CBSE · Class 12 · Chemistry · BiomoleculesWhat are proteins? Explain the primary and secondary structures of proteins in detail.

Proteins are high molecular mass, complex biopolymers composed of $\alpha$-amino acids linked together by peptide bonds. They are fundamental to the structure and function of all living cells.

1. Primary Structure: The primary structure of a protein refers to the specific sequence in which the various $\alpha$-amino acids are arranged in a polypeptide chain. Each protein has a unique sequence of amino acids. Any change in the sequence of amino acids alters the entire protein, leading to different biological properties. The sequence is determined by genetic codes.

2. Secondary Structure: The secondary structure refers to the shape or conformation in which a long polypeptide chain can exist due to hydrogen bonding between the carbonyl oxygen ($>C=O$) and the amide hydrogen ($-NH-$) of the peptide backbone. It primarily exists in two distinct forms:

   • $\alpha$-Helix: In this structure, the polypeptide chain twists into a right-handed screw (helix) stabilized by hydrogen bonds between every amine hydrogen and carbonyl oxygen of the fourth amino acid ahead in the chain.
   • $\beta$-Pleated Sheet: In this form, all polypeptide chains are stretched out nearly to maximum extension and laid side by side, held together by intermolecular hydrogen bonds, resembling the pleats of drapery. \nThus, protein structures dictate their specific biological activities.