Enzymes MCQs
MDCAT · Biology · 6 questions
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Revision summary
PMDC MDCAT syllabus: Enzymes
Enzymes are biological catalysts, almost all proteins. They speed up reactions by lowering the activation energy and are not used up; they do not change the equilibrium. The substrate binds at the active site. Emil Fischer's lock-and-key model (1894) treats the site as rigid; Koshland's induced-fit model lets it change shape slightly as the substrate binds. Many enzymes need helpers: a prosthetic group is bound tightly (haem), a coenzyme loosely (NAD, FAD), and activators are usually metal ions; the protein alone is the apoenzyme, and apoenzyme plus cofactor is the holoenzyme. Rate depends on temperature (human enzymes peak near 37 °C, then denature), pH (pepsin near 2, trypsin near 8), and substrate and enzyme concentration. Competitive inhibitors occupy the active site and can be outcompeted by more substrate; non-competitive inhibitors bind elsewhere and change the enzyme's shape.
Questions with answers
Written by Parcha from the syllabus. Samples until a subject reviewer checks them; report anything wrong from the practice screen.
Q1 · Parcha original · Sample
Enzymes speed up a chemical reaction by:
- A. Raising the temperature
- B. Lowering the activation energy
- C. Changing the equilibrium position
- D. Supplying extra substrate
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Answer: B. Lowering the activation energy
An enzyme provides another reaction pathway with a lower activation energy, so more molecules can react at body temperature. It does not change the equilibrium or the overall energy change of the reaction, and it is not used up.
Open this question on its own pageQ2 · Parcha original · Sample
A non-protein organic group that is tightly (often covalently) bound to an enzyme is called:
- A. A coenzyme
- B. An apoenzyme
- C. A prosthetic group
- D. An activator
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Answer: C. A prosthetic group
A prosthetic group (for example haem) is permanently bound to its enzyme. A coenzyme (such as NAD) binds loosely and leaves after the reaction. An apoenzyme is the protein part alone, and activators are usually metal ions.
Q3 · Parcha original · Sample
A competitive inhibitor works by:
- A. Binding to the active site
- B. Denaturing the enzyme
- C. Binding only to the product
- D. Raising the optimum pH
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Answer: A. Binding to the active site
A competitive inhibitor looks like the substrate and occupies the active site, so substrate and inhibitor compete. Adding more substrate reduces its effect. A non-competitive inhibitor binds elsewhere and changes the enzyme's shape.
Q4 · Parcha original · Sample
The lock-and-key model of enzyme action was proposed by:
- A. Daniel Koshland
- B. Louis Pasteur
- C. Emil Fischer
- D. Eduard Buchner
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Answer: C. Emil Fischer
Emil Fischer proposed the lock-and-key model in 1894: the substrate fits a rigid active site exactly. Daniel Koshland later proposed the induced-fit model, in which the active site changes shape slightly as the substrate binds.
Q5 · Parcha original · Sample
Most enzymes in the human body work best at about:
- A. 25 °C
- B. 37 °C
- C. 50 °C
- D. 70 °C
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Answer: B. 37 °C
Human enzymes have an optimum near normal body temperature, about 37 °C. Above the optimum, activity falls quickly as the enzyme denatures; below it, molecules move more slowly and collide less often.
Q6 · Parcha original · Sample
Pepsin, which digests protein in the stomach, has an optimum pH of about:
- A. 2
- B. 7
- C. 8
- D. 11
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Answer: A. 2
Pepsin works in the acidic stomach (hydrochloric acid), with an optimum near pH 2. Trypsin in the small intestine works best near pH 8.