Rucete ✏ AP Biology In a Nutshell
5. Enzymes — Practice Questions 2
This chapter introduces enzyme mechanisms, environmental impacts, and reaction energy dynamics.
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(Multiple Choice — Click to Reveal Answer)
1. What characteristic of enzymes allows them to be highly specific to their substrates?
(A) Their primary structure
(B) Their tertiary structure
(C) Their hydrophobic interactions
(D) Their binding to cofactors
Answer
(B) — The three-dimensional tertiary structure forms the specific active site for substrates.
2. Which factor most likely decreases enzyme reaction rates by changing the charge properties of the active site?
(A) Temperature increase
(B) Extreme pH
(C) Substrate saturation
(D) Addition of coenzymes
Answer
(B) — Extreme pH can alter charges in the active site, affecting enzyme-substrate binding.
3. When a competitive inhibitor is added to an enzymatic reaction, what will likely happen to Vmax (maximum rate)?
(A) Vmax increases
(B) Vmax decreases
(C) Vmax remains the same
(D) Vmax doubles
Answer
(C) — Competitive inhibitors do not affect Vmax, only Km (substrate concentration at half Vmax).
4. Which describes the effect of temperature below an enzyme’s optimum range?
(A) Increases reaction rate
(B) Decreases molecular collisions
(C) Causes permanent denaturation
(D) Alters substrate structure
Answer
(B) — Low temperatures reduce collision frequency between enzyme and substrate.
5. Which feature distinguishes cofactors from coenzymes?
(A) Cofactors are proteins.
(B) Cofactors are inorganic molecules.
(C) Coenzymes are metals.
(D) Coenzymes always inhibit reactions.
Answer
(B) — Cofactors are inorganic (like metal ions); coenzymes are organic molecules.
6. The 'lock and key' model of enzyme action suggests that:
(A) Enzymes change shape when binding substrates.
(B) Substrates induce enzyme shape change.
(C) Substrates perfectly fit into a rigid active site.
(D) Enzymes are consumed during reactions.
Answer
(C) — The lock and key model suggests a perfect fit between substrate and active site.
7. A noncompetitive inhibitor affects which parameter of enzyme kinetics?
(A) Decreases Km
(B) Increases Km
(C) Decreases Vmax
(D) Increases Vmax
Answer
(C) — Noncompetitive inhibition decreases Vmax by reducing available active enzymes.
8. Which of the following is an example of a coupled reaction?
(A) Breakdown of glucose alone
(B) Hydrolysis of ATP alone
(C) ATP hydrolysis driving sucrose formation
(D) Passive diffusion of molecules
Answer
(C) — ATP hydrolysis drives endergonic reactions like sucrose formation.
9. When enzyme concentration increases while substrate remains constant, the reaction rate will:
(A) Decrease
(B) Stay the same
(C) Initially increase
(D) Decrease and then stabilize
Answer
(C) — More enzymes initially allow faster conversion of available substrates.
10. Which best defines allosteric inhibition?
(A) Inhibitor binding to active site
(B) Inhibitor changing substrate structure
(C) Inhibitor binding elsewhere, modifying enzyme shape
(D) Inhibitor decreasing substrate concentration
Answer
(C) — Allosteric inhibitors bind away from the active site and change enzyme conformation.
11. What happens when an enzyme becomes saturated with substrate?
(A) The reaction rate keeps increasing indefinitely.
(B) The reaction rate reaches a maximum and levels off.
(C) The enzyme denatures immediately.
(D) The substrate concentration falls rapidly.
Answer
(B) — Once all active sites are occupied, the reaction rate plateaus.
12. Which best describes the induced fit model of enzyme action?
(A) The enzyme molds itself slightly around the substrate.
(B) The enzyme is rigid and does not change shape.
(C) Substrate changes permanently after binding.
(D) Enzyme and substrate repel each other.
Answer
(A) — The induced fit model suggests enzymes adjust slightly to fit substrates.
13. Which situation would most likely decrease enzyme activity?
(A) Increasing substrate concentration
(B) Raising temperature to optimum
(C) Extreme shift in pH
(D) Adding a coenzyme
Answer
(C) — Extreme pH changes can denature enzymes or alter their active site.
14. What would be the immediate effect of adding a competitive inhibitor to an enzymatic reaction?
(A) Decrease in product formation
(B) Increase in enzyme concentration
(C) Decrease in substrate availability
(D) Permanent enzyme denaturation
Answer
(A) — Competitive inhibitors temporarily reduce product formation by blocking active sites.
15. What is the effect of a noncompetitive inhibitor on the enzyme’s active site?
(A) Destroys it
(B) Blocks it directly
(C) Changes its shape
(D) Binds to the substrate
Answer
(C) — Noncompetitive inhibitors change the enzyme’s shape, altering the active site indirectly.
16. Which of the following best defines an exergonic reaction?
(A) A reaction that requires energy input
(B) A reaction that releases energy
(C) A reaction that increases activation energy
(D) A reaction that breaks enzymes
Answer
(B) — Exergonic reactions release free energy to the surroundings.
17. Why can an increase in temperature up to a point speed up enzyme activity?
(A) It destroys the enzyme faster.
(B) It increases random collisions between enzymes and substrates.
(C) It decreases the free energy of the reaction.
(D) It forces substrates into active sites mechanically.
Answer
(B) — Higher temperature increases kinetic energy, leading to more frequent collisions.
18. Which event would most likely decrease enzyme efficiency?
(A) Optimal pH maintained
(B) Excessive heat denaturing the enzyme
(C) Adding a coenzyme
(D) Saturating with substrates
Answer
(B) — Excessive heat disrupts enzyme structure, reducing activity.
19. What structural feature of an enzyme is crucial for its specificity?
(A) Size of enzyme
(B) Active site's shape and chemical properties
(C) Number of cofactors
(D) Quantity of substrate molecules
Answer
(B) — The active site's precise shape and chemistry determine substrate specificity.
20. How does the addition of a coenzyme affect an enzymatic reaction?
(A) Inhibits reaction
(B) Denatures enzyme
(C) Enhances enzyme efficiency
(D) Increases activation energy
Answer
(C) — Coenzymes often assist in stabilizing the enzyme-substrate complex, enhancing efficiency.
21. What is the primary reason enzymes speed up reactions?
(A) They increase substrate energy.
(B) They provide reactants.
(C) They lower the activation energy.
(D) They raise product energy levels.
Answer
(C) — Enzymes lower activation energy, making reactions proceed faster.
22. What is the best description of enzyme specificity?
(A) One enzyme can catalyze any reaction.
(B) Enzymes work best at high temperatures.
(C) Enzymes fit specific substrates based on shape and chemistry.
(D) Enzymes require noncompetitive inhibitors to work.
Answer
(C) — Enzymes are highly specific to their substrates' shape and chemical properties.
23. How do enzymes affect the free energy of a chemical reaction?
(A) They increase it.
(B) They decrease it.
(C) They do not change it.
(D) They destroy reactants to lower it.
Answer
(C) — Enzymes lower activation energy but do not change the overall free energy difference.
24. Which statement best explains why an enzyme might become inactive at high temperatures?
(A) Substrate concentration becomes too high.
(B) Enzyme structure is disrupted.
(C) Free energy decreases dramatically.
(D) The enzyme absorbs all energy available.
Answer
(B) — High heat can disrupt the enzyme's structure, denaturing it.
25. What would happen if the active site of an enzyme were altered?
(A) The enzyme would work faster.
(B) The enzyme would no longer bind the substrate properly.
(C) The substrate would degrade immediately.
(D) The activation energy would rise sharply.
Answer
(B) — Altering the active site usually prevents proper substrate binding, stopping function.
26. When substrate concentration is extremely high, what happens to competitive inhibition?
(A) It becomes less effective.
(B) It becomes stronger.
(C) It permanently binds the enzyme.
(D) It alters the product's structure.
Answer
(A) — High substrate concentrations outcompete competitive inhibitors.
27. Why does a noncompetitive inhibitor reduce the Vmax of an enzyme-catalyzed reaction?
(A) It permanently removes substrate.
(B) It blocks the active site directly.
(C) It decreases the number of functional enzymes.
(D) It speeds up substrate binding.
Answer
(C) — Noncompetitive inhibitors bind elsewhere and deactivate enzyme function.
28. Which scenario represents enzyme saturation?
(A) All enzyme active sites are occupied by substrates.
(B) The number of enzymes doubles.
(C) The reaction stops completely.
(D) The enzyme denatures immediately.
Answer
(A) — Enzyme saturation occurs when all active sites are occupied.
29. In an enzyme-catalyzed reaction, if temperature is lowered dramatically, what is the likely immediate effect?
(A) Enzyme denatures.
(B) Molecular collisions decrease, slowing the reaction.
(C) Substrate transforms faster.
(D) Product concentration immediately increases.
Answer
(B) — Lower temperature reduces molecular motion, slowing reactions.
30. If a molecule structurally mimics a substrate but binds irreversibly to the active site, it would be classified as:
(A) A noncompetitive inhibitor
(B) A reversible inhibitor
(C) An irreversible competitive inhibitor
(D) A coenzyme
Answer
(C) — An irreversible competitive inhibitor permanently blocks the active site.
31. In a reaction graph, which part represents the activation energy without an enzyme?
(A) The flat baseline
(B) The highest peak
(C) The distance between product and reactant energy
(D) The endpoint of the reaction
Answer
(B) — The highest peak shows the activation energy needed without enzyme catalysis.
32. Why can’t increasing substrate concentration overcome the effects of a noncompetitive inhibitor?
(A) Noncompetitive inhibitors remove substrates from the solution.
(B) Noncompetitive inhibitors bind to the substrate directly.
(C) Noncompetitive inhibitors alter enzyme shape and reduce activity regardless of substrate levels.
(D) Noncompetitive inhibitors increase substrate energy.
Answer
(C) — They change enzyme shape, not substrate access to the active site.
33. What role do cofactors most directly play in enzymatic activity?
(A) Alter substrate structure
(B) Block noncompetitive inhibitors
(C) Help maintain proper active site configuration
(D) Prevent enzymes from denaturing
Answer
(C) — Cofactors assist by stabilizing or activating the active site's structure.
34. During enzyme catalysis, temporary covalent bonds between enzyme and substrate are most likely to:
(A) Increase activation energy
(B) Speed up the reaction
(C) Decrease enzyme efficiency
(D) Denature the enzyme
Answer
(B) — Forming temporary covalent bonds helps stabilize the transition state, speeding up reactions.
35. A catalyzed reaction releases the same amount of energy as an uncatalyzed one because:
(A) Enzymes lower product energy
(B) Enzymes do not affect overall free energy change
(C) Enzymes release energy directly
(D) Enzymes act as energy reservoirs
Answer
(B) — Enzymes only lower activation energy, not the overall ΔG (free energy change).
36. Explain why enzyme saturation occurs even if more substrate is added.
Answer
At saturation, all enzyme active sites are occupied, so adding more substrate does not increase the reaction rate further.
37. Why does denaturation of an enzyme usually lead to a loss of function?
Answer
Denaturation alters the enzyme’s three-dimensional shape, preventing proper substrate binding at the active site.
38. How does a decrease in temperature impact the kinetic energy of molecules in an enzyme-catalyzed reaction?
Answer
Lower temperatures reduce molecular kinetic energy, resulting in fewer collisions between enzyme and substrate.
39. What structural feature allows noncompetitive inhibitors to regulate enzyme activity?
Answer
They bind to an allosteric site, causing a conformational change that alters the enzyme's function.
40. Compare the effects of a competitive and noncompetitive inhibitor on Km and Vmax of an enzyme.
Answer
Competitive inhibitors increase Km but do not affect Vmax; noncompetitive inhibitors decrease Vmax without changing Km.
41. Describe what happens to an enzyme’s active site when exposed to extreme heat.
Answer
Extreme heat can break hydrogen bonds and other interactions, altering the active site’s shape and preventing substrate binding.
42. How can feedback inhibition regulate enzyme activity within a metabolic pathway?
Answer
The end product of a pathway binds to an enzyme earlier in the sequence, reducing its activity and slowing the pathway.
43. Why do enzymes show an optimum pH at which their activity is highest?
Answer
At optimum pH, the enzyme’s structure and active site are most stable, allowing maximal substrate binding and catalysis.
44. What happens to enzyme efficiency if the ionic environment (salt concentration) changes drastically?
Answer
Changes in ion concentration can disrupt ionic bonds in the enzyme’s structure, leading to denaturation and decreased activity.
45. Explain why enzymes are reusable in biological reactions.
Answer
Enzymes are not consumed during reactions; they facilitate substrate conversion and remain unchanged afterward.
46. How does increasing substrate concentration affect reaction rate when enzyme concentration is kept constant?
Answer
Reaction rate increases until all enzyme active sites are saturated, beyond which adding more substrate has no effect.
47. Describe the energy profile of an enzyme-catalyzed reaction compared to an uncatalyzed reaction.
Answer
The catalyzed reaction has a lower activation energy peak but the same overall change in free energy (ΔG).
48. Why is it incorrect to say that enzymes make endergonic reactions spontaneous?
Answer
Enzymes lower activation energy but cannot alter whether a reaction is energetically favorable (spontaneous) or not.
49. Predict the effect on enzyme action if the enzyme's tertiary structure is disrupted but its active site remains intact.
Answer
Some function may remain, but overall stability and efficiency would likely decrease due to improper folding.
50. Justify why increasing temperature beyond an enzyme’s optimum leads to a rapid decline in reaction rate.
Answer
Excessive heat causes denaturation, disrupting the active site and preventing substrate binding, rapidly decreasing the reaction rate.