Free Enzymes MCQs with Answers
633 Enzymes MCQs from Biology, each with the correct answer and a written explanation of why it is correct. Free and unlimited, with no account needed.
Enzymes are biological catalysts that lower activation energy without being consumed, and their specificity is explained by the lock and key and induced fit models. Work includes the effects of temperature, pH and substrate concentration on reaction rate, plus competitive and non-competitive inhibition and how these differ.
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- 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…
Correct answer: lowering the activation energy of the reaction- 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…
Correct answer: changes shape slightly as the substrate binds- 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…
Correct answer: is denatured as its tertiary structure breaks down- 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…
Correct answer: Pepsin- 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…
Correct answer: all the active sites are occupied and the enzyme is saturated- 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…
Correct answer: binding the active site because it resembles the substrate- 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…
Correct answer: a non-competitive inhibitor- 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…
Correct answer: competitive inhibition9. 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…
Correct answer: coenzyme- 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…
Correct answer: They remain chemically unchanged at the end of the reaction- 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…
Correct answer: globular proteins- 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…
Correct answer: Emil Fischer- 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…
Correct answer: the shape and charge distribution of its active site- 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…
Correct answer: molecules move faster, so enzyme and substrate collide more often and with more energy- 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.
Correct answer: 37 degrees Celsius- 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…
Correct answer: alters the charges on the R groups, changing the shape of the active site- 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…
Correct answer: has a high affinity for that substrate, reaching half its maximum rate at a low concentration- 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…
Correct answer: bind to the sulphydryl groups of the protein and destroy its shape- 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…
Correct answer: the final product inhibits an enzyme acting early in the pathway- 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…
Correct answer: a site away from the active site where a regulatory molecule binds and changes the enzyme's shapeEnzymes MCQs: common questions
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