Enzymes — NEET Biology MCQs with Solutions
Free NEET Biology Enzymes MCQs with step-by-step solutions (124 questions). Part of Biomolecules. Practise online on Prepizo — no login needed.
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Questions with solutions
Q1 — Enzymes · easy · theory
Enzymes are biological catalysts that speed up reactions by:
A. Lowering the activation energy ✓ Correct
B. Being consumed in the reaction
C. Changing the equilibrium constant
D. Raising the activation energy
Solution: Enzymes lower the activation energy of a reaction; they are not used up and do not alter the equilibrium.
Q2 — Enzymes · easy · theory
The specific region of an enzyme where the substrate binds is the:
A. Allosteric site only
B. Cofactor
C. Active site ✓ Correct
D. Backbone
Solution: The active site is a crevice/pocket shaped to fit specific substrate(s) forming an enzyme–substrate complex.
Q3 — Enzymes · medium · theory
The non-protein component required by some enzymes for activity is a ______; if it is a tightly bound organic molecule it is a ______:
A. Substrate; product
B. Coenzyme; substrate
C. Cofactor; prosthetic group ✓ Correct
D. Inhibitor; activator
Solution: Cofactors may be prosthetic groups (tightly bound), coenzymes (loosely bound organic, e.g. NAD) or metal ions.
Q4 — Enzymes · medium · theory
Enzyme activity typically shows an optimum with respect to temperature and pH because beyond the optimum the enzyme:
A. Denatures / loses its active conformation ✓ Correct
B. Becomes a substrate
C. Gains a cofactor
D. Increases activity indefinitely
Solution: Enzymes work best at an optimum T and pH; high temperature/extreme pH denatures the protein, abolishing activity.
Q5 — Enzymes · hard · theory
A competitive inhibitor decreases enzyme activity because it:
A. Resembles the substrate and competes for the active site ✓ Correct
B. Denatures the enzyme permanently
C. Binds the allosteric site only
D. Increases substrate concentration
Solution: Competitive inhibitors (e.g. malonate for succinic dehydrogenase) resemble the substrate and can be overcome by more substrate.
Q6 — Enzymes · hard · theory
In the six IUB classes of enzymes, an enzyme catalysing the transfer of a phosphate group between substrates is a:
A. Hydrolase
B. Oxidoreductase
C. Transferase ✓ Correct
D. Lyase
Solution: Transferases move a group (e.g. phosphate) between molecules; oxidoreductases handle redox, hydrolases hydrolyse bonds, lyases add/remove groups forming double bonds.
Q7 — Enzymes · easy · numerical
Chemically, almost all enzymes are:
A. simple inorganic ions
B. lipids
C. proteins ✓ Correct
D. carbohydrates
Solution: Almost all enzymes are proteins (a few RNA molecules, ribozymes, also act catalytically), giving them their specific three-dimensional shapes.
Q8 — Enzymes · easy · numerical
The model proposing that an enzyme's active site is exactly complementary in shape to its substrate, like a lock and its key, is the:
A. induced-fit model
B. fluid-mosaic model
C. sliding-filament model
D. lock-and-key model ✓ Correct
Solution: The lock-and-key model assumes a rigid active site precisely matching the substrate; the induced-fit model allows the site to change shape on binding.
Q9 — Enzymes · easy · numerical
Many coenzymes required for enzyme activity are derived from:
A. minerals such as zinc
B. simple monosaccharides
C. fatty acids
D. vitamins ✓ Correct
Solution: Coenzymes are organic cofactors, and many of them are derived from vitamins (e.g. NAD from niacin).
Q10 — Enzymes · easy · numerical
The IUB system divides enzymes into how many major classes?
A. Four
B. Ten
C. Six ✓ Correct
D. Three
Solution: Enzymes are grouped into six classes: oxidoreductases, transferases, hydrolases, lyases, isomerases and ligases.
Q11 — Enzymes · medium · numerical
According to the induced-fit model of enzyme action:
A. the enzyme becomes a permanent part of the product
B. the active site changes shape to fit the substrate as it binds ✓ Correct
C. the active site is rigid and never alters its shape
D. the substrate permanently changes into the enzyme
Solution: The induced-fit model proposes that substrate binding induces a conformational change in the active site so it moulds around the substrate.
Q12 — Enzymes · medium · numerical
At a temperature much lower than its optimum, an enzyme is:
A. converted into a coenzyme
B. inactive but not denatured, regaining activity when warmed back to the optimum ✓ Correct
C. permanently denatured and cannot recover
D. at its maximum rate of catalysis
Solution: Low temperature only slows or halts activity temporarily; unlike heat denaturation, the enzyme resumes working once warmed to its optimum.
Q13 — Enzymes · medium · numerical
Which of the following correctly matches a cofactor type with its example?
A. Prosthetic group–NAD; Metal activator–haem of catalase; Coenzyme–zinc
B. Prosthetic group–haem of catalase; Metal activator–zinc of carbonic anhydrase; Coenzyme–NAD ✓ Correct
C. Prosthetic group–zinc; Metal activator–NAD; Coenzyme–haem of catalase
D. Prosthetic group–NADP; Metal activator–NAD; Coenzyme–haem of catalase
Solution: Haem is a tightly bound prosthetic group of catalase/peroxidase, zinc is a metal activator of carbonic anhydrase, and NAD is a freely dissociating coenzyme.
Q14 — Enzymes · medium · numerical
Malonate inhibits the enzyme succinic dehydrogenase because it:
A. permanently denatures the enzyme protein
B. binds to a site other than the active site and distorts the enzyme
C. resembles the substrate succinate and competes with it for the active site ✓ Correct
D. removes the metal cofactor from the enzyme
Solution: Malonate is a competitive inhibitor structurally similar to succinate, so it competes for the same active site of succinic dehydrogenase.
Q15 — Enzymes · medium · numerical
Match the enzyme class with the reaction it catalyses: (i) Oxidoreductase (ii) Lyase (iii) Isomerase (iv) Ligase — (p) joining two molecules using ATP energy, (q) transfer of hydrogen/electrons between substrates, (r) interconversion of isomers, (s) removal of groups leaving double bonds.
A. (i)–r, (ii)–p, (iii)–q, (iv)–s
B. (i)–p, (ii)–r, (iii)–s, (iv)–q
C. (i)–q, (ii)–s, (iii)–r, (iv)–p ✓ Correct
D. (i)–s, (ii)–q, (iii)–p, (iv)–r
Solution: Oxidoreductases transfer H/electrons, lyases remove groups leaving double bonds, isomerases interconvert isomers, and ligases join molecules using ATP.
Q16 — Enzymes · medium · numerical
Assertion (A): Enzymes are highly specific and can distinguish between closely related substrates. Reason (R): The active site has a specific three-dimensional shape complementary to the substrate.
A. A is false but R is true
B. A is true but R is false
C. Both A and R are true but R is not the correct explanation of A
D. Both A and R are true and R is the correct explanation of A ✓ Correct
Solution: Enzyme specificity arises from the precise three-dimensional complementarity between the active site and its substrate, so R correctly explains A.
Q17 — Enzymes · medium
Read the following statements and find out the incorrect statement.
A. A general rule of thumb is that rate doubles or decreases by half for every 10 C change in either direction.
B. Endothermic reactions are also called spontaneous reactions while an exothermic reaction is called energy requiring reaction. ✓ Correct
C. Rate can also be called velocity if the direction is specified.
D. Energy difference between substrate and transition state is called activation energy.
Solution: The terms are reversed: exothermic reactions are spontaneous (energy-releasing) and endothermic reactions are energy-requiring, so statement 3 is incorrect.
Q18 — Enzymes · medium
Arrange the steps of catalytic cycle of an enzyme in order and choose the right option. (i) Enzyme releases products of reaction and the enzyme is free to bind to another substrate. (ii) Active site of enzyme is in close proximity of substrate and breaks the chemical bonds of substrate. (iii) Binding of substrate induces enzyme to a shape fitting more tightly around substrate. (iv) Substrate binds to active site of enzyme fitting into the active site.
A. iii, ii, i, iv
B. iv, ii, i, iii
C. iv, iii, ii, i ✓ Correct
D. iii, iv, i, ii
Solution: The correct sequence is substrate binding to active site (iv), induced tighter fit (iii), bonds broken (ii), and products released (i).
Q19 — Enzymes · hard
One molecule of an enzyme is able to catalyse conversion of two molecules of substrate into products in 5 minutes. Ten molecules of enzyme and 25 molecules of substrate are mixed in a test tube. At the end of 10 minutes the test tube will have
A. Products, enzyme molecules and 5 molecules of substrate
B. Products and enzyme molecules ✓ Correct
C. Products only
D. Products and 5 unreacted substrate molecules
Solution: In 10 minutes each of the 10 enzymes converts 4 substrates (capacity 40 > 25), so all 25 substrates become products and the reusable enzymes remain.
Q20 — Enzymes · easy
Assertion: High temperature destroys enzymatic activity. Reason: Proteins get denatured by high temperature.
A. Assertion is true, but reason is false.
B. Both assertion and reason are true, but reason is not the correct explanation of the assertion.
C. Both assertion and reason are false.
D. Both assertion and reason are true and the reason is the correct explanation of the assertion. ✓ Correct
Solution: Enzymes are proteins, so heat denatures them and destroys their activity, making the reason the correct explanation.
Q21 — Enzymes · medium
Assertion: Allosteric enzymes show feedback inhibition. Reason: The inhibitor is competitive.
A. Both assertion and reason are true and the reason is the correct explanation of the assertion.
B. Assertion is true, but reason is false. ✓ Correct
C. Both assertion and reason are false.
D. Both assertion and reason are true, but reason is not the correct explanation of the assertion.
Solution: Allosteric enzymes show feedback inhibition, but the inhibitor acts at an allosteric (non-competitive) site, not competitively, so the reason is false.
Q22 — Enzymes · medium
Assertion: Coenzymes serve as co-factors in a number of different enzyme catalyzed reactions. Reason: Coenzymes and prosthetic groups are cofactors.
A. Both assertion and reason are true and the reason is the correct explanation of the assertion.
B. Both assertion and reason are false.
C. Both assertion and reason are true, but reason is not the correct explanation of the assertion. ✓ Correct
D. Assertion is true, but reason is false.
Solution: Both statements are true, but the reason merely restates that coenzymes are cofactors rather than explaining the assertion.
Q23 — Enzymes · medium
Assertion: Enzymes lower the activation energy. Reason: A substrate molecule can be acted upon by a particular enzyme.
A. Both assertion and reason are true, but reason is not the correct explanation of the assertion. ✓ Correct
B. Both assertion and reason are true and the reason is the correct explanation of the assertion.
C. Both assertion and reason are false.
D. Assertion is true, but reason is false.
Solution: Both statements are true, but enzyme specificity does not explain how enzymes lower activation energy, so the reason is not the correct explanation.
Q24 — Enzymes · medium
Statement I: Competitive inhibitors are often used in the control of bacterial pathogens. Statement II: When the inhibitor closely resembles the enzyme in its molecular structure, it is known as competitive inhibitor.
A. Both statement I and II are incorrect
B. Only statement I is correct ✓ Correct
C. Both statement I and II are correct
D. Only statement II is correct
Solution: Statement I is correct — competitive inhibitors (e.g. malonate for succinic dehydrogenase, sulpha drugs) are used to control bacterial pathogens. Statement II is incorrect: a competitive inhibitor closely resembles the substrate (not the enzyme) in its molecular structure.
Q25 — Enzymes · medium
Statement I: Most of the enzymes have been classified into different groups based on the types of reactions they catalyse. Statement II: Transferases are group of enzymes that catalyse transfer of a group between two substrates.
A. Only statement II is correct
B. Both statement I and II are incorrect
C. Only statement I is correct
D. Both statement I and II are correct ✓ Correct
Solution: Both statements are correct. Enzymes are classified into 6 classes based on the type of reaction catalysed, and transferases catalyse the transfer of a group (other than hydrogen) between a pair of substrates.
Q26 — Enzymes · medium
Statement I: The activity of an enzyme is sensitive to the presence of specific cofactors that compete with the binding of enzyme. Statement II: Catalytic activity is lost when the co-factor is removed from the enzyme.
A. Both statement I and II are incorrect
B. Only statement I is correct
C. Both statement I and II are correct
D. Only statement II is correct ✓ Correct
Solution: Statement I is incorrect — cofactors are required non-protein constituents that assist the enzyme; they do not compete with the enzyme's binding. Statement II is correct: catalytic activity is lost when the cofactor is removed from the enzyme (leaving the inactive apoenzyme).
Q27 — Enzymes · medium
Statement I: The essential chemical components of many coenzymes are steroid hormones. Statement II: Coenzymes NAD and NADP contain the vitamin niacin.
A. Only statement I is correct
B. Only statement II is correct ✓ Correct
C. Both statement I and II are incorrect
D. Both statement I and II are correct
Solution: Statement I is incorrect — the essential chemical components of many coenzymes are vitamins, not steroid hormones. Statement II is correct: coenzymes NAD and NADP contain the vitamin niacin.
Q28 — Enzymes · medium
Statement I: Catalase and peroxidase catalyse the breakdown of H₂O to H₂O₂ and O₂. Statement II: Haem is the coenzyme and it is a part of the active site of the enzyme catalase and peroxidase.
A. Only statement II is correct ✓ Correct
B. Only statement I is correct
C. Both statement I and II are incorrect
D. Both statement I and II are correct
Solution: Statement I is incorrect — catalase and peroxidase catalyse the breakdown of H₂O₂ (hydrogen peroxide) into H₂O and O₂, not the reverse. Statement II is correct: haem is the coenzyme and forms part of the active site of these enzymes.
Q29 — Enzymes · medium
Statement I: Co-enzymes serve as co-factors in a number of different enzyme catalysed reactions. Statement II: Association of co-enzyme with the apoenzyme is only transient, usually occurring during the course of catalysis.
A. Only statement I is correct
B. Both statement I and II are incorrect
C. Both statement I and II are correct ✓ Correct
D. Only statement II is correct
Solution: Both statements are correct. Coenzymes are organic cofactors, and their association with the apoenzyme is only transient, usually occurring during the course of catalysis.
Q30 — Enzymes · medium
Statement I: Kₘ indicates the enzyme concentration at which a reaction attains half of its maximum velocity. Statement II: Enzymes are divided into 6 classes each with 4-13 subclasses and named accordingly by a four-digit number.
A. Both statement I and II are correct
B. Both statement I and II are incorrect
C. Only statement I is correct
D. Only statement II is correct ✓ Correct
Solution: Statement I is incorrect — Kₘ (Michaelis constant) is the substrate concentration (not enzyme concentration) at which the reaction attains half its maximum velocity. Statement II is correct: enzymes are divided into 6 classes, each with 4-13 subclasses, and identified by a four-digit number.