Genomics perspective on disulfide bond formation.

Disulfide bond formation, reduction, and isomerization in substrate proteins are catalyzed by designated pathways composed of thiol-dependent enzymes. Disulfides are generated in oxidizing environments, such as bacterial periplasm and eukaryotic endoplasmic reticulum (ER), but could also be formed in the cytosol. Major contributors to the formation of intramolecular disulfides in proteins are thiol/disulfide oxidoreductases containing a conserved CxxC motif (two cysteines separated by two other residues), which in turn transfer reducing equivalents to adapter or membrane-bound oxidoreductases. Disulfide bond formation is accompanied by disulfide bond reduction and isomerization processes, allowing disulfide repair and quality control. Higher eukaryotes evolved a complex network of thiol/disulfide oxidoreductases that are involved in disulfide bond formation and isomerization and thiol-dependent protein retention. Emerging evidence suggests that these ER functions might be assisted by mammalian selenocysteine-containing oxidoreductases Sep15 and SelM.