Proteins are macromolecules having one or more polypeptide chains of amino acids. These are highly organised linear polymers of amino acids. Proteins have four structural levels - primary, secondary, tertiary and quaternary.

(i) Primary structure : The linear sequence of amino acids in a polypeptide chain is the primary structure of protein. The first amino acid is called N-terminus and last amino acid is called C-terminus. 

(ii) Secondary structure : The polypeptide chains are coiled or folded in the form of helix. The structure is maintained by a series of regularly spaced intra or inter molecular hydrogen bonds formed between the amino acids of the same or different polypeptide chains. Such a conformation is termed as secondary structure. There are 3 types of secondary structure, αhelix, β-pleated and collagen helix. 

(iii) Tertiary structure : The helical polypeptide molecules further coiled to assume a complex but specific form is called tertiary structure. They are so arranged as to hide nonpolar amino acid inside and expose the polar side chains. These structure are maintained by several types of bonds such as hydrogen bonds, vander Waal interaction, ionic bonds, disulphide bonds etc., between the polypeptide chains. 

(iv) Quaternary structure: Each polypeptide develops its own tertiary structure and functions as subunit of the protein. The different charged subunits pack together to give specific conformation. Large proteins such as haemoglobin have quaternary structure. It has four chains, two α-chains and two β-chains.