Free Enzymes MCQs with Answers

61. According to the induced fit model

• A. The active site is rigid
• B. The enzyme changes shape to fit the substrate
• C. The substrate is unchanged
• D. No conformational changes occur

Explanation: Koshland's induced fit model proposes that the enzyme's active site is flexible and changes shape to better accommodate the substrate.

62. Competitive inhibitors

• A. Bind to allosteric site
• B. Cannot be overcome by increasing substrate
• C. Bind to active site
• D. Change Vmax but not Km

Explanation: Competitive inhibitors bind to the active site, competing with substrate. They can be overcome by increasing substrate concentration.

63. The optimum pH for pepsin is approximately

• A. 2
• B. 7
• C. 8
• D. 10

Explanation: Pepsin works optimally at pH 1.5-2 in the stomach, where it is activated by HCl.

64. Which factor does NOT affect enzyme activity?

• A. Temperature
• B. pH
• C. Substrate concentration
• D. Color of the solution

Explanation: Enzyme activity is affected by temperature, pH, substrate concentration, and presence of inhibitors/activators, but not by color.

65. The Michaelis constant (Km) represents

• A. Maximum reaction velocity
• B. Substrate concentration at half Vmax
• C. Enzyme concentration
• D. Inhibitor concentration

Explanation: Km is the substrate concentration at which the reaction velocity is half of Vmax. It indicates enzyme-substrate affinity.

66. Allosteric enzymes

• A. Follow Michaelis-Menten kinetics strictly
• B. Have multiple subunits and regulatory sites
• C. Are not affected by feedback inhibition
• D. Always show competitive inhibition

Explanation: Allosteric enzymes have multiple subunits with regulatory (allosteric) sites where activators or inhibitors bind, affecting enzyme conformation.

67. Which enzyme is involved in DNA replication?

• A. RNA polymerase
• B. DNA ligase
• C. Peptidase
• D. Lipase

Explanation: DNA ligase joins Okazaki fragments by forming phosphodiester bonds between adjacent nucleotides.

68. Zymogens are

• A. Active enzymes
• B. Inactive enzyme precursors
• C. Coenzymes
• D. Enzyme inhibitors

Explanation: Zymogens (proenzymes) are inactive precursors that are activated by cleavage, e.g., pepsinogen → pepsin, trypsinogen → trypsin.

69. The enzyme carbonic anhydrase is found in

• A. Stomach
• B. Red blood cells
• C. Pancreas
• D. Liver

Explanation: Carbonic anhydrase in RBCs catalyzes CO2 + H2O ↔ H2CO3, facilitating CO2 transport in blood.

70. Enzymes activity decreases at very low or high pH because:

• A. Substrate concentration increases
• B. Enzymes become denatured
• C. Product formation increases
• D. Temperature becomes constant

Explanation: Excess hydrogen or hydroxide ions disturb the ionic and hydrogen bonds between the side chains that hold the tertiary structure, so the active site loses its precise shape and the substrate no longer fits. A small shift is reversible, but an extreme one denatures the enzyme permanently. This is why pepsin works at about pH 2 and trypsin at about pH 8, each being inactive in the other's conditions.