In the case of insulin shown here there are two polypeptide chains in the primary structure. Human Insulin Primary structure. Secondary Structure Secondary structure refers to the folding of the chain of amino acids into a helix or a pleated sheet. This structure is a pleated sheet formed by parallel chains of amino acids. These sheets are important in many structural proteins.
Many proteins have sheets and helices.
Secondary structure arises from the geometry of the bond angle between amino acids as well as hydrogen bonds between near by amino acids. Tertiary Structure Tertiary structure refers to a higher level of folding in which the helices and sheets of the secondary structure fold upon themselves. This higher level folding arises for several reasons. First, different regions of the amino acid chain are hydrophilic or hydrophobic and arrange themselves accordingly in water. Second different regions of the chain bond with each other via hydrogen bonding or disulfide linkages.