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5.A.1 The Disufide Bond Oxidoreductase D (DsbD) Family

The best characterized member of the DsbD family is DsbD of E. coli (Katzen and Beckwith, 2000; Krupp et al., 2001). The DsbD protein is membrane-embedded with a putative N-terminal TMS plus 8 additional TMSs. The smallest homologues (190 aas with 6 putative TMSs) are found in archaea while the largest are found in both Gram-negative bacteria (758 aas with 9 putative TMSs) and Gram-positive bacteria (695 aas with 6 putative TMSs). 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 (TrxB), present in the cytoplasm, thioredoxin (TrxA), also in the cytoplasm, DsbD, the integral membrane constituent of the system, and the periplasmic electron acceptors, DsbC, DsbE (CcmG) and DsbG. 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 → DsbC, DsbE, or DsbG → proteins.

Homologues include: (1) several thiol-disufide exchange proteins, (2) the cytochrome c-type biogenesis proteins, CcdA of Paracoccus pantotrophus and Bacillus subtilis (Bardischewsky and Friedrich, 2001; Le Brun et al., 2000), (3) the methylamine utilization proteins, MauF of Paracoccus denitrificans and P. versutus (Chistoserdov et al., 1992; Van Spanning et al., 1994), (4) the mercury resistance proteins (possibly Hg2+ transporters) of Mycobacterium tuberculosis and Streptomyces lividans (Brunker et al., 1996; Sedlmeier and Altenbuchner, 1992), (5) suppressors of copper sensitivity (copper tolerance proteins) of Salmonella typhimurium and Vibrio cholerae (Choudhury and Kumar, 1996; Gupta et al., 1997), (6) coomponents of peroxide reduction pathways, and (7) components of sulfenic acid reductases.  

DsbD contains three domains, each containing two reactive cysteines. One membrane-embedded domain, DsbDγ, transfers electrons from thioredoxin to the carboxy-terminal thioredoxin-like periplasmic domain DsbDγ. Alanines were substituted for each of 19 conserved amino acid residues. 11 mutants caused defects in DsbC reduction. To analyze the redox state of each DsbD domain, a thrombin-cleavable DsbD (DsbDTH) was constructed from which all three domains as separate polypeptide chains were generated. Mutants with strong defects included one mutant class that could not receive electrons from cytoplasmic thioredoxin, resulting in a DsbD that has all six of its cysteines disulfide bonded. One mutant class could not transfer electrons from DsbDβ to DsbDγ (Cho and Beckwith, 2006).

DsbD contains three cysteine pairs that undergo reversible disulfide rearrangements (Krupps et al., 2001). TrxA donates electrons to the transmembrane cysteines C163 (C3) and C285 (C5) in putative TMSs 1 and 4 in the DsbD model proposed by Katzen and Beckwith (2000). This dithiol then donates electrons to the periplasmic C-terminal thioredoxin motif (CXXC) of DsbD, thereby reducing C461 and C464 (C6 and C7, respectively). This dithiol pair attacks the periplasmic N-terminal disulfide bridge at C103 and C109 (C1 and C2, respectively) which transfers electrons to DsbC and other protein electron acceptors as noted above.

DsbD catalyses an essentially irreversible reaction due to the fact that electrons flow down their electrochemical gradient from inside the cell (negative inside) to outside the cell (positive outside). 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 (e.g., O2, NO3- or fumarate).

DsbB contains 4 essential cysteine residues, reversibly forming two disulfide bonds. Although DsbA displays no proofreading activity for repair of wrongly paired disulfides, this activity is displayed by DsbC, DsbE and DsbG (Krupp et al., 2001). Therefore, the two transmembrane pathways involving DsbD and DsbB together catalyze extracellular disulfide reduction (DsbD) and oxidation (DsbB) in a superficially reversible process that allows dithiol/disulfide exchange.

A class of DsbD proteins, named ScsB, are found in proteobacteria and Chlamydia. ScsB has a domain organization similar to that of DsbD, but its amino-terminal domain differs significantly. In DsbD, this domain directly interacts with substrates to reduce them, which suggests that ScsB acts on a different array of substrates. Using Caulobacter crescentus as a model organism, Cho et al. 2012 searched for the substrates of ScsB. ScsB provides electrons to a peroxide reduction pathway in the bacterial cell envelope. The reduction pathway comprises a thioredoxin-like protein, TlpA, and a peroxiredoxin, PprX. PprX is a thiol-dependent peroxidase that efficiently reduces both hydrogen peroxide and organic peroxides. Additional proteins that depend on ScsB for reduction include a peroxiredoxin-like protein, PrxL, and a novel protein disulfide isomerase, ScsC.  Thus, the array of proteins involved in reductive pathways in the oxidative cell envelope is broad.

The overall vectorial electron transfer reaction catalyzed by DsbD is:

2 e-cytoplasm → 2 e-periplasm

 

This family belongs to the: LysE Superfamily.

References associated with 5.A.1 family:

Appia-Ayme, C. and B.C. Berks. (2002). SoxV, an orthologue of the CcdA disulfide transporter, is involved in thiosulfate oxidation in Rhodovulum sulfidophilum and reduces the periplasmic thioredoxin SoxW. Biochem. Biophys. Res. Commun. 296: 737-741. 12176044
Bardischewsky, F. and C.G. Friedrich. (2001). Identification of CcdA in Paracoccus pantotrophus GB17: disruption of ccdA causes complete deficiency in c-type cytochromes. J. Bacteriol. 183: 257-263. 11114924
Brunker, P., D. Rother, R. Sedlmeier, J. Klein, R. Mattes, and J. Altenbuchner. (1996). Regulation of the operon responsible for broad-spectrum mercury resistance in Streptomyces lividans 1326. Mol. Genet. 251: 307-315. 8676873
Chistoserdov, A.Y., J. Boyd, F.S. Mathews, and M.E. Lidstrom. (1992). The genetic organization of the mau gene cluster of the facultative autotroph Paracoccus denitrificans. Biochem. Biophys. Res. Commun. 184: 1181-1189. 1590782
Cho, S.H., and J. Beckwith. (2006). Mutations of the Membrane-Bound Disulfide Reductase DsbD That Block Electron Transfer Steps from Cytoplasm to Periplasm in Escherichia coli. J. Bacteriol. 188: 5066-5076. 16816179
Cho, S.H., D. Parsonage, C. Thurston, R.J. Dutton, L.B. Poole, J.F. Collet, and J. Beckwith. (2012). A new family of membrane electron transporters and its substrates, including a new cell envelope peroxiredoxin, reveal a broadened reductive capacity of the oxidative bacterial cell envelope. MBio 3:. 22493033
Choudhury, P. and R. Kumar. (1996). Association of metal tolerance with multiple antibiotic resistance of enteropathogenic organisms isolated from coastal region of deltaic Sunderbans. Indian J. Med. Res. 104: 148-151. 8783519
Collet, J.-F. and J.C.A. Bardwell. (2002). Oxidative protein folding in bacteria. Mol. Microbiol. 44: 1-8. 11967064
Gupta, S.D., H.C. Wu, and P.D. Rick. (1997). A Salmonella typhimurium genetic locus which confers copper tolerance on copper-sensitive mutants of Escherichia coli. J. Bacteriol. 179: 4977-4984. 9260936
Katzen, F. and J. Beckwith. (2000). Transmembrane electron transfer by the membrane protein DsbD occurs via a disulfide bond cascade. Cell 103: 769-779. 11114333
Kimball, R.A., L. Martin, and M.H. Saier, Jr. (2003). Reversing transmembrane electron flow: The DsbD and DsbB protein families. J. Mol. Microbiol. Biotechnol. 5: 133-149. 12766342
Krupp, R., C. Chan, and D. Missiakas. (2001). DsbD-catalyzed transport of electrons across the membrane of Escherichia coli. J. Biol. Chem. 276: 3696-3701. 11085993
Le Brun, N.E., J. Bengtsson, and L. Hederstedt. (2000). Genes required for cytochrome c synthesis in Bacillus subtilis. Mol. Microbiol. 36: 638-650. 10844653
Sedlmeier, R. and J. Altenbuchner. (1992). Cloning and DNA sequence analysis of the mercury resistance genes of Streptomyces lividans. Mol. Gen. Genet. 236: 76-85. 1494353
Van Spanning, R.J., C.J. van der Palen, D.J. Slotboom, W.N. Reijnders, A.H. Stouthamer, and J.A. Duine. (1994). Expression of the mau genes involved in methylamine metabolism in Paracoccus denitrificans is under control of a LysR-type transcriptional activator. Eur. J. Biochem. 226: 201-210. 7957249