In E. coli, disulfide bonds are introduced in the periplasm by the Dsb (Disulfide bond) proteins family (Figure 1).
The primary oxidant is the soluble protein DsbA. DsbA has a CXXC catalytic site motif present within a thioredoxin fold. The cysteine residues of this motif are found oxidized in vivo. The disulfide bond of DsbA is very unstable and is rapidly transferred to secreted unfolded proteins. DsbA is then re-oxidized by the inner-membrane protein DsbB that transfers electrons from DsbA to the electron transport chain.
DsbA is a very powerful oxidant that apparently lacks proofreading activity. If the native disulfide bond pattern involves cysteine residues that are non-consecutive in the amino-acid sequence, DsbA can form incorrect disulfides. The correction of these non-native disulfides is the role of a disulfide isomerization system. This system is composed of a soluble periplasmic proteins, DsbC, which function as a disulfide isomerase protein. Like DsbA, DsbC possesses a thioredoxin fold and a CXXC catalytic site motif. In contrast to DsbA, the CXXC motif of DsbC is kept reduced in the periplasm. This allows them to attack non-native disulfides, a necessary step in the isomerization reaction. The protein that keeps DsbC reduced is the inner-membrane protein DsbD. DsbD transfers reducing equivalents from the cytoplasmic thioredoxin system to the periplasm via a succession of disulfide exchange reactions.
The E. coli periplasm contains another Dsb protein called DsbG. DsbG resembles DsbC and may be another protein disulfide isomerase. However, the role of this protein in the E. coli periplasm is still unclear.
In our lab, we are working on the characterization of the Dsb proteins by using a combination of biochemical, proteomics, structural biology and microbiological approaches. Our long-term goal is to have a detailed understanding of the role and function of every Dsb protein. This part of our research is carried on in the framework of the Brussels Center for Redox Biology.To know more... (pdf chapter of the last de Duve Institute report)