MCQs on Basic Biochemical Principles

1. What is the Michaelis-Menten constant (Km)?

• A) The maximum velocity of an enzyme reaction
• B) The substrate concentration at half-maximal velocity
• C) The enzyme turnover number
• D) The inhibitor concentration needed to block the enzyme

Answer: B) The substrate concentration at half-maximal velocity
Explanation: Km represents the substrate concentration at which an enzyme operates at half its maximum velocity (Vmax). It is an indicator of enzyme affinity, with a lower Km signifying higher affinity for the substrate.

2. What is the effect of a competitive inhibitor on enzyme kinetics?

• A) Increases Vmax and decreases Km
• B) Decreases Vmax and increases Km
• C) Increases Km without affecting Vmax
• D) Decreases both Km and Vmax

Answer: C) Increases Km without affecting Vmax
Explanation: Competitive inhibitors bind to the active site, preventing substrate binding. This increases Km (reducing affinity) but does not change Vmax because increasing substrate concentration can outcompete the inhibitor.

3. What is the physiological pH of human blood?

• A) 6.8
• B) 7.4
• C) 7.8
• D) 8.2

Answer: B) 7.4
Explanation: The normal pH of human blood is tightly regulated between 7.35-7.45. A drop below 7.35 leads to acidosis, while a rise above 7.45 leads to alkalosis, both of which can be life-threatening.

4. Which equation describes the relationship between pH, pKa, and the ratio of conjugate base to acid?

• A) Michaelis-Menten equation
• B) Henderson-Hasselbalch equation
• C) Nernst equation
• D) Lineweaver-Burk equation

Answer: B) Henderson-Hasselbalch equation
Explanation: The Henderson-Hasselbalch equation, pH = pKa + log([A⁻]/[HA]), is used to calculate pH in buffer systems based on acid-base equilibrium.

5. What happens when the pH of a solution is equal to the pKa of a weak acid?

• A) The acid is completely ionized
• B) The acid is completely unionized
• C) The concentrations of the acid and its conjugate base are equal
• D) The solution becomes highly acidic

Answer: C) The concentrations of the acid and its conjugate base are equal
Explanation: At pH = pKa, a weak acid exists in equal concentrations as its protonated (HA) and deprotonated (A⁻) forms, making it an effective buffer.

6. Which of the following is the best physiological buffer system in the human body?

• A) Acetate buffer
• B) Carbonic acid-bicarbonate buffer
• C) Phosphate buffer
• D) Ammonium buffer

Answer: B) Carbonic acid-bicarbonate buffer
Explanation: The H₂CO₃/HCO₃⁻ buffer is the most effective physiological buffer, helping maintain blood pH by regulating CO₂ levels via respiration and bicarbonate via the kidneys.

7. What does the term "Vmax" refer to in enzyme kinetics?

• A) The enzyme concentration required for the reaction
• B) The maximum reaction velocity achieved at substrate saturation
• C) The amount of substrate needed for half-maximal reaction velocity
• D) The equilibrium constant of enzyme-substrate binding

Answer: B) The maximum reaction velocity achieved at substrate saturation
Explanation: Vmax represents the fastest rate an enzyme can catalyze a reaction when all active sites are occupied by the substrate, reflecting the enzyme’s catalytic efficiency.

8. Which type of enzyme inhibition is irreversible?

• A) Competitive inhibition
• B) Non-competitive inhibition
• C) Uncompetitive inhibition
• D) Suicide inhibition

Answer: D) Suicide inhibition
Explanation: Suicide inhibitors bind covalently to an enzyme, permanently inactivating it. Examples include aspirin (inactivating COX enzymes) and penicillin (blocking bacterial transpeptidases).

9. What happens to an enzyme’s activity when the temperature exceeds its optimal range?

• A) The activity increases indefinitely
• B) The enzyme is denatured, losing function
• C) The enzyme binds more substrate
• D) The enzyme converts into a coenzyme

Answer: B) The enzyme is denatured, losing function
Explanation: High temperatures disrupt hydrogen bonds and protein structure, leading to enzyme denaturation and loss of function. Most human enzymes have an optimal temperature around 37°C.

10. What is the significance of the isoelectric point (pI) of a protein?

• A) It is the pH at which the protein has the highest activity
• B) It is the pH at which the protein carries no net charge
• C) It is the pH at which the protein is most soluble
• D) It is the pH at which the protein denatures

Answer: B) It is the pH at which the protein carries no net charge
Explanation: The isoelectric point (pI) is the pH at which a protein has an equal number of positive and negative charges, making it electrically neutral and often less soluble.

11. Which type of enzyme inhibition decreases Vmax but does not change Km?

• A) Competitive inhibition
• B) Non-competitive inhibition
• C) Uncompetitive inhibition
• D) Allosteric activation

Answer: B) Non-competitive inhibition
Explanation: In non-competitive inhibition, the inhibitor binds to an allosteric site, reducing enzyme efficiency. This decreases Vmax but does not affect Km, as substrate binding remains unchanged.

12. What is the physiological role of the phosphate buffer system?

• A) Regulates blood glucose levels
• B) Maintains intracellular pH
• C) Facilitates oxygen transport
• D) Increases enzyme activity

Answer: B) Maintains intracellular pH
Explanation: The phosphate buffer system (H₂PO₄⁻/HPO₄²⁻) is essential in intracellular fluids and the kidneys, stabilizing pH within cells.

13. What does a Lineweaver-Burk plot represent?

• A) Enzyme activity as a function of pH
• B) The inverse of enzyme velocity vs. substrate concentration
• C) The enzyme’s response to allosteric regulators
• D) The effect of temperature on enzyme activity

Answer: B) The inverse of enzyme velocity vs. substrate concentration
Explanation: The Lineweaver-Burk plot is a double reciprocal graph of enzyme kinetics used to determine Km and Vmax, aiding in inhibitor characterization.

14. What is the significance of a low Km value in an enzyme-substrate reaction?

• A) The enzyme has low affinity for the substrate
• B) The enzyme has high affinity for the substrate
• C) The enzyme requires a high substrate concentration
• D) The enzyme is non-functional

Answer: B) The enzyme has high affinity for the substrate
Explanation: A low Km indicates that the enzyme achieves half-maximal velocity at a low substrate concentration, meaning it has a high affinity for its substrate.

15. Which of the following best describes a zero-order reaction?

• A) Reaction rate depends on substrate concentration
• B) Reaction rate is independent of substrate concentration
• C) Reaction rate follows Michaelis-Menten kinetics
• D) Reaction rate increases indefinitely

Answer: B) Reaction rate is independent of substrate concentration
Explanation: In zero-order kinetics, the enzyme is saturated, and reaction rate remains constant regardless of substrate concentration, seen in ethanol metabolism by alcohol dehydrogenase.

16. What happens to pH when a strong acid is added to a buffered solution?

• A) pH changes dramatically
• B) pH remains nearly constant
• C) The solution becomes highly acidic
• D) The buffer loses its function immediately

Answer: B) pH remains nearly constant
Explanation: A buffer resists pH changes by neutralizing added acids or bases. Weak acid-conjugate base pairs (e.g., H₂CO₃/HCO₃⁻) maintain stability.

17. What is the role of a coenzyme in enzyme catalysis?

• A) It acts as a substrate
• B) It enhances enzyme stability
• C) It serves as a transient carrier of functional groups
• D) It inhibits enzyme activity

Answer: C) It serves as a transient carrier of functional groups
Explanation: Coenzymes, such as NAD+ and FAD, temporarily carry electrons or chemical groups, assisting enzymes in catalysis.

18. Which type of enzyme inhibition occurs when the inhibitor binds only to the enzyme-substrate complex?

• A) Competitive inhibition
• B) Non-competitive inhibition
• C) Uncompetitive inhibition
• D) Irreversible inhibition

Answer: C) Uncompetitive inhibition
Explanation: Uncompetitive inhibitors bind only to the enzyme-substrate complex, lowering both Km and Vmax, and are most effective at high substrate concentrations.

19. What is the role of the Hendersen-Hasselbalch equation in buffer systems?

• A) Determines enzyme kinetics
• B) Calculates the pH of a buffer solution
• C) Measures protein denaturation
• D) Determines enzyme affinity

Answer: B) Calculates the pH of a buffer solution
Explanation: The Henderson-Hasselbalch equation helps determine buffer pH using pKa and the ratio of conjugate base to acid, maintaining physiological homeostasis.

20. Which factor does NOT influence enzyme activity?

• A) Temperature
• B) pH
• C) Enzyme concentration
• D) Atomic number of the substrate

Answer: D) Atomic number of the substrate
Explanation: Temperature, pH, and enzyme concentration directly affect reaction rate, but the atomic number of a substrate has no biochemical significance in enzyme activity.

21. What is the turnover number (kcat) of an enzyme?

• A) The number of enzyme molecules in a reaction
• B) The number of substrate molecules converted per second per enzyme molecule
• C) The total time required for an enzyme reaction
• D) The number of active sites in an enzyme

Answer: B) The number of substrate molecules converted per second per enzyme molecule
Explanation: Turnover number (kcat) represents the efficiency of an enzyme, indicating how many substrate molecules a single enzyme converts to product per second when fully saturated.

22. What happens to enzyme activity when the pH moves far from the enzyme’s optimal range?

• A) Activity increases
• B) Activity remains unchanged
• C) Enzyme denaturation and loss of function occur
• D) The enzyme converts to a cofactor

Answer: C) Enzyme denaturation and loss of function occur
Explanation: Extreme pH levels disrupt enzyme structure by affecting ionic bonds and hydrogen bonding, leading to denaturation and loss of catalytic activity.

23. Which of the following is an example of a covalently regulated enzyme?

• A) Hexokinase
• B) Trypsin
• C) Pyruvate kinase
• D) Glycogen phosphorylase

Answer: D) Glycogen phosphorylase
Explanation: Glycogen phosphorylase is regulated via covalent modification (phosphorylation and dephosphorylation), allowing rapid control of glycogen breakdown in response to cellular energy needs.

24. What is the primary role of metal ions like Zn²⁺ and Mg²⁺ in enzyme function?

• A) Inhibitors
• B) Act as cofactors to stabilize enzyme-substrate interactions
• C) Disrupt enzyme function
• D) Provide energy for enzyme activation

Answer: B) Act as cofactors to stabilize enzyme-substrate interactions
Explanation: Metal ions like Zn²⁺, Mg²⁺, and Fe²⁺ function as cofactors, stabilizing enzyme structure and assisting catalysis in metalloenzymes.

25. What effect does an increase in enzyme concentration have on reaction rate, assuming unlimited substrate?

• A) The rate decreases
• B) The rate remains unchanged
• C) The rate increases proportionally
• D) The enzyme is inactivated

Answer: C) The rate increases proportionally
Explanation: More enzyme molecules mean more active sites available for substrate conversion, increasing reaction rate as long as substrate is not limiting.

26. Which type of inhibition is observed when an inhibitor binds to both the enzyme and enzyme-substrate complex?

• A) Competitive inhibition
• B) Non-competitive inhibition
• C) Uncompetitive inhibition
• D) Mixed inhibition

Answer: D) Mixed inhibition
Explanation: Mixed inhibitors bind to both the free enzyme and enzyme-substrate complex, altering both Vmax and Km, and are not overcome by increasing substrate concentration.

27. Which equation is used to calculate buffer capacity?

• A) Michaelis-Menten equation
• B) Lineweaver-Burk equation
• C) Henderson-Hasselbalch equation
• D) Nernst equation

Answer: C) Henderson-Hasselbalch equation
Explanation: The Henderson-Hasselbalch equation determines buffer capacity, crucial for maintaining physiological pH by balancing weak acids and their conjugate bases.

28. Which of the following best defines an allosteric enzyme?

• A) An enzyme that follows Michaelis-Menten kinetics
• B) An enzyme regulated by substrate concentration only
• C) An enzyme with multiple active sites modulated by effectors
• D) An enzyme that is permanently active

Answer: C) An enzyme with multiple active sites modulated by effectors
Explanation: Allosteric enzymes have regulatory and catalytic subunits, allowing feedback inhibition or activation by binding allosteric effectors.

29. What is the primary function of a buffer system in biological fluids?

• A) To remove excess water from cells
• B) To maintain a stable pH against acid or base addition
• C) To enhance enzyme activity
• D) To regulate temperature

Answer: B) To maintain a stable pH against acid or base addition
Explanation: Buffers resist pH changes by neutralizing added acids or bases, maintaining homeostasis in physiological systems like blood (bicarbonate buffer) and cells (phosphate buffer).

30. Which enzyme exhibits a sigmoidal (S-shaped) velocity vs. substrate concentration curve?

• A) Hexokinase
• B) Glucokinase
• C) Pepsin
• D) Carbonic anhydrase

Answer: B) Glucokinase
Explanation: Glucokinase follows sigmoidal kinetics, indicating cooperative binding to glucose. This property allows glucokinase to regulate glucose metabolism effectively in the liver.

31. Which factor has the most significant effect on enzyme-catalyzed reactions?

• A) Atomic weight of the substrate
• B) Concentration of the enzyme
• C) Number of peptide bonds in the enzyme
• D) Type of amino acids in the substrate

Answer: B) Concentration of the enzyme
Explanation: The rate of an enzyme-catalyzed reaction depends on enzyme concentration, as more enzyme molecules provide more active sites for substrate binding.

32. What happens when an enzyme reaches its Vmax?

• A) Substrate concentration limits the reaction
• B) Enzyme concentration limits the reaction
• C) All active sites are occupied by substrate
• D) Enzyme activity ceases

Answer: C) All active sites are occupied by substrate
Explanation: At Vmax, the enzyme is fully saturated, meaning all active sites are occupied by substrate molecules, and increasing substrate concentration will not further increase reaction rate.

33. Which of the following amino acids can participate in acid-base catalysis?

• A) Glycine
• B) Histidine
• C) Alanine
• D) Leucine

Answer: B) Histidine
Explanation: Histidine plays a major role in acid-base catalysis because its imidazole side chain can both donate and accept protons, making it an essential residue in many enzyme active sites.

34. What is the role of bicarbonate in maintaining blood pH?

• A) Acts as a strong acid
• B) Acts as a buffer by neutralizing excess H+ ions
• C) Increases oxygen solubility in blood
• D) Acts as an enzyme cofactor

Answer: B) Acts as a buffer by neutralizing excess H+ ions
Explanation: The bicarbonate buffer system (H₂CO₃/HCO₃⁻) maintains blood pH at ~7.4 by reacting with acids and bases to prevent drastic pH changes.

35. Which of the following is an example of feedback inhibition?

• A) Increased enzyme activity with increasing substrate
• B) Product of a pathway inhibiting the first enzyme
• C) Activation of an enzyme by ATP
• D) Covalent modification of an enzyme

Answer: B) Product of a pathway inhibiting the first enzyme
Explanation: Feedback inhibition occurs when the end product of a metabolic pathway inhibits an earlier enzyme, preventing overproduction and conserving energy.

36. Which parameter represents enzyme efficiency?

• A) Km only
• B) Vmax only
• C) kcat/Km
• D) Substrate concentration

Answer: C) kcat/Km
Explanation: kcat/Km is the catalytic efficiency of an enzyme, indicating how well an enzyme converts substrate into product under physiological conditions.

37. What is the major buffer system inside cells?

• A) Bicarbonate buffer
• B) Phosphate buffer
• C) Acetate buffer
• D) Citrate buffer

Answer: B) Phosphate buffer
Explanation: The phosphate buffer system (H₂PO₄⁻/HPO₄²⁻) is the primary intracellular buffer, helping maintain pH stability in the cytoplasm.

38. How does an increase in temperature initially affect enzyme activity?

• A) Decreases activity immediately
• B) Increases activity up to an optimal point
• C) Has no effect on enzyme kinetics
• D) Causes enzyme inactivation

Answer: B) Increases activity up to an optimal point
Explanation: As temperature increases, enzyme activity rises due to higher kinetic energy, but beyond the optimal temperature (~37°C in humans), denaturation occurs.

39. What is the key characteristic of an enzyme allosteric site?

• A) It binds only the substrate
• B) It is part of the active site
• C) It is a regulatory site that binds effectors
• D) It permanently deactivates the enzyme

Answer: C) It is a regulatory site that binds effectors
Explanation: Allosteric sites bind activators or inhibitors, causing conformational changes that alter enzyme activity.

40. Which equation is used to calculate the potential of an electrochemical cell?

• A) Michaelis-Menten equation
• B) Nernst equation
• C) Lineweaver-Burk equation
• D) Henderson-Hasselbalch equation

Answer: B) Nernst equation
Explanation: The Nernst equation calculates membrane potential and is critical for understanding nerve impulse transmission and pH-dependent electrochemical gradients.

41. What is the function of an enzyme’s active site?

• A) Binding to regulatory proteins
• B) Providing structural stability
• C) Catalyzing the conversion of substrates into products
• D) Preventing substrate binding

Answer: C) Catalyzing the conversion of substrates into products
Explanation: The active site is a specific region on an enzyme where substrates bind, and catalytic reactions occur, lowering activation energy.

42. What type of enzyme inhibition occurs when the inhibitor binds reversibly to the active site?

• A) Non-competitive inhibition
• B) Uncompetitive inhibition
• C) Competitive inhibition
• D) Allosteric activation

Answer: C) Competitive inhibition
Explanation: In competitive inhibition, the inhibitor competes with the substrate for the active site, increasing Km without affecting Vmax.

43. Which property of an enzyme allows it to recognize and bind to its specific substrate?

• A) Allosteric regulation
• B) Substrate flexibility
• C) Complementary shape and charge
• D) Random diffusion

Answer: C) Complementary shape and charge
Explanation: The lock-and-key and induced fit models describe how an enzyme’s active site has a shape and charge that is complementary to its specific substrate.

44. What is the function of coenzymes in enzymatic reactions?

• A) They stabilize enzyme structure
• B) They provide energy
• C) They act as temporary carriers of chemical groups or electrons
• D) They inhibit enzyme activity

Answer: C) They act as temporary carriers of chemical groups or electrons
Explanation: Coenzymes (e.g., NAD⁺, FAD, CoA) assist enzymes by temporarily carrying electrons, functional groups, or atoms during enzymatic reactions.

45. In enzyme kinetics, what does Km represent?

• A) Maximum reaction rate
• B) Substrate concentration at half Vmax
• C) Number of active sites in the enzyme
• D) Energy required for enzyme activation

Answer: B) Substrate concentration at half Vmax
Explanation: Km (Michaelis constant) reflects substrate affinity; a lower Km indicates a higher affinity between the enzyme and substrate.

46. What happens when an enzyme is exposed to high temperatures beyond its optimal range?

• A) Enzyme activity increases indefinitely
• B) Enzyme structure denatures, leading to loss of function
• C) Enzyme activity remains constant
• D) The enzyme becomes a cofactor

Answer: B) Enzyme structure denatures, leading to loss of function
Explanation: Excess heat disrupts hydrogen bonds and secondary structures, causing denaturation, rendering the enzyme inactive.

47. Which buffer system is the most important in maintaining blood pH?

• A) Phosphate buffer
• B) Ammonia buffer
• C) Bicarbonate buffer
• D) Citrate buffer

Answer: C) Bicarbonate buffer
Explanation: The bicarbonate buffer system (H₂CO₃/HCO₃⁻) maintains blood pH (~7.4) by neutralizing acids and bases, preventing drastic pH shifts.

48. What is the significance of allosteric regulation in enzyme activity?

• A) Prevents enzyme function permanently
• B) Allows regulation of enzyme activity through effectors
• C) Alters enzyme structure irreversibly
• D) Reduces substrate concentration

Answer: B) Allows regulation of enzyme activity through effectors
Explanation: Allosteric enzymes contain regulatory sites where activators or inhibitors bind, modifying enzyme activity and conformation.

49. What happens when an enzyme-catalyzed reaction reaches equilibrium?

• A) The reaction stops
• B) The forward and reverse reaction rates are equal
• C) Substrate concentration increases indefinitely
• D) Enzyme activity ceases permanently

Answer: B) The forward and reverse reaction rates are equal
Explanation: At equilibrium, product formation and substrate conversion occur at equal rates, meaning net concentration changes no longer occur.

50. Which of the following best describes enzyme specificity?

• A) Enzymes catalyze all types of reactions
• B) Enzymes only work at high temperatures
• C) Enzymes act on specific substrates based on structural compatibility
• D) Enzymes degrade over time and stop functioning

Answer: C) Enzymes act on specific substrates based on structural compatibility
Explanation: Enzymes are highly specific to their substrates, recognizing them through complementary shapes and chemical interactions.

51. Which factor affects enzyme activity by altering the charge and shape of the active site?

• A) Substrate concentration
• B) pH
• C) Enzyme concentration
• D) Presence of inhibitors

Answer: B) pH
Explanation: pH changes can alter the ionization state of amino acids in the active site, affecting substrate binding and enzyme activity.

52. What is the role of a competitive inhibitor in enzyme activity?

• A) Binds to the allosteric site
• B) Binds to the active site and prevents substrate binding
• C) Permanently inactivates the enzyme
• D) Increases enzyme activity

Answer: B) Binds to the active site and prevents substrate binding
Explanation: Competitive inhibitors resemble the substrate and compete for the active site, increasing Km without affecting Vmax.

53. What is the effect of a non-competitive inhibitor on enzyme kinetics?

• A) Increases Km and Vmax
• B) Decreases Km and increases Vmax
• C) Decreases Vmax without affecting Km
• D) Increases substrate binding

Answer: C) Decreases Vmax without affecting Km
Explanation: Non-competitive inhibitors bind to allosteric sites, altering enzyme function but not substrate binding, reducing Vmax.

54. What is the primary function of ATP in metabolism?

• A) Acts as an enzyme inhibitor
• B) Stores and transfers energy
• C) Functions as a structural protein
• D) Serves as a coenzyme

Answer: B) Stores and transfers energy
Explanation: ATP (adenosine triphosphate) is the cell’s energy currency, providing energy for biochemical reactions through phosphorylation.

55. Which type of enzyme regulation involves reversible covalent modification?

• A) Allosteric regulation
• B) Feedback inhibition
• C) Phosphorylation/dephosphorylation
• D) Competitive inhibition

Answer: C) Phosphorylation/dephosphorylation
Explanation: Kinases and phosphatases regulate enzymes through phosphorylation (activation) and dephosphorylation (inactivation), playing a key role in signal transduction.

56. What is the function of a buffer in a biochemical reaction?

• A) Catalyzes the reaction
• B) Inhibits the reaction
• C) Maintains pH stability
• D) Increases substrate concentration

Answer: C) Maintains pH stability
Explanation: Buffers resist pH changes by neutralizing added acids or bases, ensuring optimal conditions for enzyme function.

57. Which enzyme follows sigmoidal kinetics instead of Michaelis-Menten kinetics?

• A) Hexokinase
• B) Myokinase
• C) Glucokinase
• D) Carbonic anhydrase

Answer: C) Glucokinase
Explanation: Glucokinase exhibits cooperativity, leading to a sigmoidal velocity vs. substrate concentration curve.

58. What is the pH of a neutral solution at 25°C?

• A) 0
• B) 7
• C) 14
• D) 3

Answer: B) 7
Explanation: A neutral solution has equal concentrations of H⁺ and OH⁻, resulting in a pH of 7 at 25°C.

59. What happens to enzyme activity in extreme alkaline pH conditions?

• A) Activity remains the same
• B) Activity increases indefinitely
• C) The enzyme denatures and loses function
• D) The enzyme changes into a coenzyme

Answer: C) The enzyme denatures and loses function
Explanation: Extreme pH changes disrupt enzyme structure by altering ionic interactions, leading to denaturation.

60. Which equation is used to describe enzyme kinetics?

• A) Michaelis-Menten equation
• B) Nernst equation
• C) Henderson-Hasselbalch equation
• D) Van’t Hoff equation

Answer: A) Michaelis-Menten equation
Explanation: The Michaelis-Menten equation describes the relationship between substrate concentration and reaction rate in enzyme kinetics.

61. What is the function of the Henderson-Hasselbalch equation in biochemistry?

• A) Determines enzyme activity
• B) Calculates buffer pH
• C) Measures reaction velocity
• D) Describes competitive inhibition

Answer: B) Calculates buffer pH
Explanation: The Henderson-Hasselbalch equation relates pH, pKa, and the ratio of conjugate base to acid, helping determine buffer capacity.

62. What is the main function of an enzyme catalyst?

• A) Increases activation energy
• B) Decreases reaction rate
• C) Lowers activation energy and increases reaction rate
• D) Consumes ATP to drive reactions

Answer: C) Lowers activation energy and increases reaction rate
Explanation: Enzymes speed up reactions by lowering activation energy, allowing substrates to be converted to products more efficiently.

63. What is the physiological role of carbonic anhydrase?

• A) Catalyzes protein breakdown
• B) Facilitates CO₂ transport in blood
• C) Converts glucose to glycogen
• D) Hydrolyzes ATP

Answer: B) Facilitates CO₂ transport in blood
Explanation: Carbonic anhydrase rapidly converts CO₂ and H₂O into HCO₃⁻ and H⁺, aiding CO₂ transport and maintaining acid-base balance.

64. What is the meaning of pKa in acid-base chemistry?

• A) The pH at which an acid is completely dissociated
• B) The pH at which 50% of an acid is dissociated
• C) The pH of a strong acid
• D) The ionization energy of an acid

Answer: B) The pH at which 50% of an acid is dissociated
Explanation: pKa represents the acid dissociation constant, indicating the pH where half of the acid exists in dissociated form.

65. What happens to an enzyme when it is exposed to an irreversible inhibitor?

• A) It becomes permanently inactivated
• B) It increases its activity
• C) It changes its substrate specificity
• D) It dissociates from the inhibitor after some time

Answer: A) It becomes permanently inactivated
Explanation: Irreversible inhibitors form covalent bonds with enzymes, leading to permanent loss of function.

66. In a Lineweaver-Burk plot, what does the y-intercept represent?

• A) 1/Vmax
• B) 1/Km
• C) Vmax
• D) -1/Km

Answer: A) 1/Vmax
Explanation: In a Lineweaver-Burk plot, the y-intercept corresponds to 1/Vmax, helping determine enzyme kinetics parameters.

67. What type of inhibition occurs when an inhibitor binds only to the enzyme-substrate complex?

• A) Competitive inhibition
• B) Non-competitive inhibition
• C) Uncompetitive inhibition
• D) Reversible inhibition

Answer: C) Uncompetitive inhibition
Explanation: Uncompetitive inhibitors bind only to the enzyme-substrate complex, lowering both Vmax and Km.

68. What is the pH of a 1M solution of hydrochloric acid (HCl)?

• A) 1
• B) 7
• C) 14
• D) 3

Answer: A) 1
Explanation: HCl is a strong acid, fully dissociating in water, making the pH = -log(1) = 1.

69. Which factor determines the direction of a biochemical reaction?

• A) Temperature only
• B) Activation energy
• C) Free energy change (ΔG)
• D) Presence of a buffer

Answer: C) Free energy change (ΔG)
Explanation: ΔG (Gibbs free energy change) determines whether a reaction is spontaneous (ΔG < 0) or non-spontaneous (ΔG > 0).

70. How do enzymes achieve catalytic specificity?

• A) By binding any molecule available
• B) By forming covalent bonds with all substrates
• C) By having a complementary active site for specific substrates
• D) By lowering substrate concentration

Answer: C) By having a complementary active site for specific substrates
Explanation: Enzymes exhibit high specificity due to the lock-and-key or induced fit model, ensuring only specific substrates bind.

71. What is the role of a transition state in an enzyme-catalyzed reaction?

• A) It is the final product of the reaction
• B) It is a stable intermediate
• C) It represents the highest energy state before forming the product
• D) It permanently inhibits enzyme function

Answer: C) It represents the highest energy state before forming the product
Explanation: The transition state is a high-energy intermediate that occurs before the formation of products, and enzymes work to lower this energy barrier.

72. How does increasing enzyme concentration affect reaction velocity (V₀) if substrate concentration is unlimited?

• A) V₀ decreases
• B) V₀ remains constant
• C) V₀ increases linearly
• D) V₀ decreases exponentially

Answer: C) V₀ increases linearly
Explanation: When substrate is abundant, increasing enzyme concentration proportionally increases reaction velocity until Vmax is reached.

73. Which statement correctly defines an apoenzyme?

• A) An enzyme with its cofactor attached
• B) An enzyme in an inactive form without its cofactor
• C) A denatured enzyme
• D) A fully functional enzyme

Answer: B) An enzyme in an inactive form without its cofactor
Explanation: Apoenzymes require cofactors or coenzymes to become active holoenzymes.

74. What is the function of the phosphate buffer system in the body?

• A) Regulates intracellular pH
• B) Transports oxygen
• C) Enhances enzyme activity
• D) Controls calcium balance

Answer: A) Regulates intracellular pH
Explanation: The phosphate buffer system (H₂PO₄⁻/HPO₄²⁻) maintains intracellular pH, especially in cells and renal tubular fluid.

75. What is the effect of substrate concentration on enzyme activity at very high levels?

• A) Activity decreases
• B) Activity remains the same
• C) Activity reaches a plateau (Vmax)
• D) Activity continuously increases

Answer: C) Activity reaches a plateau (Vmax)
Explanation: Once all enzyme active sites are saturated with substrate, the reaction rate cannot increase further and reaches Vmax.

76. What is the role of feedback inhibition in metabolic pathways?

• A) Enhances enzyme activity
• B) Prevents the accumulation of excess products
• C) Converts ATP into ADP
• D) Inhibits only allosteric enzymes

Answer: B) Prevents the accumulation of excess products
Explanation: Feedback inhibition occurs when the final product of a metabolic pathway inhibits an earlier enzyme, preventing overproduction.

77. What happens to enzyme activity when temperature exceeds its optimal range?

• A) Activity increases indefinitely
• B) The enzyme denatures and loses function
• C) Enzyme activity is unaffected
• D) The enzyme gains additional substrates

Answer: B) The enzyme denatures and loses function
Explanation: Excess heat disrupts hydrogen bonds and tertiary structure, leading to irreversible denaturation and loss of function.

78. What is the major buffering system in the extracellular fluid?

• A) Phosphate buffer
• B) Bicarbonate buffer
• C) Protein buffer
• D) Ammonia buffer

Answer: B) Bicarbonate buffer
Explanation: The bicarbonate buffer system (H₂CO₃/HCO₃⁻) is crucial for extracellular fluid pH regulation, especially in blood.

79. Which enzyme catalyzes the interconversion of carbonic acid and bicarbonate in blood?

• A) Catalase
• B) Carbonic anhydrase
• C) Hexokinase
• D) Amylase

Answer: B) Carbonic anhydrase
Explanation: Carbonic anhydrase rapidly catalyzes the conversion of CO₂ + H₂O to H₂CO₃ (carbonic acid) and HCO₃⁻ (bicarbonate) for CO₂ transport.

80. How does a buffer system resist changes in pH?

• A) By releasing or absorbing protons (H⁺)
• B) By breaking down proteins
• C) By inactivating enzymes
• D) By altering temperature

Answer: A) By releasing or absorbing protons (H⁺)
Explanation: Buffers maintain pH by absorbing excess H⁺ when acidic and releasing H⁺ when basic, keeping pH stable.

81. Which type of enzyme inhibition can be reversed by increasing substrate concentration?

• A) Competitive inhibition
• B) Non-competitive inhibition
• C) Uncompetitive inhibition
• D) Irreversible inhibition

Answer: A) Competitive inhibition
Explanation: Competitive inhibitors bind to the active site, preventing substrate binding, but increasing substrate concentration can outcompete the inhibitor, restoring enzyme activity.

82. What is the physiological significance of the Michaelis constant (Km)?

• A) Measures enzyme stability
• B) Indicates substrate affinity
• C) Determines enzyme half-life
• D) Represents reaction velocity

Answer: B) Indicates substrate affinity
Explanation: Km is the substrate concentration at which an enzyme achieves half of its maximum velocity (Vmax). A lower Km means higher substrate affinity.

83. What is the effect of Allosteric regulation on enzyme activity?

• A) Alters enzyme structure and activity
• B) Permanently inhibits the enzyme
• C) Decreases enzyme affinity for all substrates
• D) Does not affect enzyme function

Answer: A) Alters enzyme structure and activity
Explanation: Allosteric regulation involves binding of effectors at non-active sites, causing conformational changes that enhance or inhibit enzyme activity.

84. What is the primary role of Coenzymes in enzymatic reactions?

• A) Act as secondary inhibitors
• B) Provide additional binding sites
• C) Serve as carriers of chemical groups or electrons
• D) Denature the enzyme

Answer: C) Serve as carriers of chemical groups or electrons
Explanation: Coenzymes (e.g., NAD⁺, FAD) transfer functional groups or electrons to facilitate enzyme-catalyzed reactions.

85. How does pH affect enzyme activity?

• A) It does not affect enzyme function
• B) It changes the shape and charge of the active site
• C) It permanently inhibits the enzyme
• D) It increases enzyme stability indefinitely

Answer: B) It changes the shape and charge of the active site
Explanation: Enzymes have an optimal pH; deviations alter ionization of amino acids in the active site, affecting substrate binding and catalytic activity.

86. What is the characteristic of a zero-order enzyme reaction?

• A) Rate depends on substrate concentration
• B) Rate remains constant regardless of substrate concentration
• C) Reaction stops after initial phase
• D) Rate depends on enzyme inhibition

Answer: B) Rate remains constant regardless of substrate concentration
Explanation: In zero-order kinetics, the enzyme is saturated with substrate, making the reaction independent of substrate concentration.

87. What is the relationship between temperature and enzyme activity?

• A) Activity always increases with temperature
• B) Activity increases up to an optimal temperature and then declines
• C) Activity is unaffected by temperature
• D) Higher temperatures permanently activate enzymes

Answer: B) Activity increases up to an optimal temperature and then declines
Explanation: Enzymes function optimally within a temperature range; excessive heat leads to denaturation and loss of function.

88. What is the role of hemoglobin as a buffer in the blood?

• A) Enhances oxygen transport
• B) Binds to H⁺ ions to maintain pH
• C) Converts CO₂ into glucose
• D) Acts as a competitive inhibitor

Answer: B) Binds to H⁺ ions to maintain pH
Explanation: Hemoglobin buffers blood pH by binding H⁺ ions, helping maintain acid-base balance during CO₂ transport.

89. Which factor determines enzyme specificity?

• A) Random interactions with substrates
• B) The shape of the active site
• C) Enzyme degradation rate
• D) Presence of ATP

Answer: B) The shape of the active site
Explanation: Enzyme specificity is dictated by the active site’s shape, which complementarily binds only specific substrates.

90. Which metabolic condition results from excessive hydrogen ion accumulation?

• A) Alkalosis
• B) Acidosis
• C) Hyperglycemia
• D) Hyperthermia

Answer: B) Acidosis
Explanation: Acidosis occurs when excess H⁺ ions lower blood pH, leading to physiological disturbances.

91. Which enzyme model explains the flexibility of the enzyme’s active site?

• A) Lock and Key Model
• B) Induced Fit Model
• C) Michaelis-Menten Model
• D) Enzyme-Substrate Model

Answer: B) Induced Fit Model
Explanation: The Induced Fit Model suggests that the enzyme changes shape upon substrate binding, enhancing specificity and catalytic efficiency.

92. What is the term for an enzyme that catalyzes oxidation-reduction reactions?

• A) Hydrolase
• B) Transferase
• C) Oxidoreductase
• D) Isomerase

Answer: C) Oxidoreductase
Explanation: Oxidoreductases facilitate electron transfer in oxidation-reduction reactions, playing a crucial role in metabolic pathways like cellular respiration.

93. What happens to Vmax in the presence of a non-competitive inhibitor?

• A) Increases
• B) Decreases
• C) Remains unchanged
• D) Becomes infinite

Answer: B) Decreases
Explanation: Non-competitive inhibitors bind irreversibly to enzymes at a site other than the active site, reducing the number of active enzymes and lowering Vmax.

94. What is the normal physiological pH of human blood?

• A) 6.8
• B) 7.0
• C) 7.4
• D) 8.0

Answer: C) 7.4
Explanation: The normal blood pH is tightly regulated at ~7.35-7.45, primarily by the bicarbonate buffer system to prevent acidosis or alkalosis.

95. Which enzyme kinetics parameter is unaffected by a competitive inhibitor?

• A) Vmax
• B) Km
• C) Reaction rate
• D) Enzyme activity

Answer: A) Vmax
Explanation: Competitive inhibitors increase Km (reduce substrate affinity) but do not change Vmax, as high substrate concentrations can outcompete the inhibitor.

96. What happens to an enzyme when exposed to extreme pH conditions?

• A) It becomes more active
• B) It gets denatured
• C) It permanently binds to the substrate
• D) It loses specificity

Answer: B) It gets denatured
Explanation: Extreme pH levels disrupt ionic bonds and tertiary structure, causing enzyme denaturation and loss of function.

97. What is the function of a zwitterion in buffering solutions?

• A) It enhances enzyme activity
• B) It acts as a weak acid and a weak base
• C) It denatures proteins
• D) It inhibits metabolic reactions

Answer: B) It acts as a weak acid and a weak base
Explanation: Zwitterions contain both positive and negative charges, allowing them to act as buffers by resisting pH changes.

98. Which of the following statements about buffer solutions is TRUE?

• A) Buffers completely neutralize acids and bases
• B) Buffers resist changes in pH by donating or accepting protons
• C) Buffers only work in acidic conditions
• D) Buffers are only used in laboratory settings

Answer: B) Buffers resist changes in pH by donating or accepting protons
Explanation: Buffers maintain pH stability by absorbing excess H⁺ or OH⁻, preventing drastic pH fluctuations in biological systems.

99. What effect does increasing substrate concentration have on a reaction with a fixed amount of enzyme?

• A) Increases reaction rate indefinitely
• B) Decreases reaction rate
• C) Increases reaction rate until Vmax is reached
• D) Has no effect

Answer: C) Increases reaction rate until Vmax is reached
Explanation: Reaction velocity increases with substrate concentration but eventually plateaus at Vmax when all enzyme active sites are occupied.

100. What is the primary function of the bicarbonate buffer system?

• A) Maintain osmotic pressure
• B) Regulate body temperature
• C) Maintain blood pH homeostasis
• D) Assist in protein synthesis

Answer: C) Maintain blood pH homeostasis
Explanation: The bicarbonate buffer system (H₂CO₃/HCO₃⁻) is the primary extracellular buffer, preventing excessive acid-base imbalances in the blood.