MIXED DISULFIDE INTERMEDIATES DURING THE REDUCTION OF DISULFIDES BY ESCHERICHIA-COLI THIOREDOXIN

The reduction of disulfides by thioredoxin involves a two-step mechanism. The first step features an intermolecular attack of Cys32 of thioredoxin on the disulfide with formation of a protein mixed disulfide and release of 1 equiv of thiol, The second step involves intramolecular breakdown of the mixed disulfide intermediate via attack of Cys35 with concomitant formation of the oxidized protein and release of a second equivalent of thiol. Study of mixed disulfide intermediates for Escherichia coli thioredoxin is exceedingly difficult because the second step is highly favorable. We have studied these intermediates via two approaches. First, Cys35 can be mutated to the similar but chemically nonreactive residue serine, This precludes breakdown of the intermediate. Second, ''mass action trapping'' techniques can be used because the second step of the mechanism is first-order in the forward direction and second-order in the reverse direction. This has yielded a thermodynamic breakdown of the reaction into its two component steps. Results for reaction of thioredoxin and 2-hydroxyethyl disulfide indicate that about half of the free energy change for the entire process is associated with the first step. Comparison with a small molecule cysteine analog suggests that significant interactions stabilize the mixed disulfide intermediate. Two-dimensional NMR analysis of the C35S thioredoxin 32C-beta-mercaptoethanol mixed disulfide shows packing interactions between the mixed disulfide moiety and Trp31 and Ile75. Additionally, studies with C35S thioredoxin show that substitution of the cysteine residue slightly perturbs the equilibrium for the first step in the reaction. Analysis using P34S thioredoxin, a mutant with a drastically higher reduction potential, indicates that most, although not all, of the change is affected in the second step of the reaction. This type of analysis should be useful in structure-function studies for thioredoxin and other oxidoreductases in the thioredoxin superfamily.