Which amino acids can form disulfide bonds?

Disulfide bonds are covalent bonds that can form between the sulfur atoms of two cysteine residues in a protein. Cysteine (A) contains a thiol (-SH) group in its side chain, which allows it to undergo oxidation and form a covalent link with another cysteine's thiol group, resulting in a disulfide bond (R-S-S-R). This bond is important for stabilizing the three-dimensional structure of proteins, particularly in extracellular environments where oxidative conditions may promote such interactions.

Other amino acids listed, such as lysine, aspartate, and histidine, do not have the necessary thiol side chains to participate in disulfide bond formation. Lysine has an amino group in its side chain; aspartate has a carboxylate group; and histidine contains an imidazole ring. None of these can create the covalent bond that is characteristic of disulfide linkages. Therefore, cysteine is uniquely suited for forming these important structural features in proteins.