Direct identification of the primary nucleophile of thioredoxin f

Thioredoxin, by virtue of the proximal active-site sulfhydryls (Trp-Cys-Gly-Pro-Cys), catalyzes thiol-disulfide exchange with specific target enzymes. Considerable data (chemical modification, spectroscopic, and crystallographic) have implicated the cysteinyl residue nearest the N terminus of thioredoxin as the primary nucleophile; however, direct proof has been lacking. Proof is now provided by characterization of site-directed mutants of thioredoxin f with respect to activation of chloroplastic fructose-1,6-bisphosphatase (FBPase). The C49S mutant retains the capacity to activate FBPase, whereas the C46S mutant is totally lacking in this regard. Based on kinetics of FBPase activation, wild-type and C49S thioredoxins exhibit half-saturation values of 0.9 and 4 microM, respectively. Lack of activation by C46S is not because of failure to interact with FBPase, for it exhibits a Ki of 5 microM in competition with wild-type thioredoxin. Therefore, in the normal thioredoxin-catalyzed reduction pathway, Cys-46 is the nucleophile required to attack the disulfide of the substrate and Cys-49 serves to cleave the mixed disulfide intermediate, thus allowing for the release of oxidized thioredoxin and the reduced target enzyme.