Disulfide oxidoreductase D
The Disulfide bond oxidoreductase D family is a member of the Lysine Exporter (LysE) Superfamily. A representative list of proteins belonging to the DsbD family can be found in the .
Homology
Homologues include:several thiol-disulfide exchange proteins
the cytochrome c-type biogenesis proteins, CcdA of Paracoccus pantotrophus and Bacillus subtilis.
the methylamine utilization proteins, MauF of Paracoccus denitrificans and P. versutus.
the mercury resistance proteins of Mycobacterium tuberculosis and Streptomyces lividans.
suppressors of copper sensitivity of Salmonella typhimurium and Vibrio cholerae.
components of peroxide reduction pathways, and
components of sulfenic acid reductases.
Disulfide bond oxidoreductase D (DsbD)
The best characterized member of the DsbD family is DsbD of E. coli. The DsbD protein is membrane-embedded with a putative N-terminal transmembrane segment plus 8 additionalTMSs. The smallest homologues are found in archaea, while the largest are found in both Gram-negative bacteria and Gram-positive bacteria.The overall vectorial electron transfer reaction catalyzed by DsbD is:
2 e → 2 e
Structure
DsbB contains 4 essential cysteine residues, reversibly forming two disulfide bonds. Although DsbA displays no proofreading activity for repair of wrongly paired disulfides, DsbC, DsbE and DsbG have been found to demonstrate proofreading activity. Therefore, the two transmembrane pathways involving DsbD and DsbB together catalyze extracellular disulfide reduction and oxidation in a superficially reversible process that allows dithiol/disulfide exchange.System reduction pathway
In the E. coli DsbD system, electrons are transferred from NADPH in the cytoplasm to periplasmic dithiol/disulfide-containing proteins via an electron transfer chain that sequentially involves NADPH, thioredoxin reductase, thioredoxin, DsbD, and the periplasmic electron acceptors.All of these last three proteins can donate electrons to oxidized disulfide-containing proteins in the periplasm of a Gram-negative bacterium or presumably in the external milieu of a Gram-positive bacterium or an archaeon.
Thus, the pathway is:
NADPH → TrxB → TrxA → DsbD → → proteins.
DsbD contains three cysteine pairs that undergo reversible disulfide rearrangements. TrxA donates electrons to the transmembrane cysteines C163 and C285 in putative TMSs 1 and 4 in the DsbD model proposed by Katzen and Beckwith. This dithiol then donates electrons to the periplasmic C-terminal thioredoxin motif of DsbD, thereby reducing C461 and C464. This dithiol pair attacks the periplasmic N-terminal disulfide bridge at C103 and C109 which transfers electrons to DsbC and other protein electron acceptors as noted above.
Reverse pathway
DsbD catalyses an essentially irreversible reaction due to the fact that electrons flow down their electrochemical gradient from inside the cell to outside the cell. In order to reverse the reaction, electrons are transferred from dithiol proteins in the periplasm to an electron acceptor in the cytoplasm as follows:reduced proteinperiplasm → DsbAperiplasm → DsbBmembrane → quinonesmembrane → reductasemembrane→ terminal electron acceptorcytoplasm.