The amino acid is cysteine. Cysteine has a thiol group as its side chain, and the oxidation of these thiol groups allows for the formation of disulfide bridges between two cysteins to occur in proteins. The formation of a disulfide bridge increases the rigidity of the protein's folded structure, helping to maintain the protein's structure and to prevent unfolding. While the reducing environment found within the cell often prevents the formation of these disulfide bridges by preventing oxidation these bonds can form in the extracellular environment, and so the disulfide bridges formed by cysteines contribute to maintaining cellular stability outside the cell.
