Disulfide Bond Formation in the Periplasm

This chapter reviews the process of disulfide bond formation in the periplasm by following the life of a protein, from the “birth” of a protein at the ribosome to its entrance into the periplasm as an unfolded infant, to the oxidation of the maturing protein. In eukaryotic cells the protein disulfide isomerase (PDI) represents the counterpart of DsbA, being responsible for the formation of disulfide bonds in the endoplasmic reticulum. DsbA can act to generate disulfide bonds in proteins as nascent polypeptide chains emerge into the periplasm. Several features of disulfide bond formation discovered in recent years illustrate complexity. First, disulfide bond formation likely occurs on nascent poly-peptide chains as they are being translocated into the periplasm in bacteria and into the endoplasmic reticulum in eukaryotic cells. Second, for efficient formation of disulfide bonds, proteins require enzyme catalysts whose prototypes are DsbA in bacterial cells and protein disulfide isomerase in eukaryotic cells. Third, the formation of the correct disulfide bonds requires enzyme catalysts such as DsbC. Bioinformatic analysis conducted in the authors' lab revealed that at least 90% of the periplasmic proteins with more than one cysteine contain disulfide bonds. For those periplasmic proteins with only a single free cysteine, their crystal structures showed these cysteines to be buried within the protein structure (AlsB, AraF, FhuD, GlpQ, and Tpx). These findings are consistent with the proposal that any protein that appears in the periplasm with more than one cysteine will be acted on by DsbA.