Tertiary structure of a polypeptide is stablizied by all of

Tertiary structure of a polypeptide is stablizied by all of the following except:

a) ionic bonds

b) hydrogen bonds

c) disulfide bonds

d) covalent bonds

e) there are no exceptions

Solution

Ans. Correct option: A. Ionic bond

The overall 3D conformation of a polypeptide is called its tertiary structure. Tertiary structure is stabilized by hydrogen bonds (generally, between polar amino acids), ionic interactions (between oppositely charged side chains), van der Waal’s forces (among hydrophobic residues) and disulfide bridge (-S-S-) between cysteine residues brought together in close proximity during folding.

Note that disulfide bond (or, disulfide bridge) is a covalent bond.

There is not formation of ionic bond at any level of protein structure. Ionic bond is a chemical bond arising due to subsequent loss and gain of electrons between the participating atoms. No ionic bond forms in protein.

However, there is significant role of “ionic interactions” between oppositely charged residues in the protein. Note that, during the process, the oppositely charged groups attract each other BUT there is NOT net gain or loss of electrons between the participating pairs. So, an ionic interaction is NOT an ionic bond.