structure of protiens

Proteins are macromolecules made up of amino acids which are linked to each other by covalent bonds called peptide bonds. Peptide bonds are formed between carboxyl group of one amino acid and amino group of the other amino acid. There are four levels in the protein structure on the basis of the complexity in their structure:
Primary structure: linear polypeptide chain
Secondary structure: Areas of folding or coiling within a protein
Tertiary structure: Final three dimension structure produced by coiling and linkages and non covalent interactions between amino acids. 
Quaternary structure: complex protein formed by non covalent interactions between multiple polypeptide chains.