Alpha helix and Beta pleated sheet are examples of?

Structural organization of proteins Every protein has a unique three dimensional structure, which is referred to as its native conformation and made up of only 20 different amino acids. Protein structure can be classified into four level of organization.1) Primary structuresThe linear sequence of amino acid residues and location of disulphide bridges, if any, in a polypeptide chain constitute its primary structure. In simple words, primary structure of proteins refers to the specific sequence of amino acids. Primary structure is maintained by covalent 'peptide' bond.2) Secondary structureFor stability of primary structure, hydrogen bonding between the hydrogen of NH and oxygen of C = 0 groups of the polypeptide chain occurs, which give rise to twisting, folding or bending of the primary structure. Thus, regular folding and twisting of the polypeptide chain brought about by hydrogen bonding is called secondary structure. Impoant types of secondary structure are a-helix, O-pleated sheet and 0-bends.3) Teiary structureThe peptide chain, with its secondary structure, may be fuher folded and twisted about itself forming three-dimensional arrangement of polypeptide chain, i.e., teiary structure refers to the overall folding pattern of polypeptide which forms three dimensional shape. Teiary structure (three dimensional shape) is maintained by weak non-covalent interactions which include hydrogen bonds, hydrophobic interactions, ionic bond (electrostatic bonds or salt bridges) and Van-der wall forces. Covalent linkage (disulphide bond) also plays some (but minor) role.4) Quaternary structureMany proteins are made up of more than one polypeptide chains (polymers). Each polypeptide chain is known as protomer (or subunit). The subunit are linked with each other by non-covalent bonds. The structure formed by union of subunits is known as quaternary structure, i.e., spatial relation of subunits (peptide chains) with one another is called quaternary structure. Mainly three non-covalent bonds stabilize quaternary structure : Hydrophobic, hydrogen and ionic (electrostatic).Dimeric proteins contain two polypeptide chains. Homodimers contain two copies of same polypeptide chain, while in a heterodimer the polypeptides differ.