A single dipeptide sequence modulates the redox properties of a whole enzyme family.
BACKGROUND
Disulfide exchange reactions are catalyzed by thiol/disulfide oxidoreductases. These enzymes possess a thioredoxin fold and contain a catalytic disulfide with the sequence Cys-X-X-Cys at the N terminus of an alpha helix. Despite these similarities, the various members differ strongly in their redox potentials (-122 mV to -270 mV). Using the strong oxidant DsbA from Escherichia coli as a model system, we investigated whether the redox properties of these enzymes can be modulated rationally by exchange of the X-X dipeptide.


RESULTS
The X-X dipeptide of DsbA (Cys30-Pro31-His32-Cys33) was exchanged by the dipeptides of eukaryotic protein disulfide isomerase (PDI; Gly-His), glutaredoxin (Pro-Tyr), and thioredoxin (Gly-Pro) from E. coli. All variants were less oxidizing than wild-type DsbA and their redox potentials were in the order of the related natural enzymes (DsbA > PDI > glutaredoxin > thioredoxin). The equilibrium constant between glutathione and the thioredoxin-like variant increased 1200-fold compared with wild-type DsbA. The variants also showed a strong increase in the pKa of the nucleophilic cysteine (Cys30). As for glutaredoxin and thioredoxin, the catalytic disulfide stabilized the corresponding variants while destabilizing wild-type DsbA and the PDI-like variant.


CONCLUSIONS
The X-X dipeptide in the active site of thiol/disulfide oxidoreductases appears to be the main determinant of the redox properties of these enzymes. This empirical finding should be very useful for the design of new thiol/disulfide oxidoreductases with altered redox potentials and for studying the function of these enzymes in vivo.
Huber-Wunderlich
M
M
Institut für Molekularbiologie und Biophysik, Eidgenössische Technische Hochschule, Hönggerberg CH-8093, Zürich, Switzerland.
Glockshuber
R
R
D004151:Dipeptides; D004795:Enzyme Stability; D054477:Glutaredoxins; D016297:Mutagenesis, Site-Directed; D010084:Oxidation-Reduction; D010088:Oxidoreductases; D019704:Protein Disulfide-Isomerases / Q000235:genetics / Q000378:metabolism*; D011506:Proteins / Q000378:metabolism; D013379:Substrate Specificity; D019749:Sulfur-Sulfur Bond Isomerases / Q000235:genetics / Q000378:metabolism*; D013879:Thioredoxins / Q000378:metabolism
D004151:Dipeptides; D054477:Glutaredoxins; D011506:Proteins; D013879:Thioredoxins; D010088:Oxidoreductases; D019749:Sulfur-Sulfur Bond Isomerases; D019704:Protein Disulfide-Isomerases