Which amino acids are capable of forming disulfide bonds?

The ability of amino acids to form disulfide bonds is primarily attributed to cysteine. Cysteine contains a thiol (-SH) group in its side chain. When two cysteine molecules come close together, they can undergo an oxidation reaction, resulting in the formation of a covalent bond between their sulfur atoms. This bond is known as a disulfide bond (–S–S–), and it plays a crucial role in stabilizing the three-dimensional structure of proteins, particularly in extracellular proteins where the environment is more oxidizing.

In contrast, histidine, lysine, and glutamine do not possess the thiol group necessary for disulfide bond formation. Histidine has an imidazole side chain that does not lend itself to forming such bonds. Lysine has an amino group in its side chain, while glutamine has an amide group, neither of which can oxidize to form disulfide links. Thus, only cysteine is capable of forming these essential bonds in protein structures.