Final answer:
The disulfide bond is the only covalent bond seen in tertiary and quaternary structures, providing essential structural stability to proteins by linking cysteine amino acids.
Step-by-step explanation:
The only covalent bond seen in tertiary and quaternary protein structures is the disulfide bond. The purpose of this bond is to provide structural stability to the protein. Disulfide bonds form when cysteine amino acids, which contain -SH (sulfhydryl) groups, come into proximity as the protein folds, and the sulfhydryl groups are oxidized to create a disulfide (-S-S-) linkage. This covalent bond significantly enhances the stability of the protein's three-dimensional structure, ensuring that it maintains its functional conformation even under stress.