Free Factors Affecting Enzyme Action MCQs with Answers

1. 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.

2. 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.

3. 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.

4. Between 0 degrees Celsius and the optimum, a rise of 10 degrees roughly doubles the rate of an enzyme catalysed reaction because

• A. the enzyme molecules multiply
• B. the activation energy of the reaction falls further
• C. the active site changes shape to fit better
• D. molecules move faster, so enzyme and substrate collide more often and with more energy

Explanation: Heat increases kinetic energy, so there are more successful collisions between enzyme and substrate per second and more enzyme substrate complexes form. Above the optimum this gain is overtaken by denaturation and the rate falls steeply, which is what gives the curve its sharp peak. The enzyme is a catalyst, so its number does not change during the reaction.

5. The optimum temperature of most human enzymes is about

• A. 37 degrees Celsius
• B. 25 degrees Celsius
• C. 60 degrees Celsius
• D. 100 degrees Celsius

Explanation: Human enzymes have evolved to work fastest at normal body temperature, which is why homeostatic control of temperature matters so much. A high fever above roughly 40 degrees begins to denature enzymes and becomes dangerous. Bacteria living in hot springs have enzymes with far higher optima, and one of them, Taq polymerase, is what makes the polymerase chain reaction possible.

6. A change in pH away from the optimum reduces enzyme activity because it

• A. reduces the number of substrate molecules present
• B. alters the charges on the R groups, changing the shape of the active site
• C. lowers the temperature of the mixture
• D. converts the enzyme into a substrate

Explanation: Excess hydrogen or hydroxide ions interfere with the ionic and hydrogen bonds between the side chains that hold the tertiary structure, so the active site distorts 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 pH 2 and trypsin at pH 8 and neither is active in the other's conditions.

7. An enzyme with a low Km value for its substrate

• A. reaches Vmax only at very high substrate concentrations
• B. has been irreversibly inhibited
• C. has a high affinity for that substrate, reaching half its maximum rate at a low concentration
• D. is working below its optimum temperature

Explanation: Km is the substrate concentration at which the reaction runs at half of Vmax, so a small Km means the active sites are half occupied even when substrate is scarce, which is the definition of high affinity. This is why Km is used to compare how tightly different enzymes hold the same substrate. Vmax measures how fast the enzyme works when saturated and is a separate quantity.

8. When substrate is present in excess, the rate of an enzyme catalysed reaction is

• A. independent of enzyme concentration
• B. directly proportional to the enzyme concentration
• C. inversely proportional to the enzyme concentration
• D. zero

Explanation: With substrate always available, every additional enzyme molecule adds its own turnover to the total, so doubling the enzyme doubles the rate and the graph is a straight line through the origin. The line only levels off if the substrate begins to run short and becomes limiting instead. This is the reverse of the saturation curve obtained when enzyme is fixed and substrate is increased.

9. The essential difference between denaturation and inhibition of an enzyme is that denaturation

• A. always occurs at the active site only
• B. can be reversed by adding more substrate
• C. involves the loss of the enzyme's three dimensional shape and is usually permanent
• D. increases the rate of the reaction

Explanation: Denaturation unfolds the whole protein, so the enzyme is destroyed as a catalyst and no amount of substrate will restore it, whereas most inhibition leaves the enzyme intact and is reversible when the inhibitor is removed or, for competitive inhibitors, outcompeted. High temperature and extreme pH denature, while specific molecules inhibit. Only competitive inhibition is relieved by extra substrate.

10. 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.