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Amino acids, peptide bonds and protein structural levels: NEET MCQs with answers

75 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 α-amino acids is optically inactive due to the absence of a chiral carbon atom?

  1. Alanine
  2. Valine
  3. Leucine
  4. Glycine
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Answer: (D) Glycine

Glycine has the structure H₂N-CH₂-COOH. Its α-carbon is bonded to two hydrogen atoms, making it achiral and therefore optically inactive. All other standard α-amino acids have four different groups on the α-carbon and are optically active.

medium

2. Which structural feature characterizes fibrous proteins such as keratin and collagen?

  1. Polypeptide chains coiled into spherical shapes with high solubility in water
  2. A compact globular shape maintained exclusively by metallic coordinate bonds
  3. Polypeptide chains running parallel and held together by disulfide and hydrogen bonds, generally insoluble in water
  4. Complete absence of secondary and tertiary structural levels
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Answer: (C) Polypeptide chains running parallel and held together by disulfide and hydrogen bonds, generally insoluble in water

In fibrous proteins, polypeptide chains run parallel and are held together by hydrogen and disulfide bonds, producing a fiber-like structure. These proteins are typically insoluble in water (e.g., keratin in hair/nails and collagen in tendons).

hard

3. Quaternary structure is an essential feature of certain proteins. Which statement best accounts for the quaternary structure of adult human hemoglobin?

  1. It contains a single linear polypeptide chain that folds into an α-helical rod.
  2. It consists of two identical subunits linked covalently by continuous polysaccharide chains.
  3. It is held together exclusively by strong peptide bonds linking the individual subunits into a continuous circle.
  4. It is an assembly of four subunits (two α-chains and two β-chains) arranged in a specific spatial geometry.
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Answer: (D) It is an assembly of four subunits (two α-chains and two β-chains) arranged in a specific spatial geometry.

Some proteins consist of two or more polypeptide chains referred to as sub-units. The spatial arrangement of these subunits with respect to each other is known as quaternary structure. Hemoglobin is an oligomeric protein composed of four sub-units: two identical α-chains and two identical β-chains.

medium

4. Match List-I with List-II: List-I (Amino Acid / Protein) A. Histidine B. Glutamic acid C. Insulin D. Myosin List-II (Classification / Type) I. Fibrous protein II. Basic amino acid III. Acidic amino acid IV. Globular protein Choose the correct answer from the options given below:

  1. A-II, B-III, C-I, D-IV
  2. A-III, B-II, C-I, D-IV
  3. A-II, B-III, C-IV, D-I
  4. A-IV, B-I, C-II, D-III
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Answer: (C) A-II, B-III, C-IV, D-I

Histidine is a basic amino acid due to its nitrogenous imidazole ring. Glutamic acid has an extra carboxylic group and is acidic. Insulin is a soluble globular protein, and myosin is a water-insoluble fibrous protein present in muscles.

easy

5. Which of the following levels of protein structure remains unaffected when egg albumin undergoes irreversible thermal coagulation?

  1. Secondary structure
  2. Tertiary structure
  3. Quaternary structure
  4. Primary structure
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Answer: (D) Primary structure

Thermal coagulation of egg albumin is an example of protein denaturation. During denaturation, secondary, tertiary, and quaternary structures are disrupted due to the cleavage of weak hydrogen bonds and other non-covalent interactions, but the covalent primary peptide backbone remains intact.

hard

6. Consider the following four statements regarding proteins: (a) Quaternary structure arises when a protein is composed of two or more polypeptide sub-units. (b) Curdling of milk is caused by the denaturation of lactalbumin and casein. (c) Native protein refers to a protein found in a biological system with a unique three-dimensional conformation and biological activity. (d) During denaturation, the sequence of amino acids in the polypeptide chain is completely scrambled. Which of the above statements are correct?

  1. (a), (b), and (c) only
  2. (a) and (c) only
  3. (b) and (d) only
  4. (a), (b), (c), and (d)
Show answer and explanation

Answer: (A) (a), (b), and (c) only

Statements (a), (b), and (c) are factually correct statements from NCERT. Statement (d) is incorrect because denaturation disrupts secondary and tertiary structures while primary structure (the covalent sequence of amino acids) remains entirely intact.

easy

7. The number of peptide bonds in a cyclic peptide made of 6 amino-acid residues is:

  1. 7
  2. 4
  3. 6
  4. 5
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Answer: (C) 6

In a cyclic peptide every residue is joined to the next, including the last to the first: 6 peptide bonds.

easy

8. The number of peptide bonds in a linear peptide made of 26 amino-acid residues is:

  1. 26
  2. 25
  3. 27
  4. 24
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Answer: (B) 25

A linear chain of 26 residues has 26 − 1 = 25 peptide bonds.

easy

9. The number of peptide bonds in a linear peptide made of 34 amino-acid residues is:

  1. 34
  2. 35
  3. 33
  4. 32
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Answer: (C) 33

A linear chain of 34 residues has 34 − 1 = 33 peptide bonds.

easy

10. The number of peptide bonds in a linear peptide made of 40 amino-acid residues is:

  1. 41
  2. 38
  3. 39
  4. 40
Show answer and explanation

Answer: (C) 39

A linear chain of 40 residues has 40 − 1 = 39 peptide bonds.

hard

11. A pentapeptide on complete acid hydrolysis yields Glycine, Alanine, and Phenylalanine in a 2:2:1 molar ratio. Partial hydrolysis yields the dipeptides Gly-Ala, Ala-Gly, and Ala-Phe. If the N-terminal amino acid is Glycine, what is the correct sequence of the pentapeptide?

  1. Gly-Ala-Gly-Ala-Phe
  2. Gly-Ala-Phe-Ala-Gly
  3. Gly-Gly-Ala-Ala-Phe
  4. Ala-Gly-Ala-Phe-Gly
Show answer and explanation

Answer: (A) Gly-Ala-Gly-Ala-Phe

The N-terminal residue is Glycine. With fragments Gly-Ala, Ala-Gly, and Ala-Phe, overlapping the sequences: Gly-Ala must be followed by Ala-Gly (linking through Ala), giving Gly-Ala-Gly, then followed by Ala-Phe (linking through Gly to Ala, then Phe), yielding the sequence Gly-Ala-Gly-Ala-Phe. This gives 2 Gly, 2 Ala, and 1 Phe.

easy

12. In an aqueous solution at physiological pH, an amino acid predominantly exists as a dipolar ion known as a zwitterion. Which statement correctly describes this zwitterionic form?

  1. It contains a positively charged -NH₃⁺ group and a negatively charged -COO⁻ group.
  2. It contains an uncharged -NH₂ group and an uncharged -COOH group.
  3. It has a net negative charge and migrates towards the anode in an electric field.
  4. It lacks amphoteric character because it carries both charges simultaneously.
Show answer and explanation

Answer: (A) It contains a positively charged -NH₃⁺ group and a negatively charged -COO⁻ group.

In aqueous solution, the carboxyl group loses a proton and the amino group accepts a proton, forming the dipolar zwitterion H₃N⁺-CHR-COO⁻. It is electrically neutral overall and retains amphoteric behavior by reacting with both acids and bases.

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