By stabilizing the transition state
(B) By changing reactant free energy
(C) By decreasing the amount of product formed
(D) By increasing entropy in the system
Answer
(A) — Enzymes stabilize the transition state, reducing the energy required to reach it.
29. When a competitive inhibitor is present, what change would you expect in a Michaelis-Menten plot?
(A) Vmax decreases, Km stays constant.
(B) Km increases, Vmax stays constant.
(C) Both Km and Vmax decrease.
(D) Km decreases and Vmax increases.
Answer
(B) — Competitive inhibitors increase Km but do not affect Vmax.
30. Which experimental condition would most likely denature an enzyme permanently?
(A) Slight pH shift from optimal
(B) Mild temperature increase
(C) Extreme heat exposure
(D) Doubling substrate concentration
Answer
(C) — Extreme heat can permanently unfold and denature enzymes.
31. In feedback inhibition, the end product of a metabolic pathway acts as:
(A) A competitive inhibitor
(B) A noncompetitive inhibitor
(C) A substrate mimic
(D) An allosteric activator
Answer
(B) — Feedback inhibition often involves noncompetitive (allosteric) inhibition.
32. Why does the presence of a noncompetitive inhibitor lower the Vmax of an enzyme-catalyzed reaction?
(A) It binds to the substrate.
(B) It blocks the active site permanently.
(C) It reduces the total number of functioning enzyme molecules.
(D) It denatures the substrate molecule.
Answer
(C) — Noncompetitive inhibitors disable some enzymes without competing with substrates.
33. If the concentration of substrate is very low, which type of inhibitor will have the greatest impact on enzyme activity?
(A) Competitive inhibitor
(B) Noncompetitive inhibitor
(C) Cofactor
(D) Coenzyme
Answer
(A) — At low substrate concentrations, competitive inhibitors are very effective.
34. Which of the following is true regarding coupled reactions in metabolism?
(A) Endergonic reactions provide energy to drive exergonic reactions.
(B) Two endergonic reactions are directly linked.
(C) Exergonic reactions supply energy to drive endergonic reactions.
(D) Only exergonic reactions occur spontaneously without coupling.
Answer
(C) — Exergonic reactions (like ATP hydrolysis) are coupled to drive unfavorable endergonic reactions.
35. Which observation best supports the conclusion that an enzyme was denatured rather than merely inhibited?
(A) The enzyme activity resumed after substrate concentration increased.
(B) The enzyme structure was permanently altered.
(C) The reaction rate increased after removing the inhibitor.
(D) The enzyme activity increased with a higher temperature.
Answer
(B) — Denaturation involves irreversible structural changes, unlike reversible inhibition.
36. Explain how competitive inhibitors impact enzyme kinetics in terms of Km and Vmax.
Answer
Competitive inhibitors increase Km (indicating a need for more substrate) but do not affect Vmax, since inhibition can be overcome by high substrate concentration.
37. Describe why noncompetitive inhibitors lower Vmax but not Km.
Answer
Noncompetitive inhibitors reduce the number of functional enzymes available, decreasing Vmax, but they do not affect substrate binding affinity, so Km remains unchanged.
38. How does enzyme saturation limit the effect of increasing substrate concentration?
Answer
When all active sites are occupied, additional substrate cannot further increase the reaction rate, leading to a plateau at Vmax.
39. Explain why high temperatures cause irreversible enzyme denaturation.
Answer
Excessive heat disrupts hydrogen bonds and other stabilizing interactions, permanently altering the enzyme’s three-dimensional structure.
40. In terms of energy, why are exergonic reactions favorable even without enzymes?
Answer
Exergonic reactions release free energy (negative ΔG), making them thermodynamically favorable, although enzymes are needed to lower activation energy and speed them up.
41. How does pH affect the ionic bonds within an enzyme?
Answer
Deviations from optimal pH disrupt ionic bonds stabilizing the enzyme’s structure, potentially denaturing the enzyme and reducing its activity.
42. Why are coenzymes critical for some enzyme-catalyzed reactions?
Answer
Coenzymes assist in transferring chemical groups or electrons, enhancing the enzyme’s ability to catalyze reactions efficiently.
43. What distinguishes a substrate from a product in an enzymatic reaction?
Answer
The substrate is the reactant that binds to the enzyme’s active site, whereas the product is the molecule formed after the reaction is catalyzed.
44. Predict the outcome of an enzymatic reaction if the enzyme’s tertiary structure is slightly altered but the active site remains intact.
Answer
The enzyme may retain partial activity, but its stability and overall efficiency might decrease due to improper folding.
45. How does the induced fit model differ from the lock-and-key model?
Answer
In the induced fit model, the enzyme adjusts its shape slightly to fit the substrate better, while in the lock-and-key model, the enzyme’s shape is rigid and perfectly matches the substrate.
46. Why might a reaction’s rate initially increase with temperature but then decrease after a threshold?
Answer
Higher temperatures initially increase kinetic energy and collision frequency, but excessive heat denatures enzymes, leading to decreased reaction rates.
47. Define the term "transition state" in an enzyme-catalyzed reaction.
Answer
The transition state is a high-energy, unstable intermediate between reactants and products that must be reached for the reaction to proceed.
48. Why does lowering activation energy increase reaction speed?
Answer
Lowering activation energy allows more reactant molecules to achieve the transition state more quickly, accelerating the overall reaction.
49. How do allosteric inhibitors regulate enzyme activity?
Answer
Allosteric inhibitors bind to a site other than the active site, causing a conformational change that reduces enzyme activity.
50. Explain why enzyme-catalyzed reactions are often more efficient than uncatalyzed reactions under biological conditions.
Answer
Enzymes lower activation energy, allowing reactions to proceed rapidly at the moderate temperatures and conditions found in living organisms.