Biochemistry: Chapter 6 Exam
1. What is the primary function of enzymes in biological systems?
A) Increase temperature
B) Facilitate chemical reactions
C) Provide structural support
D) Store energy
2. Which of the following statements is true about enzymes?
A) They are consumed during the reaction.
B) They operate only in non-aqueous solutions.
C) They can increase the rate of both forward and reverse reactions.
D) They alter the equilibrium constant (Keq) of a reaction.
3. Which scientist first crystallized an enzyme and identified it as a protein?
A) Eduard Buchner
B) James Sumner
C) Louis Pasteur
D) Frederick Kuhne
4. What is a cofactor?
A) A type of enzyme
B) A non-protein component required for enzyme activity
C) A substrate
D) A byproduct of enzymatic reactions
5. Which of the following is NOT a common type of cofactor?
A) Inorganic ions
B) Coenzymes
C) Nucleic acids
D) Prosthetic groups
6. What type of enzyme catalyzes the transfer of a phosphate group?
A) Ligase
B) Oxidoreductase
C) Transferase
D) Hydrolase
7. The formation of the enzyme-substrate (ES) complex is crucial for:
A) Lowering the reaction temperature
B) Increasing the local concentration of reactants
C) Changing the pH of the reaction
D) Enhancing the stability of products
8. What term describes the energy barrier that must be overcome for a reaction to
proceed?
A) Activation energy
B) Transition state
C) Free energy
D) Binding energy
9. Enzymes lower the activation energy of a reaction by:
A) Changing the temperature
B) Stabilizing the transition state
C) Increasing product concentration
D) Decreasing substrate concentration
10. What does the enzyme commission (E.C.) number represent?
A) The temperature at which the enzyme functions
B) The substrate specificity of the enzyme
C) The classification of the enzyme based on the reaction it catalyzes
D) The size of the enzyme molecule
11. Which of the following mechanisms is NOT a way that enzymes increase reaction
rates?
A) Providing a reactive surface for substrates
B) Stabilizing the transition state
C) Increasing the energy of substrates
D) Orienting substrates for optimal contact
12. Which of the following is true about the transition state?
A) It is more stable than the ground state.
B) It represents a high-energy state that reactants must reach.
C) It is the same as the product.
D) It occurs after the reaction is complete.
13. What is the role of prosthetic groups in enzymes?
A) They are a type of substrate.
B) They assist in the reaction but are not covalently bonded.
C) They are covalently attached non-protein components required for enzyme activity.
D) They enhance the protein structure but do not affect enzyme activity.
14. In the enzyme-catalyzed reaction ATP + D-Glucose → ADP +
D-Glucose-6-phosphate, what type of enzyme is catalyzing the reaction?
A) Hydrolase
B) Ligase
C) Transferase
D) Isomerase
15. Which enzyme activity would be affected by a lack of essential cofactors?
A) None, enzymes function independently.
B) Only those requiring prosthetic groups
C) Only those that do not require cofactors
D) All enzyme activities
16. What is the standard free energy change ΔG° for a reaction?
A) The energy change at any given concentration
B) The energy change under standard conditions at pH 7
C) The energy change in the presence of an enzyme
D) The total energy change of the system
17. The catalytic power of enzymes can enhance reaction rates by how many orders
of magnitude?
A) 1 to 5
B) 5 to 17
C) 10 to 20
D) 2 to 10
18. Which of the following best describes the relationship between enzymes and
thermodynamic feasibility?
A) Enzymes make thermodynamically impossible reactions possible.
B) Enzymes can only catalyze reactions that are thermodynamically feasible.
C) Enzymes alter the thermodynamic properties of a reaction.
D) Enzymes are not influenced by thermodynamics.
19. The mechanism by which enzymes reduce activation energy includes:
A) Creating high-temperature environments
B) Stabilizing the ground state of substrates
C) Facilitating transient covalent interactions
D) Permanently altering substrate structures
20. Which statement about enzyme specificity is true?
A) Enzymes can catalyze any reaction.
B) Enzymes have an active site that is specific for certain substrates.
C) Enzymes are specific for only one substrate and no other molecules.
D) Enzymes lose their specificity upon denaturation.
21. What is the first step in an enzyme-catalyzed reaction?
a) Formation of the EP complex
b) Binding of substrate to enzyme
c) Dissociation of the enzyme
d) Formation of products
22. In the equation E + S⇌ ES, what does ES represent?
a) Enzyme
b) Substrate
c) Enzyme-Substrate complex
d) Product
23. The ES complex is converted into which complex?
a) E + S
b) E + P
c) EP
d) ES'
24. Which of the following describes the equilibrium constant Keq?
a) It is influenced by enzyme concentration
b) It is a measure of the ratio of products to reactants at equilibrium
c) It changes with temperature only
d) It depends on the amount of substrate
25. What does ΔG′∘\Delta G'^\circΔG′∘ represent in enzyme kinetics?
a) The activation energy
b) The standard free energy change of the reaction
c) The energy released during catalysis
d) The concentration of products
26. Which hypothesis proposed that enzymes have a tight fit with substrates? a)
Induced fit hypothesis
b) Lock and key hypothesis
c) Transition state hypothesis
d) Michaelis-Menten hypothesis
27. What was a major flaw in the lock and key hypothesis?
a) It did not explain substrate specificity
b) It suggested that enzymes stabilize substrates in their ground state
c) It ignored the role of cofactors
d) It was too complex
28. According to the modern notion of enzyme catalysis, which state has the highest
energy?
a) Ground state
b) Substrate state
c) Product state
d) Transition state
29. How does binding energy contribute to lowering activation energy?
a) By increasing substrate concentration
b) By forming transient covalent bonds
c) By stabilizing the ground state
d) By distorting the substrate molecule
30. Which of the following is NOT a mechanism of enzyme catalysis?
a) Acid-base catalysis
b) Covalent catalysis
c) Metal ion catalysis
d) Thermal catalysis
31. In acid-base catalysis, what role does histidine often play?
a) Proton donor
b) Electrophile
c) Nucleophile
d) Product
32. In covalent catalysis, what happens to the active site during the reaction?
a) It remains unchanged
b) It is permanently modified
c) It forms a transient covalent bond with substrate
d) It dissociates from the substrate
33. How do metal ions facilitate catalysis?
a) By increasing the activation energy
b) By generating heat
c) By stabilizing reaction intermediates
d) By lowering substrate concentration
34. What is the effect of binding reactant molecules to the active site on entropy?
a) Increases entropy
b) Decreases entropy
c) Has no effect on entropy
d) Fluctuates entropy
35. Which catalytic strategy involves replacing water interactions with
enzyme-substrate interactions?
a) Covalent catalysis
b) Acid-base catalysis
c) Metal ion catalysis
d) Desolvation
36. What is the purpose of the induced fit model proposed by Koshland?
a) To explain substrate specificity
b) To describe how enzymes stabilize substrates
c) To illustrate how enzymes change shape upon substrate binding
d) To define the lock and key hypothesis
37. Which amino acids commonly participate in acid-base catalysis?
a) Glutamate and aspartate
b) Methionine and cysteine
c) Glycine and proline
d) Leucine and isoleucine
38. Which enzyme is an example of covalent catalysis?
a) Chymotrypsin
b) Carbonic anhydrase
c) Lactate dehydrogenase
d) Amylase
39. What is the significance of the transition state in enzyme reactions?
a) It is the state with the lowest energy
b) It is a stable intermediate
c) It is the highest energy state during the reaction
d) It occurs after product formation
40. What role do metal ions play in enzymes that utilize them?
a) They provide structural support
b) They act as catalysts in all reactions
c) They enhance substrate binding and stabilize intermediates
d) They replace the enzyme’s active site
41. Which of the following is NOT an approach to study the mechanism of an
enzyme-catalyzed reaction?
a) Determining the three-dimensional structure of the enzyme
b) Mutating amino acid residues in the active site
c) Measuring the temperature of the reaction
d) Analyzing the reaction rate under varying conditions
42. What does enzyme kinetics help to understand?
a) The physical structure of enzymes
b) The reaction rates under defined conditions
c) The evolutionary history of enzymes
d) The molecular weight of substrates
43. The initial reaction rate (V0) is measured during which time frame?
a) After 5 minutes
b) Within the first 60 seconds
c) During the maximum rate of reaction
d) After the reaction reaches equilibrium
44. At what substrate concentration does V0approach maximum velocity Vmax?
a) Low substrate concentration
b) Intermediate substrate concentration
c) High substrate concentration
d) Zero substrate concentration
45. What is the shape of the substrate saturation curve for most enzymes?
a) Linear
b) Rectangular hyperbola
c) Exponential
d) Parabolic
46. According to the Michaelis-Menten theory, which step is the rate-limiting step?
a) Formation of ES complex
b) Breakdown of ES complex to free enzyme and product
c) Binding of substrate to enzyme
d) Dissociation of products
47. What does the Michaelis constant Km represent?
a) The maximum velocity of the reaction
b) The substrate concentration at which V0 = Vmax/2
c) The rate of product formation
d) The total enzyme concentration
48. In the Lineweaver-Burk plot, what does the Y-intercept represent?
a) Km
b) Vmax
c) 1/Vmax
d) 1/Km
49. How does an increase in substrate concentration affect the initial reaction rate V0
at low substrate concentrations?
a) Decreases V0
b) Has no effect
c) Increases V0linearly
d) Causes V0 to plateau
50. What is the effect of Kmon substrate affinity?
a) Higher Kmindicates higher substrate affinity
b) Lower Kmindicates lower substrate affinity
c) Higher Kmindicates lower substrate affinity
d) Kmhas no effect on substrate affinity
51. The turnover number Kcat indicates what aspect of enzyme function?
a) The total concentration of enzyme
b) The number of substrate molecules converted to product per unit time
c) The rate of substrate binding
d) The activation energy of the reaction
52. How is the Michaelis-Menten equation expressed?
a) V0 = Kmax ⋅ [S] / Km + [S]
b) V0 = Km ⋅ [S]
c) V0 = Vmax / Km ⋅ [S]
d) V0 = Vmax ⋅ [S] / Km + [S]
53. What is the significance of the Lineweaver-Burk plot?
a) It provides a direct measure of Km
b) It allows determination of Vmaxand types of enzyme inhibition
c) It simplifies the calculation of reaction rates
d) It eliminates the need for substrate concentration measurements
54. Which of the following statements is true regarding Vmax?
a) It is reached when all enzyme molecules are bound to substrate
b) It is the minimum rate of reaction
c) It cannot be affected by enzyme concentration
d) It represents the steady-state concentration of products
55. What does a higher Kcatvalue indicate about an enzyme?
a) It has a lower turnover number
b) It is less efficient
c) It converts substrate to product more rapidly
d) It has a lower affinity for substrate
56. Which of the following statements is correct about enzyme inhibition?
a) It always increases Vmax
b) Competitive inhibition increases Km
c) Non-competitive inhibition decreases Km
d) Uncompetitive inhibition affects both Kmand Vmax
57. What is the role of the active site in an enzyme?
a) It is the site of product release
b) It is where substrates bind and reactions occur
c) It stabilizes the enzyme structure
d) It is involved in enzyme regulation
58. How is Kcatrelated to Vmaxand total enzyme concentration [Et]?
a) Kcat = Vmax ⋅ [Et]
b) Kcat = Vmax / [Et]
c) Kcat = [Et] ⋅ Vmax
d) Kcat [Et] / Vmax
59. What does a plot of -1/Kmversus 1/[Vmax] represent?
a) Reaction rate
b) Substrate concentration
c) Lineweaver-Burk plot
d) Michaelis-Menten curve
60. In the context of enzyme kinetics, what does desolvation refer to?
a) Binding of substrate to the active site
b) Removal of water molecules from the enzyme's active site
c) Reaction of enzyme with products
d) Stabilization of enzyme structure
61. What is the primary purpose of studying the three-dimensional structure of an
enzyme?
A) To determine its molecular weight
B) To understand its catalytic mechanism
C) To identify its substrate specificity
D) To measure its kinetic efficiency
62. Which parameter is not affected by competitive inhibition?
A) Vmax
B) Km
C) Initial reaction rate (Vo)
D) The affinity of the enzyme for its substrate
63. The Michaelis-Menton equation describes the relationship between:
A) Enzyme concentration and product formation
B) Substrate concentration and enzyme activity
C) Temperature and enzyme activity
D) Enzyme inhibitors and substrate binding
64. In enzyme kinetics, the term Km refers to:
A) Maximum velocity of the enzyme
B) Substrate concentration at half Vmax
C) Total enzyme concentration
D) The rate constant for product formation
65. What is the effect of increasing substrate concentration on the initial reaction rate
(Vo) at high substrate levels?
A) Vo decreases
B) Vo increases linearly
C) Vo reaches a plateau
D) Vo becomes unpredictable
66. The specificity constant (Kcat/Km) is used to compare:
A) The maximum velocity of different enzymes
B) The affinity of the same enzyme for different substrates
C) The efficiency of different enzymes
D) The rate of reaction at varying temperatures
67. What type of inhibition is characterized by an inhibitor binding only to the ES
complex?
A) Competitive inhibition
B) Uncompetitive inhibition
C) Mixed inhibition
D) Non-competitive inhibition
68. In the Lineweaver-Burk plot, what does the y-intercept represent?
A) -1/Km
B) Vmax
C) 1/Vmax
D) Km
69. What is the effect of uncompetitive inhibition on Vmax?
A) Vmax remains unchanged
B) Vmax increases
C) Vmax decreases
D) Vmax becomes infinite
70. Which of the following is an irreversible inhibitor?
A) Methotrexate
B) Iodoacetamide
C) Ethanol
D) Sulfanilamide
71. In the presence of a competitive inhibitor, the apparent Km of an enzyme:
A) Decreases
B) Increases
C) Remains the same
D) Becomes zero
72. Which factor is primarily used to determine the order of binding of substrates in
enzyme kinetics?
A) Temperature
B) pH
C) Reaction rate
D) Inhibitor concentration
73. What is the primary function of enzyme inhibitors in biological systems?
A) To enhance enzyme activity
B) To slow down or stop enzyme activity
C) To promote substrate binding
D) To increase product formation
74. The turnover number (Kcat) is defined as:
A) The rate at which substrate is converted to product
B) The number of substrate molecules converted per unit time per enzyme molecule
C) The total concentration of the enzyme
D) The maximum substrate concentration in a reaction
75. A plot of Vo versus [S] that yields a hyperbolic curve is indicative of:
A) Non-competitive inhibition
B) Uncompetitive inhibition
C) Michaelis-Menton kinetics
D) Mixed inhibition
76. What does a higher Km value indicate about an enzyme's affinity for its substrate?
A) Higher affinity
B) Lower affinity
C) No change in affinity
D) The enzyme is non-functional
77. What type of reaction pathway is characterized by a single displacement?
A) Enzyme inhibition
B) Bisubstrate reaction
C) Irreversible reaction
D) Uncompetitive inhibition
78. In competitive inhibition, what happens to Vmax?
A) It remains the same
B) It decreases
C) It increases
D) It becomes zero
79. The reaction mechanism in which the enzyme alternates between two states with
different affinities for the substrates is known as:
A) Double displacement
B) Single displacement
C) Mixed inhibition
D) Competitive inhibition
80. Which statement about specificity constants is true?
A) It has units of M/s
B) It can exceed the diffusion-controlled limit
C) It is a second-order rate constant
D) It does not provide insight into enzyme efficiency
81. What type of enzymes exhibit increased or decreased catalytic activity in
response to signals?
A) Competitive enzymes
B) Allosteric enzymes
C) Monomeric enzymes
D) Zymogens
82. Which of the following statements is true about allosteric enzymes?
A) They are generally monomeric.
B) They have multiple active sites.
C) They can exhibit cooperativity.
D) They do not change conformation upon substrate binding.
83. What happens when a ligand modulator binds to an allosteric enzyme?
A) The active site remains unchanged.
B) There is a conformational change affecting the active site.
C) The enzyme becomes less active.
D) The enzyme denatures.
84. If both the ligand modulator and the substrate are the same molecule, the
allosteric enzyme is said to be:
A) Heterotropic
B) Homotropic
C) Monomeric
D) Zymogenic
85. What is the term for the phenomenon where binding of one ligand molecule
facilitates the binding of other molecules to a protein?
A) Allosteric regulation
B) Cooperativity
C) Feedback inhibition
D) Covalent modification
86. Which type of allosteric enzyme exhibits a sigmoid substrate-saturation curve?
A) Homotropic
B) Heterotropic
C) Monomeric
D) Zymogenic
87. What does K0.5 represent in the context of homotropic allosteric enzymes?
A) Substrate concentration at which maximum velocity (Vmax) is achieved
B) Substrate concentration at which half the Vmax is achieved
C) Enzyme concentration
D) Inhibitor concentration
88. In heterotropic allosteric enzymes, positive modulation results in:
A) Increased K0.5 without change in Vmax
B) Decreased K0.5 without change in Vmax
C) Decreased Vmax without change in K0.5
D) Increased Vmax without change in K0.5
89. Which of the following covalent modifications is commonly involved in regulating
enzyme activity?
A) Hydrolysis
B) Phosphorylation
C) Oxidation
D) Glycolysis
90. What role does phosphorylation play in enzyme activity?
A) It always activates the enzyme.
B) It imparts a negative charge that can lead to conformational changes.
C) It is irreversible.
D) It has no effect on enzyme activity.
91. The inactive precursor of an enzyme is known as:
A) Allosteric modulator
B) Zymogen
C) Isoenzyme
D) Active enzyme
92. Which enzyme is activated by proteolytic cleavage in the intestine?
A) Chymotrypsinogen
B) Pepsinogen
C) Trypsinogen
D) Amylase
93. The binding of pancreatic trypsin inhibitor serves to:
A) Activate trypsin
B) Inhibit trypsin
C) Increase trypsin activity
D) Promote the synthesis of trypsinogen
94. Which type of regulation is characterized by irreversible cleavage of precursor
proteins?
A) Allosteric regulation
B) Covalent modification
C) Proteolytic cleavage
D) Feedback inhibition
95. Which of the following is an example of negative allosteric regulation?
A) Substrate binding increasing enzyme activity
B) End product inhibition slowing down the pathway
C) Activation of enzymes by phosphorylation
D) Cooperativity enhancing binding
96. What is the effect of a positive modulator on a heterotropic allosteric enzyme?
A) Decreases Vmax
B) Increases K0.5
C) Increases Vmax
D) Decreases enzyme affinity for substrate
97. Enzyme activation often occurs through:
A) Competitive inhibition
B) Proteolytic cleavage
C) Allosteric modulation
D) Both B and C
98. What happens during the phosphorylation of a protein?
A) The protein is always deactivated.
B) A phosphate group is added, changing the protein's charge and properties.
C) It converts the protein to an enzyme.
D) It does not affect the protein's function.
99. Which enzyme can be regulated by ADP-ribosylation?
A) Trypsin
B) Dinitrogen reductase
C) Cyclooxygenase
D) G-proteins
100. The phenomenon where a substrate binding to one subunit of an enzyme
facilitates the binding of substrates to other subunits is called:
A) Enzyme saturation
B) Cooperativity
C) Feedback inhibition
D) Allosteric inhibition
ANSWER KEY
1. B) Facilitate chemical reactions
2. C) They can increase the rate of both forward and reverse reactions.
3. B) James Sumner
4. B) A non-protein component required for enzyme activity
5. C) Nucleic acids
6. C) Transferase
7. B) Increasing the local concentration of reactants
8. A) Activation energy
9. B) Stabilizing the transition state
10. C) The classification of the enzyme based on the reaction it catalyzes
11. C) Increasing the energy of substrates
12. B) It represents a high-energy state that reactants must reach.
13. C) They are covalently attached non-protein components required for enzyme
activity.
14. C) Transferase
15. D) All enzyme activities
16. B) The energy change under standard conditions at pH 7
17. B) 5 to 17
18. B) Enzymes can only catalyze reactions that are thermodynamically feasible.
19. C) Facilitating transient covalent interactions
20. B) Enzymes have an active site that is specific for certain substrates.
21. b) Binding of substrate to enzyme
22. c) Enzyme-Substrate complex
23. c) EP
24. b) It is a measure of the ratio of products to reactants at equilibrium
25. b) The standard free energy change of the reaction
26. b) Lock and key hypothesis
27. b) It suggested that enzymes stabilize substrates in their ground state
28. d) Transition state
29. d) By distorting the substrate molecule
30. d) Thermal catalysis
31. a) Proton donor
32. c) It forms a transient covalent bond with substrate
33. c) By stabilizing reaction intermediates
34. b) Decreases entropy
35. d) Desolvation
36. c) To illustrate how enzymes change shape upon substrate binding
37. a) Glutamate and aspartate
38. a) Chymotrypsin
39. c) It is the highest energy state during the reaction
40. c) They enhance substrate binding and stabilize intermediates
41. c) Measuring the temperature of the reaction
42. b) The reaction rates under defined conditions
43. b) Within the first 60 seconds
44. c) High substrate concentration
45. b) Rectangular hyperbola
46. b) Breakdown of ES complex to free enzyme and product
47. b) The substrate concentration at which V0 = Vmax/2
48. c) 1/Vmax
49. c) Increases V0 linearly
50. c) Higher KmK_mKmindicates lower substrate affinity
51. b) The number of substrate molecules converted to product per unit time
52. d) V0 = Vmax ⋅ [S] / Km + [S]
53. b) It allows determination of Vmaxand types of enzyme inhibition
54. a) It is reached when all enzyme molecules are bound to substrate
55. c) It converts substrate to product more rapidly
56. b) Competitive inhibition increases Km
57. b) It is where substrates bind and reactions occur
58. b) Kcat = Vmax / [Et]
59. c) Lineweaver-Burk plot
60. b) Removal of water molecules from the enzyme's active site
61. B Three-dimensional structure of the enzyme
62. A Initial reaction rate (Vo)
63. B Steady state
64. B Michaelis-Menton curve
65. C Rate limiting step
66. C Michaelis constant (Km)
67. B Substrate concentration
68. C Enzyme-catalyzed reaction
69. C Rate at which ES complex is broken down
70. B Specificity constant
71. B Turnover number (Kcat)
72. C Single displacement reaction
73. B Competitive inhibition
74. B Uncompetitive inhibition
75. C Mixed inhibition
76. B Irreversible inhibition
77. B Kinetic efficiency
78. A Lineweaver-Burk plot
79. A Diffusion-controlled limit
80. C Iodoacetamide
81. B) Allosteric enzymes
82. C) They can exhibit cooperativity.
83. B) There is a conformational change affecting the active site.
84. B) Homotropic
85. B) Cooperativity
86. A) Homotropic
87. B) Substrate concentration at which half the Vmax is achieved
88. B) Decreased K0.5 without change in Vmax
89. B) Phosphorylation
90. B) It imparts a negative charge that can lead to conformational changes.
91. B) Zymogen
92. C) Trypsinogen
93. B) Inhibit trypsin
94. C) Proteolytic cleavage
95. B) End product inhibition slowing down the pathway
96. C) Increases Vmax
97. D) Both B and C
98. B) A phosphate group is added, changing the protein's charge and properties.
99. B) Dinitrogen reductase
100. B) Cooperativity
