Final answer:
The sulfhydryl group in cysteine amino acids forms disulfide linkages which stabilize the tertiary structure of proteins through the formation of covalent bonds.
Step-by-step explanation:
The functional group that forms a covalent bond between peptide chains and stabilizes protein tertiary structure is the sulfhydryl group. This group is found within cysteine amino acids. When two cysteine molecules are brought close together during the folding of a protein, their sulfhydryl groups can undergo an oxidation reaction, leading to the formation of a covalent bond known as a disulfide linkage (illustrated as part (c) in various figures). This disulfide bond is a key stabilizing force in a protein's tertiary structure, alongside other interactions such as ionic bonding, hydrogen bonding, and dispersion forces.