The protein structures can be classified into four levels, namely the primary structure, secondary structure, tertiary structure, and quaternary structure. The primary structure is the simplest of all the structures. When all the hydrogen bonds are disrupted, the secondary, tertiary and the quaternary structures gets disrupted, which leads the protein to the most simplest structural form, that is the primary structure. In this structure, the a carbon atom is bonded to hydrogen atom, carboxyl group, amino group, and an 'R' group.