title: Characterization of Escherichia coli thioredoxin variants mimicking the active-sites of other thiol/disulfide oxidoreductases.
abstract: Thiol/disulfide oxidoreductases like thioredoxin, glutaredoxin, DsbA, or protein disulfide isomerase (PDI) share the thioredoxin fold and a catalytic disulfide bond with the sequence Cys-Xaa-Xaa-Cys (Xaa corresponds to any amino acid). Despite their structural similarities, the enzymes have very different redox properties, which is reflected by a 100,000-fold difference in the equilibrium constant (K(eq)) with glutathione between the most oxidizing member, DsbA, and the most reducing member, thioredoxin. Here we present a systematic study on a series of variants of thioredoxin from Escherichia coli, in which the Xaa-Xaa dipeptide was exchanged by that of glutaredoxin, PDI, and DsbA. Like the corresponding natural enzymes, all thioredoxin variants proved to be stronger oxidants than the wild-type, with the order wild-type < PDI-type < DsbA-type < glutaredoxin-type. The most oxidizing, glutaredoxin-like variant has a 420-fold decreased value of K(eq), corresponding to an increase in redox potential by 75 mV. While oxidized wild-type thioredoxin is more stable than the reduced form (delta deltaG(ox/red) = 16.9 kJ/mol), both redox forms have almost the same stability in the variants. The pH-dependence of the reactivity with the alkylating agent iodoacetamide proved to be the best method to determine the pKa value of thioredoxin's nucleophilic active-site thiol (Cys32). A pKa of 7.1 was measured for Cys32 in the reduced wild-type. All variants showed a lowered pKa of Cys32, with the lowest value of 5.9 for the glutaredoxin-like variant. A correlation of redox potential and the Cys32 pKa value could be established on a quantitative level. However, the predicted correlation between the measured delta deltaG(ox/red) values and Cys32 pKa values was only qualitative.
authors.affiliation: Institut für Molekularbiologie und Biophysik, Eidgenössische Technische Hochschule Hönggerberg, Zürich, Switzerland.
authors.forename: E
authors.initials: E
authors.lastname: Mössner
authors.forename: M
authors.initials: M
authors.lastname: Huber-Wunderlich
authors.forename: R
authors.initials: R
authors.lastname: Glockshuber
mesh_terms: D001665:Binding Sites; D002942:Circular Dichroism; D003545:Cysteine / Q000378:metabolism; D004926:Escherichia coli / Q000378:metabolism*; D054477:Glutaredoxins; D010084:Oxidation-Reduction; D010088:Oxidoreductases / Q000378:metabolism*; D011490:Protein Disulfide Reductase (Glutathione); D011994:Recombinant Proteins / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism; D013056:Spectrophotometry, Ultraviolet; D013816:Thermodynamics; D013879:Thioredoxins / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism*
chemical_list: D054477:Glutaredoxins; D011994:Recombinant Proteins; D013879:Thioredoxins; D010088:Oxidoreductases; D011490:Protein Disulfide Reductase (Glutathione); D003545:Cysteine