(i) Primary structure: Proteins may contain one or more polypeptide chains. Each polypeptide chain has a large number of α – amino acids which are linked to one another in a specific sequence. The specific sequence in which the various α – amino acids present in a protein are linked to one another is called its primary structure. Any change in the sequence of α – amino acids creates a different protein.
(ii) Secondary structure: It refers to shape in which a long polypeptide chain exists. A protein may assume a – helix structure or β – pleated structure. The α – helix structure results due to regular coiling of polypeptide chain which is stabilized by intramolecular hydrogen bonding. In β pleated sheet structure, all peptide chains are stretched to a nearly maximum extention and then arranged side by side and held together by intramolecular hydrogen bonding.
(iii) Tertiary structure: The tertiary structure of proteins represent overall folding of the polypeptide chains, i.e., further folding of the secondary structure. The main force which stabilises 2° and 3° of proteins are hydrogen bonds, disulphide linkages, Vanderwaals forces of attraction and electrostatic force of attraction.
(iv) Quaternary structure: Same of the proteins are composed of two or more polypeptide chains referred to as sub-units. The spatial arrangement of these sub units with respect to each other is called quaternary structure.
