Free Enzymes MCQs with Answers

11. Chemically, almost all enzymes are

• A. globular proteins
• B. lipids
• C. polysaccharides
• D. nucleotides

Explanation: An enzyme is a globular protein whose precisely folded tertiary structure creates the active site, which is why anything that unfolds the protein destroys the catalytic power. The globular rather than fibrous form matters because it makes the molecule soluble and compact. The one exception is the small group of catalytic RNA molecules called ribozymes.

12. 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.

13. 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.

14. 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.

15. 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.

16. 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.

17. 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.

18. Heavy metal ions such as mercury and silver inhibit many enzymes irreversibly because they

• A. compete with the substrate for the active site
• B. raise the pH of the solution
• C. supply the enzyme with excess energy
• D. bind to the sulphydryl groups of the protein and destroy its shape

Explanation: These ions attack the sulphur containing side chains, breaking disulphide bridges and permanently distorting the tertiary structure, so the damage cannot be reversed by removing the inhibitor or adding more substrate. This is essentially poisoning rather than regulation, and it is why mercury contamination is so dangerous. A competitive inhibitor, by contrast, binds reversibly at the active site.

19. In feedback inhibition of a metabolic pathway

• A. the final product inhibits an enzyme acting early in the pathway
• B. the first substrate inhibits the last enzyme
• C. the enzyme is destroyed after each reaction
• D. the pathway runs faster as product accumulates

Explanation: When enough product has been made it binds an allosteric site on an early enzyme and switches it off, so the cell stops spending resources on something it already has, and production restarts when the product is used up. This is negative feedback applied to metabolism and is the standard way biosynthetic pathways are regulated. Inhibiting an early step also prevents intermediates from piling up.

20. An allosteric site on an enzyme is

• A. a second active site that binds the same substrate
• B. a site away from the active site where a regulatory molecule binds and changes the enzyme's shape
• C. the region that is denatured first by heat
• D. the point where the enzyme attaches to the cell membrane

Explanation: A molecule binding at an allosteric site alters the conformation of the whole enzyme, and therefore the shape of the active site, either activating or inhibiting it. Non competitive inhibition and feedback inhibition both work this way, which is why adding more substrate cannot overcome them. The site is chemically distinct from the active site and binds a different molecule.