Disulfide bond formation

Disulfide bonds do not form readily in reducing conditions (the cytosol): they require oxidising conditions (the ER in eukaryotes or the bacterial periplasm). They form where 2 cysteine residues are brought close together during protein folding. The biological catalysts are protein disulfide isomerase (PDI) and its relatives. PDIs can make, break and shuffle disulfide bonds. Covalent S-S bonds contribute to the stability of protein tertiary structures (of SECRETED proteins, not cytosolic proteins). Many are essential for the activity of proteins.