Roles of a conserved arginine residue of DsbB in linking protein disulfide-bond-formation pathway to the respiratory chain of Escherichia coli

Formation and transfer of disulphide bonds in living cells

Protein disulphide bonds are formed in the endoplasmic reticulum of eukaryotic cells and the periplasmic space of prokaryotic cells. The main pathways that catalyse the formation of protein disulphide bonds in prokaryotes and eukaryotes are remarkably similar, and they share several mechanistic features. The recent identification of new redox-active proteins in humans and yeast that mechanistically parallel the more established redox-active enzymes indicates that there might be further uncharacterized redox pathways throughout the cell.

DsbB catalyzes disulfide bond formation de novo

DsbA and DsbB are responsible for disulfide bond formation. DsbA is the direct donor of disulfides, and DsbB oxidizes DsbA. DsbB has the unique ability to generate disulfides by quinone reduction. It is thought that DsbB oxidizes DsbA via thiol disulfide exchange. In this mechanism, a disulfide is formed across the N-terminal pair of cysteines (Cys-41/Cys-44) in DsbB by quinone reduction. This disulfide is then transferred on to the second pair of cysteine residues in DsbB (Cys-104/Cys-130) and then finally transferred to DsbA. We have shown here the redox potential of the two disulfides in DsbB are -271 and -284 mV, respectively, and considerably less oxidizing than the disulfide of DsbA at -120 mV. In addition, we have found the Cys-104/Cys-130 disulfide of DsbB to actually be a substrate for DsbA in vitro. These findings indicate that the disulfides in DsbB are unsuitable to function as the oxidant of DsbA. Furthermore, we have shown that mutants in DsbB that lack either pair or all of its cysteines are also capable of oxidizing DsbA. These unexpected findings raise the possibility that the oxidation of DsbA by DsbB does not occur via thiol disulfide exchange as is widely assumed but rather, directly via quinone reduction.

Paradoxical redox properties of DsbB and DsbA in the protein disulfide-introducing reaction cascade

Protein disulfide bond formation in the bacterial periplasm is catalyzed by the Dsb enzymes in conjunction with the respiratory quinone components. Here we characterized redox properties of the redox active sites in DsbB to gain further insights into the catalytic mechanisms of DsbA oxidation. The standard redox potential of DsbB was determined to be -0.21 V for Cys41/Cys44 in the N-terminal periplasmic region (P1) and -0.25 V for Cys104/Cys130 in the C-terminal periplasmic region (P2), while that of Cys30/Cys33 in DsbA was -0.12 V. To our surprise, DsbB, an oxidant for DsbA, is intrinsically more reducing than DsbA. Ubiquinone anomalously affected the apparent redox property of the P1 domain, and mutational alterations of the P1 region significantly lowered the catalytic turnover. It is inferred that ubiquinone, a high redox potential compound, drives the electron flow by interacting with the P1 region with the Cys41/Cys44 active site. Thus, DsbB can mediate electron flow from DsbA to ubiquinone irrespective of the intrinsic redox potential of the Cys residues involved.

Four cysteines of the membrane protein DsbB act in concert to oxidize its substrate DsbA

Protein disulfide bond formation in Escherichia coli is catalyzed by the periplasmic protein DsbA. A cytoplasmic membrane protein DsbB maintains DsbA in the oxidized state by transferring electrons from DsbA to quinones in the respiratory chain. Here we show that DsbB activity can be reconstituted by co-expression of N- and C-terminal fragments of the protein, each containing one of its redox-active disulfide bonds. This system has allowed us (i) to demonstrate that the two DsbB redox centers interact directly through a disulfide bond formed between the two DsbB domains and (ii) to identify the specific cysteine residues involved in this covalent interaction. Moreover, we are able to capture an intermediate in the process of electron transfer from one redox center to the other. These results lead us to propose a model that describes how the cysteines cooperate in the early stages of oxidation of DsbA. DsbB appears to adopt a novel mechanism to oxidize DsbA, using its two pairs of cysteines in a coordinated reaction to accept electrons from the active cysteines in DsbA.

Oxidative protein folding in bacteria

Ten years ago it was thought that disulphide bond formation in prokaryotes occurred spontaneously. Now two pathways involved in disulphide bond formation have been well characterized, the oxidative pathway, which is responsible for the formation of disulphides, and the isomerization pathway, which shuffles incorrectly formed disulphides. Disulphide bonds are donated directly to unfolded polypeptides by the DsbA protein; DsbA is reoxidized by DsbB. DsbB generates disulphides de novo from oxidized quinones. These quinones are reoxidized by the electron transport chain, showing that disulphide bond formation is actually driven by electron transport. Disulphide isomerization requires that incorrect disulphides be attacked using a reduced catalyst, followed by the redonation of the disulphide, allowing alternative disulphide pairing. Two isomerases exist in Escherichia coli, DsbC and DsbG. The membrane protein DsbD maintains these disulphide isomerases in their reduced and thereby active form. DsbD is kept reduced by cytosolic thioredoxin in an NADPH-dependent reaction.

Mutations in the thiol-disulfide oxidoreductases BdbC and BdbD can suppress cytochrome c deficiency of CcdA-defective Bacillus subtilis cells

Cytochromes of the c type in the gram-positive bacterium Bacillus subtilis are all membrane anchored, with their heme domains exposed on the outer side of the cytoplasmic membrane. They are distinguished from other cytochromes by having heme covalently attached by two thioether bonds. The cysteinyls in the heme-binding site (CXXCH) in apocytochrome c must be reduced in order for the covalent attachment of the heme to occur. It has been proposed that CcdA, a membrane protein, transfers reducing equivalents from thioredoxin in the cytoplasm to proteins on the outer side of the cytoplasmic membrane. Strains deficient in the CcdA protein are defective in cytochrome c and spore synthesis. We have discovered that mutations in the bdbC and bdbD genes can suppress the defects caused by lack of CcdA. BdbC and BdbD are thiol-disulfide oxidoreductases. Our experimental findings indicate that these B. subtilis proteins functionally correspond to the well-characterized Escherichia coli DsbB and DsbA proteins, which catalyze the formation of disulfide bonds in proteins in the periplasmic space.

Identification of the ubiquinone-binding domain in the disulfide catalyst disulfide bond protein B

Disulfide bond (Dsb) formation is catalyzed in the periplasm of prokaryotes by the Dsb proteins. DsbB, a key enzyme in this process, generates disulfides de novo by using the oxidizing power of quinones. To explore the mechanism of this newly described enzymatic activity, we decided to study the ubiquinone-protein interaction and identify the ubiquinone-binding domain in DsbB by cross-linking to photoactivatable quinone analogues. When purified Escherichia coli DsbB was incubated with an azidoubiquinone derivative, 3-azido-2-methyl-5-[(3)H]methoxy-6-decyl-1,4-benzoquinone ([(3)H]azido-Q), and illuminated with long wavelength UV light, the decrease in enzymatic activity correlated with the amount of 3-azido-2-methyl-5-methoxy-6-decyl-1,4-benzoquinone (azido-Q) incorporated into the protein. One azido-Q-linked peptide with a retention time of 33.5 min was obtained by high performance liquid chromatography of the V8 digest of [(3)H]azido-Q-labeled DsbB. This peptide has a partial NH(2)-terminal amino acid sequence of NH(2)-HTMLQLY corresponding to residues 91-97. This sequence occurs in the second periplasmic domain of the inner membrane protein DsbB in a loop connecting transmembrane helices 3 and 4. We propose that the quinone-binding site is within or very near to this sequence.

Roles of thiol-redox pathways in bacteria

Disulfide bonds in proteins play various important roles. They are either formed as structural features to stabilize the protein or are found only transiently as part of a catalytic or regulatory cycle. In vivo, the formation and reduction of disulfide bonds is catalyzed by specialized thiol-disulfide exchanging enzymes that contain an active site with the sequence motif Cys-X-X-Cys. These proteins have structurally evolved to catalyze predominantly either oxidative reactions or reductive steps. There is mounting evidence that, in addition to the thiol redox potential, the spatial distribution within different cell compartments and the overall redox state of the cell are equally important. In the cytoplasm, multiple pathways play overlapping roles in the reduction of disulfide bonds and additionally, the expression of several components of thiol-redox pathways was shown to respond to the changes in the cellular thiol-redox equilibrium. In the periplasm, two systems coexist, one catalyzing thiol oxidation and the other disulfide reduction. Recent results suggest that two different mechanisms are used to translocate reducing power from the cytoplasm or to dissipate the electrons after oxidation.