The structure and shape of proteins can be studied at four different levels, i.e., primary, secondary, tertiary and quaternary.
1) Primary structure of proteins – It refers to the sequence of amino acids in a polypeptide chain.
2) They are found to exist in two different types of structures such as α -helix and β-pleated sheet structure. In α -helix the polypeptide chain forms all possible hydrogen bonds by twisting into a right-handed screw with the -NH groups of each amino acid residue hydrogen-bonded to the group of an adjacent tum of the helix. In β -pleated sheet structure all peptide chains are stretched out to nearly maximum extension and laid side by side which are held together by intermolecular hydrogen bonds.
3) Tertiary structure of proteins – It represents overall folding of the polypeptide chains.
4) Quaternary structure of proteins – The spacial arrangement of two or more polypeptide chains.