Oxidation-reduction properties of Escherichia coli thioredoxin reductase altered at each active site cysteine residue.

Thioredoxin is a small oxidation-reduction (redox) mediator protein. Its reduction by NADPH is catalyzed by the flavoenzyme thioredoxin reductase. Site-directed mutagenesis has provided forms of the reductase in which Cys135 and Cys138 have each been changed to a serine residue (Prongay, A. J., Engelke, D. R., and Williams, C. H., Jr. (1989) J. Biol. Chem. 264, 2656-2664). Cys135 and Cys138 form the redox-active disulfide in the oxidized enzyme. The redox properties of the two altered forms of Escherichia coli thioredoxin reductase have been determined from pH 6.0 to 9.0. Photoreduction of TRR(Ser135,Cys138) produces the blue, neutral semiquinone species, which disproportionates (Kf = 0.73) to an apparent maximum of 29% of the total enzyme as the semiquinone. In contrast, the semiquinone formed on TRR(Cys135,Ser138) during a photoreductive titration does not disproportionate and 70% of the enzyme is stabilized as the semiquinione. Reductive titrations have demonstrated that 1 mol of sodium dithionite (2 electrons)/mol of FAD is required to fully reduce TRR(Ser135,Cys138) whereas 2 mol of dithionite/mol of FAD are required to fully reduce TRR(Cys135,Ser138). The oxidation-reduction midpoint potentials for the 1-electron and 2-electron reductions of TRR(Ser135,Cys138) have been determined by NADH/NAD+ titrations in the presence of a mediator, benzyl viologen. The midpoint potential for the 2-electron reduction of TRR(Ser135,Cys138) is -280 mV, at pH 7.0 and 20 degrees C. Thus, the redox potential is similar to that of the FAD/FADH2 couple in the dithiol form of wild type enzyme, -270 mV (corrected to 20 degrees C) (O'Donnell, M. E., and Williams, C. H., Jr. (1983) J. Biol. Chem. 258, 13795-13805). The delta Em/delta pH is -57.1 mV, which corresponds to a proton stoichiometry of 2 H+/2 e-.A maximum of 19% of the enzyme forms a stable semiquinone species during the titration, and the potentials for the oxidized enzyme/semiquinone couple, E2, and the semiquinone/reduced enzyme couple, E1, are -306 and -256 mV, respectively, at pH 7.0 and 20 degrees C. These studies provide evidence that the residue at position 138 exerts a greater effect on the FAD than does the residue at position 135.