Free Enzymes as Biocatalysts MCQs with Answers

1. Enzymes are described as biological catalysts because they

• A. are used up in the reactions they catalyse
• B. raise the activation energy of a reaction
• C. supply energy to drive unfavourable reactions
• D. speed up reactions by lowering the activation energy and are recovered unchanged

Explanation: An enzyme provides an alternative pathway with a smaller energy barrier, so a far larger proportion of collisions are effective at body temperature, and it emerges from each cycle ready to work again. It cannot make an energetically unfavourable reaction happen, only accelerate one that is already possible. A single enzyme molecule may convert thousands of substrate molecules per second.

2. The high specificity of an enzyme for its substrate is due to

• A. the shape and chemistry of its active site
• B. its molar mass
• C. the temperature of the reaction
• D. the concentration of the substrate

Explanation: Only a substrate whose shape and charge distribution complement the active site can bind, which is why sucrase acts on sucrose but not on the closely related maltose. The induced fit model refines this picture by allowing the site to mould itself around the substrate as binding occurs. Specificity is a structural property and is unaffected by temperature or concentration.

3. Enzyme activity is lost above about 45 degrees Celsius because

• A. the substrate evaporates
• B. the enzyme is denatured as the weak bonds holding its tertiary structure break
• C. the enzyme is used up faster
• D. the activation energy increases

Explanation: Heat disrupts the hydrogen and ionic interactions that hold the chain in shape, so the active site loses its precise geometry and can no longer bind the substrate, and the loss is usually permanent. Below the optimum, raising the temperature increases the rate in the normal way, which is why the curve rises to a peak and then falls sharply. The peak for human enzymes lies near 37 degrees.

4. A competitive inhibitor reduces the rate of an enzyme catalysed reaction by

• A. denaturing the enzyme permanently
• B. binding an allosteric site
• C. occupying the active site because it resembles the substrate
• D. lowering the temperature

Explanation: Substrate and inhibitor compete for the same site, so the inhibition can be overcome by increasing the substrate concentration and the maximum rate is eventually still reached. A non competitive inhibitor binds elsewhere, changes the shape of the enzyme and cannot be outcompeted, so it genuinely lowers the maximum rate. Many drugs are designed as competitive inhibitors of a specific enzyme.

5. Heavy metal ions such as mercury and lead are poisonous partly because they

• A. increase enzyme activity uncontrollably
• B. are converted into proteins by the body
• C. supply too much energy to the cell
• D. bind to sulphur containing groups in enzymes and denature them irreversibly

Explanation: These ions attack the sulphydryl side chains of cysteine, breaking disulphide bridges and destroying the tertiary structure, so the affected enzymes lose all activity permanently. Because the damage cannot be reversed by removing the metal, the effect is poisoning rather than simple inhibition. This is also why drinking milk or egg white gives some first aid benefit, as those proteins bind the metal first.

6. Each enzyme works fastest at a particular pH because a change in pH

• A. alters the charges on the side chains, changing the shape of the active site
• B. changes the molar mass of the enzyme
• C. removes the substrate from the solution
• D. converts the enzyme into a carbohydrate

Explanation: Excess hydrogen or hydroxide ions interfere with the ionic and hydrogen bonds between side chains, distorting the active site so the substrate no longer fits, and a large shift denatures the enzyme entirely. This is why pepsin works at about pH 2 in the stomach while trypsin needs about pH 8 in the small intestine. Small deviations are reversible, large ones are not.

7. An enzyme lowers the activation energy of a reaction by

• A. raising the temperature of the substrate
• B. binding the substrate at its active site and stabilising the transition state
• C. supplying energy to the reactants
• D. changing the equilibrium position

Explanation: Binding orients the substrate correctly and strains the bonds that are about to break, offering a lower energy route to the products. The enzyme is regenerated unchanged and does not shift the equilibrium, only the speed at which it is reached. This last point is what separates a catalyst from a reactant.

8. Compared with an inorganic catalyst such as manganese dioxide, an enzyme is

• A. far less efficient
• B. highly specific and effective at mild temperature and pH
• C. unaffected by temperature
• D. consumed during the reaction

Explanation: Enzymes achieve rate enhancements that industrial catalysts cannot approach and do so at body temperature and near neutral pH, but they work on one substrate or one class of substrate only. Their protein nature also makes them sensitive to heat and pH, which an inorganic catalyst is not. Like all catalysts, they emerge unchanged.

9. The turnover number of an enzyme measures

• A. how many substrate molecules one enzyme molecule converts per unit time
• B. the number of enzymes in a cell
• C. the temperature at which it denatures
• D. its molar mass

Explanation: Carbonic anhydrase converts around 600,000 molecules of carbon dioxide per second, which is why the reaction in red blood cells is essentially instantaneous. A high turnover number means a small quantity of enzyme suffices. It is a measure of catalytic efficiency rather than of abundance.

10. Which statement about the active site of an enzyme is correct?

• A. It occupies most of the enzyme's surface
• B. It is a small region whose shape and charges complement the substrate
• C. It binds any molecule of similar mass
• D. It is destroyed after each reaction

Explanation: Only a few residues, often far apart in the primary sequence but brought together by folding, form the active site, which is why the whole three dimensional structure matters. Complementarity of shape and charge is what produces specificity. The site is regenerated after each catalytic cycle.