The influence of His94 and Pro149 in modulating the activity of V. cholerae DsbA

Structure and function of DsbA, a key bacterial oxidative folding catalyst

Since its discovery in 1991, the bacterial periplasmic oxidative folding catalyst DsbA has been the focus of intense research. Early studies addressed why it is so oxidizing and how it is maintained in its less stable oxidized state. The crystal structure of Escherichia coli DsbA (EcDsbA) revealed that the oxidizing periplasmic enzyme is a distant evolutionary cousin of the reducing cytoplasmic enzyme thioredoxin. Recent significant developments have deepened our understanding of DsbA function, mechanism, and interactions: the structure of the partner membrane protein EcDsbB, including its complex with EcDsbA, proved a landmark in the field. Studies of DsbA machineries from bacteria other than E. coli K-12 have highlighted dramatic differences from the model organism, including a striking divergence in redox parameters and surface features. Several DsbA structures have provided the first clues to its interaction with substrates, and finally, evidence for a central role of DsbA in bacterial virulence has been demonstrated in a range of organisms. Here, we review current knowledge on DsbA, a bacterial periplasmic protein that introduces disulfide bonds into diverse substrate proteins and which may one day be the target of a new class of anti-virulence drugs to treat bacterial infection.

Generation of monoclonal antibodies against Escherichia coli DsbA

Disulfide oxidoreductase A (also known as disulfide bond formation protein A, or DsbA) is produced in Escherichia coli (E. coli) periplasm, which plays an important role in correct formulation of disulfide bonds during protein exportation in vivo. DsbA prokaryotic expression vectors are designed for periplasmic co-expression of recombinant proteins with an improving secretion. In the present study, the first domain of human CD226 (CD226D1) was expressed as a His-tagged fusion protein with DsbA and purified by Ni-NTA resin. Three monoclonal antibodies (MAbs) against DsbA were raised by using the recombinant fusion protein as immunogen. We have demonstrated that one of them can detect DsbA via Western blotting, in addition to its ability to immunoprecipitate DsbA fusion protein. Furthermore, anti-DsbA MAb can be employed to prepare antibody-coupled affinity column for purification of DsbA fusion protein from E. coli lysate.

Thioredoxins and glutaredoxins as facilitators of protein folding

Thiol-disulfide oxidoreductase systems of bacterial cytoplasm and eukaryotic cytosol favor reducing conditions and protein thiol groups, while bacterial periplasm and eukaryotic endoplasmatic reticulum provide oxidizing conditions and a machinery for disulfide bond formation in the secretory pathway. Oxidoreductases of the thioredoxin fold superfamily catalyze steps in oxidative protein folding via protein-protein interactions and covalent catalysis to act as chaperones and isomerases of disulfides to generate a native fold. The active site dithiol/disulfide of thioredoxin fold proteins is CXXC where variations of the residues inside the disulfide ring are known to increase the redox potential like in protein disulfide isomerases. In the catalytic mechanism thioredoxin fold proteins bind to target proteins through conserved backbone-backbone hydrogen bonds and induce conformational changes of the target disulfide followed by nucleophilic attack by the N-terminally located low pK(a) Cys residue. This generates a mixed disulfide covalent bond which subsequently is resolved by attack from the C-terminally located Cys residue. This review will focus on two members of the thioredoxin superfamily of proteins known to be crucial for maintaining a reduced intracellular redox state, thioredoxin and glutaredoxin, and their potential functions as facilitators and regulators of protein folding and chaperone activity.

Mutational alterations of the key cis proline residue that cause accumulation of enzymatic reaction intermediates of DsbA, a member of the thioredoxin superfamily

The DsbA-DsbB pathway introduces disulfide bonds into newly translocated proteins. Conversion of the conserved cis proline 151 of DsbA to several hydrophilic residues results in accumulation of mixed disulfides between DsbA and its dedicated oxidant, DsbB. However, only a proline-to-threonine change causes accumulation of mixed disulfides of DsbA with its substrates.

Molecular Characterization of the Principal Substrate Binding Site of the Ubiquitous Folding Catalyst Protein Disulfide Isomerase

Disulfide bond formation in the endoplasmic reticulum of eukaryotes is catalyzed by the ubiquitously expressed enzyme protein disulfide isomerase (PDI). The effectiveness of PDI as a catalyst of native disulfide bond formation in folding polypeptides depends on the ability to catalyze disulfide-dithiol exchange, to bind non-native proteins, and to trigger conformational changes in the bound substrate, allowing access to buried cysteine residues. It is known that the b' domain of PDI provides the principal peptide binding site of PDI and that this domain is critical for catalysis of isomerization but not oxidation reactions in protein substrates. Here we use homology modeling to define more precisely the boundaries of the b' domain and show the existence of an intradomain linker between the b' and a' domains. We have expressed the recombinant b' domain thus defined; the stability and conformational properties of the recombinant product confirm the validity of the domain boundaries. We have modeled the tertiary structure of the b' domain and identified the primary substrate binding site within it. Mutations within this site, expressed both in the isolated domain and in full-length PDI, greatly reduce the binding affinity for small peptide substrates, with the greatest effect being I272W, a mutation that appears to have no structural effect.