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. enzyme
- B. substrate
- C. product
- D. coenzyme
Explanation: Emil Fischer's analogy pictures the enzyme's rigid active site as the lock and the substrate as the one key that fits it, explaining…
Correct answer: enzyme- A. E + P → EP → ES → E + S
- B. ES → E + S → E + P
- C. E + S → ES → EP → E + P
- D. EP → ES → E + S
Explanation: Enzyme and substrate first form the enzyme-substrate complex, which converts to an enzyme-product complex before releasing product and…
Correct answer: E + S → ES → EP → E + P- A. activation energy
- B. speed of completion
- C. overall energy difference between substrates and products
- D. reaction pathway
Explanation: Catalysts lower the activation energy and speed the reaction, but the energy of substrates and products is fixed by chemistry, so the…
Correct answer: overall energy difference between substrates and products- A. are very large molecules
- B. are made of DNA
- C. work only at very high temperatures
- D. are not consumed and can be reused repeatedly
Explanation: Since the enzyme emerges unchanged from each reaction, one molecule can process thousands of substrate molecules per second.
Correct answer: are not consumed and can be reused repeatedly- A. slow every reaction down
- B. damage cellular molecules and speed all reactions indiscriminately
- C. destroy the substrates
- D. prevent products from forming
Explanation: Raising temperature speeds every reaction and denatures proteins, so it cannot give the selective control life needs.
Correct answer: damage cellular molecules and speed all reactions indiscriminately- A. approximately double the rate of the reaction
- B. halve the rate of the reaction
- C. leave the rate unchanged
- D. denature the substrate
Explanation: With abundant substrate every added enzyme molecule finds work at once, so rate is directly proportional to enzyme concentration.
Correct answer: approximately double the rate of the reaction- A. 8
- B. 2
- C. 5
- D. 11
Explanation: The alkaline environment of the duodenum, fed by bicarbonate from the pancreas, suits trypsin at pH 8.
Correct answer: 8- A. increase the substrate concentration
- B. supply the necessary coenzyme
- C. keep the pH constant throughout
- D. raise the temperature slowly
Explanation: Because pH changes can alter ionisation of the active site and denature the enzyme, a fair test holds pH steady with a buffer.
Correct answer: keep the pH constant throughout- A. it makes the blood too alkaline
- B. vital enzymes begin to denature and lose function
- C. it stops all diffusion in the body
- D. it destroys red blood cells instantly
Explanation: Proteins start to unfold irreversibly a few degrees above body temperature, so sustained high fever disrupts the enzyme systems of the…
Correct answer: vital enzymes begin to denature and lose function- A. halve the rate
- B. stop the reaction
- C. have no effect on the rate
- D. double the rate
Explanation: Each 10 degree rise roughly doubles the rate because molecules collide more often and with more energy, until heat damage overtakes this…
Correct answer: double the rate- A. A straight line rising forever
- B. A curve that rises and then levels off as active sites fill
- C. A curve that falls from the start
- D. A horizontal line from the beginning
Explanation: At low substrate, rate climbs steeply with each added molecule, but once every active site are occupied the enzyme is saturated and the…
Correct answer: A curve that rises and then levels off as active sites fill- A. Strong heating well above its optimum
- B. Cooling the solution to 0 degrees Celsius
- C. Diluting the enzyme solution
- D. Adding more substrate
Explanation: Strong heat breaks the weak bonds holding the tertiary structure, so the active site is permanently distorted.
Correct answer: Strong heating well above its optimum- A. denature every bacterial protein at once
- B. dissolve the bacterial membrane
- C. raise the temperature inside the bacterium
- D. resemble PABA and competitively block the bacterial enzyme that uses it
Explanation: The drug mimics para-aminobenzoic acid, a substrate bacteria need to make folic acid, so it occupies the enzyme's active site and starves…
Correct answer: resemble PABA and competitively block the bacterial enzyme that uses it- A. bind permanently to the enzyme by covalent bonds
- B. denature the whole protein
- C. mimic the shape of the enzyme's natural substrate
- D. block the synthesis of the enzyme
Explanation: A competitive inhibitor succeeds only if it fits the active site, so medicinal chemists build molecules shaped like the substrate.
Correct answer: mimic the shape of the enzyme's natural substrate- A. can be removed simply by dilution
- B. binds only at the allosteric site
- C. attaches by weak hydrogen bonds
- D. forms permanent covalent bonds with the enzyme
Explanation: Irreversible poisons chemically modify amino acids at or near the active site, so the enzyme is permanently disabled and the cell must…
Correct answer: forms permanent covalent bonds with the enzyme- A. the substrate of the first reaction
- B. an allosteric site on the first enzyme of the pathway
- C. the active site of every enzyme in the cell
- D. the DNA that codes for the pathway
Explanation: The end product docks at a regulatory site on the pathway's first committed enzyme, changing its shape and shutting production until the…
Correct answer: an allosteric site on the first enzyme of the pathway- A. bacterial DNA
- B. bacterial ribosomes
- C. bacterial cell wall
- D. bacterial flagellum
Explanation: Penicillin blocks the transpeptidase that crosslinks the peptidoglycan wall, so a growing bacterium bursts under its own osmotic pressure.
Correct answer: bacterial cell wall- A. Increasing the concentration of substrate
- B. Increasing the concentration of inhibitor
- C. Raising the temperature above the optimum
- D. Lowering the pH well below the optimum
Explanation: Because substrate and inhibitor compete for the same active site, flooding the mixture with substrate outcompetes the inhibitor and the…
Correct answer: Increasing the concentration of substrate- A. a coenzyme
- B. an activator
- C. a substrate
- D. an enzyme inhibitor
Explanation: By disabling cyclooxygenase, aspirin stops the making of prostaglandins, the chemicals that promote pain, fever and inflammation.
Correct answer: an enzyme inhibitor- A. The entire enzyme surface
- B. A specific region where substrate binds
- C. The coenzyme binding site
- D. Where allosteric regulators bind
Explanation: The active site is a specific pocket or cleft on the enzyme where the substrate binds and catalysis occurs.
Correct answer: A specific region where substrate binds