Free Mode of Enzyme Action MCQs with Answers

1. Enzymes increase the rate of a reaction by

• A. raising the temperature of the reaction mixture
• B. lowering the activation energy of the reaction
• C. changing the equilibrium position in favour of the products
• D. supplying the energy the reaction needs

Explanation: An enzyme provides an alternative route with a lower activation energy, so a much larger fraction of molecules has enough energy to react at body temperature. It does not change the free energy difference between reactants and products, which means the equilibrium position stays exactly where it was and only the time taken to reach it falls. Enzymes supply no energy of their own.

2. According to the induced fit model of enzyme action, the active site

• A. is already an exact complement of the substrate before binding
• B. changes shape slightly as the substrate binds
• C. plays no part in catalysis
• D. binds any substrate regardless of shape

Explanation: Koshland's induced fit model replaced the rigid lock and key picture: the active site is flexible and moulds itself around the substrate as binding occurs, straining the bonds that are about to break. The lock and key model is the one that treats the site as pre-formed and rigid. Specificity is retained in both models, so an enzyme still does not bind just any substrate.

3. The lock and key hypothesis of enzyme action was proposed by

• A. Koshland
• B. Emil Fischer
• C. Michaelis
• D. James Sumner

Explanation: Fischer suggested in 1894 that the substrate fits the active site as a key fits a lock, which explained specificity but treated the site as rigid. Koshland's induced fit model replaced it in 1958 by allowing the site to mould itself around the substrate. Sumner is remembered for crystallising urease and proving that enzymes are proteins.

4. The specificity of an enzyme depends on

• A. the total number of amino acids in the molecule
• B. the temperature at which it works
• C. the shape and charge distribution of its active site
• D. the concentration of the substrate available

Explanation: Only a substrate whose shape and charges are complementary to the active site can bind, which is why sucrase acts on sucrose but not on maltose despite the two being very similar disaccharides. The shape of that site is determined by the folding of the chain, and so ultimately by the sequence of amino acids. Temperature and substrate concentration change the rate but never the specificity.

5. Activation energy is best defined as the energy

• A. released when the products are formed
• B. stored in the bonds of the substrate
• C. needed to bring the reactants to the transition state so the reaction can proceed
• D. used by the enzyme to bind the substrate

Explanation: Even an energetically favourable reaction needs an initial input to break existing bonds and reach the unstable transition state, and that barrier is what keeps sugars from spontaneously oxidising in the air. An enzyme lowers the barrier by stabilising the transition state, so far more molecules can cross it at body temperature. The energy difference between reactants and products is a separate quantity that the enzyme does not change.

6. The active site of an enzyme is

• A. The entire enzyme surface
• B. A specific region where substrate binds
• C. The coenzyme binding site
• D. Where allosteric regulators bind

Explanation: The active site is a specific pocket or cleft on the enzyme where the substrate binds and catalysis occurs.

7. 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.

8. 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.