MP Board · Class 12 · Chemistry · BiomoleculesWhat are proteins? Discuss their classification based on structure and shape, and explain the different levels of protein structure (primary, secondary, tertiary, and quaternary) in detail.

Definition of Proteins\nProteins are high molecular mass, complex biopolymers composed of $\alpha$-amino acids linked together by peptide bonds. They are fundamental building blocks of all living systems and are essential for growth, repair, and regulation of physiological processes.

Classification Based on Structure and Shape

1. Fibrous Proteins:
   • When the polypeptide chains run parallel and are held together by hydrogen and disulfide bonds, fiber-like structures are formed.
   • These are insoluble in water.
   • Examples: Keratin (present in hair, wool, silk) and myosin (present in muscles).
2. Globular Proteins:
   • These structures result when the chains of polypeptides coil around to give a spherical shape.
   • These are usually soluble in water.
   • Examples: Insulin and albumin.

Levels of Protein Structure

1. Primary Structure:
   • The primary structure of a protein refers to the specific sequence in which amino acids are arranged in a polypeptide chain.
   • Any change in the sequence of amino acids creates a completely different protein.
2. Secondary Structure:
   • It refers to the shape in which a long polypeptide chain can exist. It arises due to regular folding of the backbone of the peptide chain due to hydrogen bonding between $-\text{C=O}$ and $-\text{NH}-$ groups.
   • It exists in two different types: $\alpha$-helix and $\beta$-pleated sheet structure.
3. Tertiary Structure:
   • It represents the overall three-dimensional folding of the polypeptide chain.
   • It gives rise to two major molecular shapes: fibrous and globular. The main forces stabilizing the tertiary structure are hydrogen bonds, ionic bonds, disulfide linkages, and van der Waals forces.
4. Quaternary Structure:
   • Some proteins consist of two or more polypeptide chains referred to as subunits. The spatial arrangement of these subunits relative to each other is termed the quaternary structure.