Free Enzymes 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. Above its optimum temperature the rate of an enzyme catalysed reaction falls sharply because the enzyme

• A. is used up by the reaction
• B. is denatured as its tertiary structure breaks down
• C. becomes a substrate itself
• D. loses its active site to the product

Explanation: Heat breaks the hydrogen bonds and other weak interactions holding the tertiary structure, so the active site loses its shape and can no longer bind the substrate. This is denaturation and it is usually irreversible, which is why the curve falls steeply rather than plateauing. Enzymes are catalysts, so they are recovered unchanged and are never used up.

4. Which enzyme works at an optimum pH of about 2?

• A. Salivary amylase
• B. Pepsin
• C. Trypsin
• D. Catalase

Explanation: Pepsin is secreted into the stomach where hydrochloric acid keeps the pH near 2, and its structure is stable and active in exactly those conditions. Trypsin works in the small intestine at about pH 8, salivary amylase near neutral, and catalase around pH 7. An enzyme moved far from its optimum pH loses activity because the charges on the R groups in the active site change.

5. At a constant enzyme concentration, increasing substrate concentration eventually stops increasing the reaction rate because

• A. the substrate begins to inhibit the enzyme
• B. all the active sites are occupied and the enzyme is saturated
• C. the enzyme is denatured by the excess substrate
• D. the products are used up

Explanation: Once every active site is engaged, adding more substrate cannot raise the rate, because the enzyme is already working as fast as it can turn substrate over. This plateau is Vmax, and reaching it means the enzyme concentration, not the substrate, is now limiting. Substrate does not denature the enzyme, and products accumulate rather than run out.

6. A competitive inhibitor slows an enzyme catalysed reaction by

• A. binding the active site because it resembles the substrate
• B. binding a site away from the active site and distorting it
• C. permanently destroying the enzyme
• D. lowering the pH of the surroundings

Explanation: A competitive inhibitor is structurally similar to the substrate and occupies the active site, so substrate and inhibitor compete for the same place. Because the competition depends on relative concentrations, adding more substrate overcomes the inhibition and Vmax is unchanged. The description in the second option is non-competitive inhibition, which binds an allosteric site instead.

7. Increasing the substrate concentration will NOT relieve the effect of

• A. a competitive inhibitor
• B. a non-competitive inhibitor
• C. a low enzyme concentration
• D. a substrate below the saturation point

Explanation: A non-competitive inhibitor binds at a site other than the active site and changes the enzyme's shape, so the substrate cannot compete it away no matter how much is added and Vmax is genuinely reduced. Competitive inhibition, by contrast, is overcome by flooding the system with substrate. This difference is the standard way of telling the two apart experimentally.

8. Malonate inhibits succinate dehydrogenase because it resembles succinate. This is an example of

• A. non-competitive inhibition
• B. competitive inhibition
• C. feedback inhibition
• D. denaturation

Explanation: Malonate is close enough in structure to succinate to occupy the same active site without being converted, which is the definition of competitive inhibition, and it is the textbook example. The inhibition can be reversed by raising the succinate concentration. Feedback inhibition is a regulatory mechanism in which the end product of a pathway inhibits an earlier enzyme, usually allosterically.

9. A non-protein organic molecule that an enzyme needs in order to function, such as NAD, is called a

• A. cofactor metal ion
• B. coenzyme
• C. prosthetic group of a haemoglobin type
• D. substrate

Explanation: Coenzymes are organic helper molecules, often derived from vitamins, that bind loosely and carry chemical groups or electrons between reactions, and NAD is the classic hydrogen carrier of respiration. Inorganic helpers such as zinc or magnesium ions are cofactors. A prosthetic group is a helper bound tightly and permanently, such as the haem in catalase.

10. Which statement about enzymes is correct?

• A. They are consumed in the reactions they catalyse
• B. They remain chemically unchanged at the end of the reaction
• C. They can catalyse reactions that would otherwise be thermodynamically impossible
• D. They work equally well at every pH

Explanation: An enzyme releases the product and returns to its original state, ready to bind the next substrate molecule, which is why a small amount catalyses the conversion of a great deal of substrate. Enzymes only speed up reactions that were already energetically favourable; they cannot drive an unfavourable one. Each has a narrow pH optimum outside which activity falls off.