Proteins are most abundant biomolecules of the living organism. The main source of proteins are milk, cheese, pulses, peanuts, fish meat etc. Amino acids are building block of proteins. Amino acids contain amino (–NH2) and carboxyl (–COOH) functional groups. Depending upon the relative position of amino group with respect to carboxyl group, the amino acids can be classified as α, β, γ, δ and so on. Only alpha amino acid are obtained on hydrolysis of protein. Amino acids are classified as acidic, or basic or neutral depending upon the relative number of amino and carboxyl group in their molecule.
Amino acids which can be synthesised in the body are known as non-essential amino acids. On the other hand, those which can not be synthesised in the body and must be obtained through diet are known as essential amino acids. Proteins are condensation product of amino acids which are linked through peptide bond or peptide linkage.
Proteins can be classified into two types on the basis of their molecular shape. (a) Fibrous and (b) Globular proteins. Proteins found in a biological system with unique three-dimensional structure and biological activity is called a native protein. When a protein in its native form is subjected to physical changes, such as change in temperature, or chemical changes, such as change in pH, hydrogen bonds are disturbed. As a result, globules unfold, helices uncoil, and protein loses its biological activity. This is called denaturation of protein.
Protein denaturation results in
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → disruption of the secondary and tertiary structure of the protein.
A protein has up to four levels of structure:
Primary: the sequence of amino acids, held by strong covalent peptide bonds.
Secondary: the α-helix or β-pleated sheet, held by hydrogen bonds between C=O and N–H groups.
Tertiary: the three-dimensional folding of the chain, held by hydrogen bonds, disulphide links, van der Waals and electrostatic forces.
Quaternary: the arrangement of two or more chains (subunits).
Denaturation breaks only the hydrogen bonds that hold the secondary and tertiary levels; the peptide bonds, and so the primary structure, stay intact.
Why the other options are incorrect
Option (1) disruption of the primary, secondary, or tertiary structure of the protein — Incorrect. The primary structure (peptide bonds) is not disrupted.
Option (3) disruption of all structures including primary, secondary, tertiary, and quaternary structures of protein — Incorrect. Again the primary structure survives; breaking peptide bonds is hydrolysis, not denaturation.
Option (4) restoration of original biological activity — Incorrect. Denaturation destroys the biological activity.
NCERT (2026-27 edition):
Denaturation of Proteins
