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.
Secondary structure of protein refers to
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → the regular folding of the backbone of polypeptide chain due to hydrogen bonding between carbonyl and amino group of the peptide bond.
The structure of a protein is described at four levels:
Primary structure: the sequence of amino acids in the chain. A change of even one amino acid gives a different protein.
Secondary structure: the regular shape of the chain backbone, held by hydrogen bonds between the C=O and N–H groups of peptide bonds. Its two forms are the α-helix (H-bonds within one coiled chain) and the β-pleated sheet (H-bonds between neighbouring chains).
Tertiary structure: the three-dimensional folding, held by hydrogen bonds, disulphide links, van der Waals and electrostatic forces between groups far apart in the chain. It gives the fibrous or globular shape.
Quaternary structure: the arrangement of two or more polypeptide chains (subunits).
Why the other options are incorrect
Option (1) the three-dimensional structure especially the bonds between amino acid residues that are distant from each other in the polypeptide chain — Incorrect. This describes the tertiary structure.
Option (3) denatured proteins and prosthetic group structures — Incorrect. Denaturation destroys the secondary structure, and a prosthetic group is a non-protein part; neither is a level of structure.
Option (4) the linear sequence of amino acid residues in a polypeptide chain — Incorrect. This is the primary structure.
NCERT (2026-27 edition):
Secondary Structure of Proteins
