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Protein denaturation and enzymes: NEET MCQs with answers

50 questions on this topic of Biomolecules (NEET 2027 syllabus). Here are 12; tap one to see its answer and explanation.

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easy

1. Which of the following physical or chemical agents can cause denaturation of proteins without initiating hydrolysis of the polypeptide backbone?

  1. Addition of organic solvents like acetone or ethanol
  2. Digestion with concentrated trypsin solution
  3. Prolonged boiling with 6 M hydrochloric acid
  4. Incubation with carboxypeptidase enzyme
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Answer: (A) Addition of organic solvents like acetone or ethanol

Addition of organic solvents like ethanol or acetone alters the dielectric constant and disrupts hydrophobic and hydrogen-bonding interactions, leading to denaturation (precipitation) without cleaving peptide bonds. In contrast, 6 M HCl or digestive peptidases (trypsin, carboxypeptidase) hydrolyze the peptide bonds of the backbone.

medium

2. Match the biocatalyst in List-I with the reaction it catalyzes in List-II: List-I: (A) Pepsin (B) Invertase (C) Zymase (D) Urease List-II: (i) Conversion of glucose to ethanol (ii) Hydrolysis of urea into NH₃ and CO₂ (iii) Hydrolysis of sucrose to glucose and fructose (iv) Hydrolysis of proteins into peptides in stomach Choose the correct match:

  1. A-(iii), B-(iv), C-(i), D-(ii)
  2. A-(iv), B-(i), C-(iii), D-(ii)
  3. A-(iv), B-(iii), C-(i), D-(ii)
  4. A-(ii), B-(iii), C-(i), D-(iv)
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Answer: (C) A-(iv), B-(iii), C-(i), D-(ii)

Pepsin hydrolyzes proteins to peptides in the acidic stomach medium. Invertase hydrolyzes sucrose into glucose and fructose. Zymase ferments glucose into ethanol and CO₂. Urease decomposes urea into ammonia and carbon dioxide.

hard

3. A solution of a pure enzyme has a turnover number (kcat) of 2.5 × 10⁴ s⁻¹. If the total concentration of the enzyme in the reaction vessel is 2.0 × 10⁻⁸ mol L⁻¹ and the substrate is present in saturating amounts, what is the maximum velocity (Vmax) of the reaction?

  1. 8.0 × 10⁻¹³ mol L⁻¹ s⁻¹
  2. 1.25 × 10¹² mol L⁻¹ s⁻¹
  3. 5.0 × 10⁻⁴ mol L⁻¹ s⁻¹
  4. 5.0 × 10⁻² mol L⁻¹ s⁻¹
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Answer: (C) 5.0 × 10⁻⁴ mol L⁻¹ s⁻¹

The maximum reaction velocity under substrate saturation is given by Vmax = kcat × [E]total. Substituting the values: Vmax = (2.5 × 10⁴ s⁻¹) × (2.0 × 10⁻⁸ mol L⁻¹) = 5.0 × 10⁻⁴ mol L⁻¹ s⁻¹.

medium

4. During denaturation of a protein:

  1. Peptide bonds are formed
  2. Activity increases
  3. Secondary and tertiary structures are disrupted, but the primary structure remains intact
  4. The primary structure is destroyed
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Answer: (C) Secondary and tertiary structures are disrupted, but the primary structure remains intact

Heat or pH changes break hydrogen bonds and other interactions that hold the folded structure, so the protein loses biological activity but its amino acid sequence remains.

NCERT Class 12

easy

5. Which of the following statements regarding the physical state and structure of proteins during denaturation is correct?

  1. Helical and pleated sheet structures are preserved while hydrophobic cores expand.
  2. Peptide bonds hydrolyze readily to yield a mixture of free α-amino acids.
  3. The primary structure is destroyed due to breaking of covalent linkages.
  4. The secondary and tertiary structures unfold while the covalent peptide backbone remains intact.
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Answer: (D) The secondary and tertiary structures unfold while the covalent peptide backbone remains intact.

During denaturation, physical or chemical changes disrupt hydrogen bonds and other non-covalent forces, causing secondary and tertiary structures to be destroyed. The primary structure (sequence of amino acids linked by covalent peptide bonds) remains unaltered.

hard

6. An uncatalyzed hydrolytic reaction proceeds with an activation energy of E₁(uncat) = 90 kJ/mol. In the presence of a specific hydrolase enzyme, the activation energy drops to E₁(cat) = 45 kJ/mol at 300 K. By what factor does the rate constant change according to the Arrhenius equation? (Take R = 8.314 J K⁻¹ mol⁻¹ and e¹⁸ ≈ 6.56 × 10⁷)

  1. Rate increases by ~1.8 × 10⁴ times
  2. Rate increases by 2 times
  3. Rate decreases by ~6.56 × 10⁷ times
  4. Rate increases by ~6.56 × 10⁷ times
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Answer: (D) Rate increases by ~6.56 × 10⁷ times

According to the Arrhenius relation, k(cat)/k(uncat) = exp[(E₁(uncat) - E₁(cat)) / (RT)]. Here, ΔE₁ = 90000 - 45000 = 45000 J/mol. RT = 8.314 × 300 ≈ 2494.2 J/mol. The exponent is 45000 / 2494.2 ≈ 18.04, leading to a rate acceleration of e¹⁸ ≈ 6.56 × 10⁷ times.

medium

7. Enzymes speed up biochemical reactions by:

  1. Increasing temperature
  2. Lowering the activation energy
  3. Raising the activation energy
  4. Changing the equilibrium constant
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Answer: (B) Lowering the activation energy

For example, the activation energy for acid hydrolysis of sucrose is 6.22 kJ mol⁻¹, but only 2.15 kJ mol⁻¹ when catalysed by the enzyme sucrase.

NCERT Class 12

medium

8. Which of the following statements regarding the non-protein constituent of a conjugated enzyme is INCORRECT?

  1. A prosthetic group is loosely bound to the apoenzyme and is easily dialyzable.
  2. The protein portion of the enzyme alone without its co-factor is called an apoenzyme.
  3. The complete active enzyme along with its non-protein part is called a holoenzyme.
  4. Coenzymes are organic molecules often derived from water-soluble vitamins.
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Answer: (A) A prosthetic group is loosely bound to the apoenzyme and is easily dialyzable.

Prosthetic groups are tightly and permanently bound to the apoenzyme (often covalently or through very strong non-covalent forces) and cannot be removed by simple dialysis. Coenzymes, by contrast, are more transiently associated.

medium

9. Given below are two statements: Statement I: Enzymes are highly specific biocatalysts that generally act on only a specific substrate or class of substrates. Statement II: The exceptional specificity of an enzyme is due to the presence of uniquely shaped active sites lined with specific chemical groups. In the light of the above statements, choose the correct answer:

  1. Both Statement I and Statement II are true
  2. Both Statement I and Statement II are false
  3. Statement I is true but Statement II is false
  4. Statement I is false but Statement II is true
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Answer: (A) Both Statement I and Statement II are true

Enzymes possess specific active sites characterized by unique shapes and spatial arrangements of functional groups. These active sites bind only complementary substrate molecules in a lock-and-key fashion, accounting for their high specificity.

medium

10. Which of the following interactions is NOT cleaved or disrupted when a native globular protein undergoes denaturation by a moderate change in pH?

  1. Salt bridges (ionic interactions)
  2. Peptide bonds
  3. Intramolecular hydrogen bonds
  4. Hydrophobic interactions
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Answer: (B) Peptide bonds

Alteration of pH affects the ionization of basic and acidic side chains, disrupting ionic bonds, hydrogen bonds, and hydrophobic interactions. However, peptide linkages are strong covalent bonds that can only be cleaved by chemical or enzymatic hydrolysis.

easy

11. Curdling of milk is a common phenomenon in daily life. This process primarily occurs due to:

  1. Hydrolysis of lactose into glucose and galactose
  2. Denaturation of milk protein casein caused by lactic acid
  3. Oxidation of fats present in milk
  4. Cleavage of peptide bonds in casein by rennin
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Answer: (B) Denaturation of milk protein casein caused by lactic acid

Bacteria present in milk produce lactic acid, which lowers the pH of the system. This change in pH causes the denaturation and subsequent coagulation of the milk protein casein.

easy

12. Which of the following organic reagents disrupts protein tertiary structure primarily by breaking disulfide (-S-S-) bridges?

  1. Dilute acetic acid
  2. Sodium chloride
  3. 2-Mercaptoethanol
  4. Glucose solution
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Answer: (C) 2-Mercaptoethanol

2-Mercaptoethanol (or dithiothreitol) is a reducing agent that reduces covalent disulfide cross-links (-S-S-) between cysteine residues into free sulfhydryl (-SH) groups, facilitating protein denaturation.

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