The rate-limiting step of sulfiredoxin is associated with the transfer of the γ-phosphate of ATP to the sulfinic acid of overoxidized typical 2-Cys peroxiredoxins.

The eukaryotic sulfiredoxin (Srx) catalyzes the reduction of overoxidized typical 2-Cys peroxiredoxins PrxSO(2) via ATP/Mg(2+)-dependent phosphorylation of the sulfinic acid group, followed by formation of a PrxSO-SSrx thiolsulfinate intermediate. Using real-time kinetics of wild-type and C84A Srxs, and pH-rate profiles with ATP/Mg(2+) analogues, we show that the rate-limiting step of the reaction is associated with the chemical process of transfer of the γ-phosphate of ATP to the sulfinic acid, in contrast to that described by Jönsson et al. Two pK(apps) of 6.2 and 7.5 were extracted from the bell-shaped pH-rate profile, corresponding to the γ-phosphate of ATP, and to an acid-base catalyst, respectively.