Ch#5 Enzymes
Multiple choice questions (MCQs)
Select the correct answers for the following questions.
1. What is the primary role of an enzyme in a chemical reaction?
a) Increase the activation energy
b) Decrease the activation energy
c) Change the products formed
d) Get consumed in the reaction
2. The active site of an enzyme is made up of:
a) Lipid molecules
b) A few amino acids
c) Carbohydrate chains
d) Nucleic acids
3. Which term refers to the complete, functional form of an enzyme including its cofactor?
a) Apoenzyme
b) Holoenzyme
c) Prosthetic group
d) Coenzyme
4. Metal ions such as Zn²⁺ and Mg²⁺ that assist enzyme function are called:
a) Coenzymes
b) Prosthetic groups
c) Cofactors
d) Inhibitors
5. The Lock and Key model of enzyme action was proposed by:
a) Daniel Koshland
b) Emil Fischer
c) Louis Pasteur
d) Robert Koch
6. Which model suggests that the enzyme’s active site is flexible and changes shape upon
substrate binding?
a) Induced Fit Model
b) Lock and Key Model
c) Fluid Mosaic Model
d) Active Site Model
7. Enzymes that function inside the cell are often found in the:
a) Cell wall
b) Cytoplasm
c) Nucleus
d) Mitochondria
8. Pepsin is secreted in an inactive form called:
a) Pepsinase
b) Pepsinogen
c) Propepsin
d) Trypsinogen
9. Which vitamin often acts as a coenzyme?
a) Vitamin A
Written and composed by: Ahsan Iqbal
b) Vitamin C
c) Vitamin D
d) Niacin (B3)
10. The energy required to start a chemical reaction is known as:
a) Free energy
b) Kinetic energy
c) Activation energy
d) Potential energy
11. Human enzymes work best at around:
a) 25°C
b) 37°C
c) 50°C
d) 70°C
12. Enzymes from thermophilic bacteria function optimally at:
a) Below 20°C
b) Around 37°C
c) 70°C or higher
d) 100°C exactly
13. Denaturation of an enzyme occurs due to:
a) Low pH
b) High temperature
c) Excess substrate
d) Low enzyme concentration
14. Pepsin works best in _____ environment.
a) Acidic
b) Alkaline
c) Neutral
d) Variable
15. Trypsin is most active in _____ medium.
a) Acidic
b) Alkaline
c) Neutral
d) Cold
16. Which enzyme has the lowest optimum pH?
a) Trypsin
b) Salivary amylase
c) Pepsin
d) Catalase
17. If enzyme concentration increases and substrate is unlimited, the reaction rate will:
a) Decrease
b) Increase
c) Remain same
d) Stop
18. At high substrate concentration, the reaction rate becomes constant because:
a) Enzymes are denatured
b) All active sites are occupied
c) pH changes
d) Temperature drops
19. A substance that reduces enzyme activity by blocking the active site is called a/an:
Written and composed by: Ahsan Iqbal
a) Activator
b) Inhibitor
c) Coenzyme
d) Substrate
20. Cyanide is an example of a:
a) Competitive inhibitor
b) Non-competitive inhibitor
c) Reversible inhibitor
d) Coenzyme
21. Competitive inhibitors resemble the enzymes
a) Product
b) Cofactor
c) Substrate
d) Active site
22. Malonic acid inhibits succinate dehydrogenase. This is an example of:
a) Feedback inhibition
b) Competitive inhibition
c) Non-competitive inhibition
d) Allosteric inhibition
23. In non-competitive inhibition, the inhibitor binds to:
a) The active site
b) The substrate
c) A site other than the active site
d) The coenzyme
24. Feedback inhibition usually involves the _____ inhibiting an early enzyme in the pathway.
a) Substrate
b) Cofactor
c) Final product
d) Competitive inhibitor
25. Penicillin is an example of a
a) Irreversible inhibitor
b) Reversible inhibitor
c) Coenzyme
d) Prosthetic group
26. Enzymes that catalyze oxidation-reduction reactions are called:
a) Transferases
b) Hydrolases
c) Oxidoreductases
d) Ligases
27. Hexokinase is an example of which class of enzymes?
a) Hydrolases
b) Transferases
c) Lyases
d) Isomerases
28. Enzymes that add water to break down molecules are:
a) Lyases
b) Ligases
c) Hydrolases
d) Isomerases
Written and composed by: Ahsan Iqbal
29. Which enzyme class joins molecules using energy from ATP?
a) Hydrolases
b) Lyases
c) Ligases
d) Transferases
30. Enzymes that rearrange atoms within a molecule are
a) Oxidoreductases
b) Transferases
c) Isomerases
d) Ligases
31. Pyruvate decarboxylase, which removes CO₂, is
a) Lyase
b) Ligase
c) Hydrolase
d) Transferase
32. Enzymes that break down proteins are called
a) Lipases
b) Proteases
c) Carbohydrases
d) Nucleases
33. The enzyme that breaks down starch is
a) Lactase
b) Maltase
c) Amylase
d) Sucrase
34. Lipases act on
a) Proteins
b) Carbohydrates
c) Lipids
d) Nucleic acids
35. Sucrase breaks down sucrose into:
a) Glucose + Galactose
b) Glucose + Fructose
c) Glucose + Glucose
d) Fructose + Galactose
36. Enzymes that break down nucleic acids are:
a) Proteases
b) Lipases
c) Nucleases
d) Carbohydrases
37. The function of a coenzyme is often to:
a) Denature the substrate
b) Provide a stable rigid structure
c) Transport electrons or functional groups
d) Inhibit competitive reactions
38. The protein part of an enzyme without its cofactor is called the:
a) Holoenzyme
b) Apoenzyme
c) Prosthetic group
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d) Co-substrate
39. A tightly bound, non-protein organic cofactor is a:
a) Coenzyme
b) Prosthetic group
c) Metal ion
d) Activator
40. Which is NOT a type of cofactor?
a) Organic ions
b) Prosthetic groups
c) Coenzymes
d) Polypeptides
41. NAD+ is an important coenzyme that functions as a:
a) Protein carrier
b) Hydrogen acceptor
c) Phosphate donor
d) Vitamin precursor
42. The molecule that enters the active site of an enzyme is the:
a) Inhibitor
b) Product
c) Substrate
d) Coenzyme
43. The temporary structure formed when a substrate binds to an enzyme is the:
a) Enzyme-product complex
b) Enzyme-cofactor complex
c) Enzyme-substrate complex
d) Active complex
44. Enzymes speed up reactions by:
a) Making endothermic reactions exothermic
b) Increasing the collision energy of molecules
c) Lowering the activation energy
d) Changing the equilibrium constant
45. After catalyzing a reaction, the enzyme:
a) Is used up and must be replaced
b) Becomes part of the product
c) Is unchanged and can be reused
d) Changes its specificity
46. The specificity of an enzyme is primarily due to the shape of its:
a) Allosteric site
b) Inhibitor site
c) Active site
d) Coenzyme
47. At very low temperatures, enzyme activity is low because:
a) Enzymes are denatured
b) Molecules move too slowly
c) pH is too high
d) Substrates are destroyed
48. The graph of enzyme activity vs. temperature typically shows:
a) A straight line increase
b) A sharp peak
Written and composed by: Ahsan Iqbal
c) A steady decrease
d) A U-shaped curve
49. The graph of enzyme activity vs. pH for most enzymes is:
a) A straight line
b) A bell-shaped curve
c) A logarithmic curve
d) An exponential curve
50. If the pH is changed significantly above or below the optimum, the enzyme may:
a) Work faster
b) Become denatured
c) Change its substrate
d) Act as an inhibitor
51. The maximum rate of reaction (Vmax) is reached when:
a) All enzyme active sites are saturated with substrate
b) The enzyme concentration is very low
c) The temperature is very low
d) An inhibitor is added
52. A non-competitive inhibitor binds to the enzyme and:
a) Changes the shape of the active site
b) Competes with the substrate for the active site
c) Increases the activation energy
d) Becomes the new substrate
53. An example of a competitive inhibitor is
a) Cyanide
b) Malonate
c) Penicillin
d) Heavy metals
54. An example of a non-competitive inhibitor is
a) Malonate for succinate dehydrogenase
b) Cyanide for cytochrome oxidase
c) Statin drugs for HMG-CoA reductase
d) Sulfa drugs for bacterial enzymes
55. Irreversible inhibition usually involves
a) Weak hydrogen bonds
b) Strong covalent bonds
c) Temporary ionic bonds
d) Hydrophobic interactions
56. Reversible inhibition can be overcome by
a) Adding more inhibitor
b) Diluting the solution
c) Increasing the substrate concentration
d) Boiling the enzyme
57. In feedback inhibition, the end product of a pathway acts as an inhibitor for
a) The last enzyme in the pathway
b) Any random enzyme in the pathway
c) The first committed step enzyme
d) All enzymes simultaneously
58. ATP inhibiting an early step in its own synthesis is an example of
a) Competitive inhibition
Written and composed by: Ahsan Iqbal
b) Feedback inhibition
c) Allosteric activation
d) Denaturation
59. Aspirin works as a drug because it
a) Acts as a coenzyme
b) Inhibits prostaglandin-producing enzymes
c) Competes with substrate for active sites
d) Denatures pain receptors
60. Which statement is TRUE about enzymes?
a) They provide energy for reactions.
b) They are consumed in the reactions they catalyze.
c) They are highly specific for their substrates.
d) They work equally well at any pH.
61. Which factor does NOT affect enzyme activity?
a) Light intensity
b) Temperature
c) pH
d) Enzyme concentration
62. Ribozymes are unique because they are
a) Enzymes made of RNA
b) Faster than protein enzymes
c) Not affected by temperature
d) Used only in DNA replication
Written and composed by: Ahsan Iqbal
