Pepsin is secreted as inactive pepsinogen and activated in the stomach by ____.
Correct answer C. hydrochloric acid
Topic / 45 questions
Enzymes MCQs
Enzymes MCQs are multiple-choice questions on the nature of enzymes, how they lower activation energy, the factors that change their rate of reaction and the ways they can be inhibited. Every question on MCQs360 shows the correct answer instantly, so you can find gaps in your understanding straight away.
- Biological Molecules 74
- Coordination and Control 72
- Biology Diagnostic 70
- Cell Structure and Function 67
- Biotechnology 65
- Reproduction 58
- Bioenergetics 55
- Circulation 50
- Inheritance 48
- Digestion 45
- Immunity 45
- Support and Movement 45
- Acellular Life (Viruses) 45
- Homeostasis 45
- Respiration and Gaseous Exchange 45
- Evolution 45
- Enzymes 45
- Cell Cycle 41
- Chromosome and DNA 40
- Biology Introduction 38
- Ecosystem 27
Catalase protects cells by converting hydrogen peroxide into ____.
Correct answer D. water and oxygen
DNA ligase, which joins two DNA fragments using energy from ATP, belongs to which class?
Correct answer C. Ligases (synthetases)
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An enzyme that converts glucose-6-phosphate into fructose-6-phosphate by rearranging atoms within the molecule is ____.
Correct answer D. an isomerase
Kinases that move a phosphate group from ATP to another molecule are examples of ____.
Correct answer C. transferases
Dehydrogenases that remove hydrogen from a substrate are placed in which class of enzymes?
Correct answer B. Oxidoreductases
A competitive inhibitor reduces enzyme activity by ____.
Correct answer B. resembling the substrate and occupying the active site
The Michaelis constant (Km) of an enzyme is the substrate concentration at which the reaction rate is ____.
Correct answer C. half of the maximum rate
The effect of a competitive inhibitor can be reduced by ____.
Correct answer A. increasing the substrate concentration
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Malonate inhibits succinate dehydrogenase because it ____.
Correct answer D. resembles succinate and competes for the active site
A non-competitive inhibitor acts by ____.
Correct answer C. binding at a site other than the active site and altering its shape
Which statement is true of non-competitive inhibition?
Correct answer A. Adding more substrate does not restore the maximum rate
Cyanide is poisonous because it irreversibly inhibits ____.
Correct answer C. cytochrome oxidase in the respiratory chain
Organophosphate nerve gases and some insecticides are toxic because they inhibit ____.
Correct answer A. acetylcholinesterase
An inhibitor that forms strong covalent bonds with an enzyme and permanently inactivates it is called ____.
Correct answer D. an irreversible inhibitor
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In feedback inhibition, the end product of a metabolic pathway ____.
Correct answer A. inhibits an enzyme early in the pathway
A site on an enzyme, separate from the active site, where regulatory molecules bind is the ____.
Correct answer C. allosteric site
The protein part of an enzyme that is inactive without its cofactor is called the ____.
Correct answer B. apoenzyme
An organic cofactor that is tightly and permanently bound to its enzyme is a ____.
Correct answer D. prosthetic group
Which one of these cofactors is classed as a coenzyme?
Correct answer B. NAD⁺
About Enzymes MCQs
The opening questions cover the characteristics of enzymes: their protein nature, specificity, the active site and the role of cofactors, including activators, coenzymes and prosthetic groups. You will also meet terms such as apoenzyme and holoenzyme and the main classes of enzymes, such as hydrolases and oxidoreductases. Mechanism questions compare the lock and key and induced fit models and the formation of the enzyme-substrate complex. Further sets test how temperature, pH, enzyme concentration and substrate concentration affect reaction rate, with graph-based questions on optimum points and saturation. The final area covers competitive and non-competitive inhibition, irreversible inhibitors such as cyanide, and feedback inhibition.
Key facts to remember
- Enzymes speed up reactions by lowering the activation energy, and they are not used up in the reaction.
- An apoenzyme combined with its cofactor forms an active holoenzyme.
- A prosthetic group is tightly bound to its enzyme, while a coenzyme is loosely attached.
- Emil Fischer proposed the lock and key model, and Daniel Koshland proposed the induced fit model.
- Competitive inhibitors bind to the active site, so their effect can be overcome by raising substrate concentration.
- Most human enzymes work best at about 37°C, but pepsin works best at an acidic pH of about 2.
Exams that include Enzymes MCQs
Pakistan
- MDCAT
- NUMS
- AKU
- FSc boards
India
- NEET
Bangladesh
- HSC
- Medical Admission Test
How to prepare for Enzymes MCQs
- Sketch the rate graphs for temperature, pH, substrate and enzyme concentration and explain the shape of each one.
- Make a two-column table comparing competitive and non-competitive inhibition.
- Learn the differences between activators, coenzymes and prosthetic groups with one example of each.
- Practise reading graph-based questions carefully, noting which variable is fixed and which is changing.
Frequently asked questions
Which exams ask Enzymes MCQs?
In Pakistan, Enzymes questions appear in MDCAT, the NUMS and AKU entry tests and FSc board papers. Students in India meet the same content in NEET, and in Bangladesh it is tested in HSC Biology and the Medical Admission Test.
Why does the rate of an enzyme reaction level off at high substrate concentration?
At high substrate concentration all the active sites are occupied at any moment, so the enzyme is saturated. Adding more substrate cannot raise the rate further unless more enzyme is added, and the rate reaches its maximum value.
What happens to an enzyme at high temperature?
Above the optimum temperature, the hydrogen and other bonds that hold the enzyme's three-dimensional shape break. The active site changes shape and can no longer bind the substrate, so the enzyme is denatured and the rate falls sharply.
Last reviewed October 2026