The tertiary structure of a protein is held in shape by all of the following EXCEPT

Explanation

Peptide bonds join the amino group of one residue to the carboxyl group of the next along the backbone, so they build the primary structure and never form between R groups. The fold itself is maintained by the weaker hydrogen, ionic and hydrophobic interactions between side chains, reinforced by the covalent disulphide bridge where two cysteines meet. That is why gentle heating or a pH change can unfold a protein without breaking it apart.