Free Enzymes MCQs with Answers

71. Enzymes belong to which class of biomolecules?

• A. Carbohydrates
• B. Lipids
• C. Proteins
• D. Nucleic acids

Explanation: Almost every enzyme is a globular protein whose precisely folded tertiary structure creates the active site, which is why heat and extreme pH destroy catalytic power. The one exception is the small group of catalytic RNA molecules called ribozymes, which is why nucleic acids are offered here. Carbohydrates and lipids never act as catalysts.

72. Effect of increased substrate concentration on enzyme activity:

• A. Decrease the rate of reaction
• B. Increase the reaction rate until all active sites are saturated
• C. Have no effect on the reaction rate
• D. Increase the rate of reaction in a straight diagonal line

Explanation: Rate rises steeply at first because more collisions occur between substrate and free active sites, then levels off at Vmax once every site is occupied and the enzyme is working as fast as it can. The curve is therefore hyperbolic rather than a straight line, which is what the fourth option gets wrong. Beyond saturation the enzyme concentration rather than the substrate is limiting.

73. According to Lock and Key model, the active site is regarded as:

• A. Rigid and specific
• B. Flexible and specific
• C. Rigid and non specific
• D. Flexible and non specific

Explanation: Fischer's lock and key model treats the active site as a pre formed rigid shape that only the matching substrate fits, which explains specificity but not the conformational changes later observed. Koshland's induced fit model keeps the specificity but makes the site flexible, moulding itself around the substrate as it binds. Both models agree that the enzyme is specific.

74. Reversible inhibitors differ from irreversible inhibitors because they:

• A. Bind permanently and cannot be removed
• B. Bind temporarily and can be removed
• C. Permanently inactivate the enzyme
• D. Change enzyme structure permanently

Explanation: A reversible inhibitor binds through weak interactions and detaches again, so activity returns when it is removed or outcompeted, which is how competitive and non competitive inhibitors behave. An irreversible inhibitor forms a covalent bond, as heavy metal ions and nerve gases do, and the enzyme is destroyed for good. The other three options all describe the irreversible case.