So-called 1-Cys peroxiredoxins (Prx) employ only one cysteine residue for the reduction of hydroperoxides and require an external thiol for the reduction of a reactive sulfenic acid during the catalytic cycle. Hence, 1-Cys Prx, which often belong to the structural Prx5-or the Prx6-type subfamily, are potentially promiscuous enzymes that could react with a variety of thiols. Furthermore, the dependence on an external thiol could affect the susceptibility of 1-Cys Prx to hyperoxidation, i.e., the formation of a sulfinic or sulfonic acid. Here, we compared the reaction mechanisms and kinetics of the Prx5-and Prx6-type enzymes PfAOP and PfPrx6 from the malaria parasite Plasmodium falciparum to address the hyperoxidation susceptibility and potential substrate promiscuity of 1-Cys Prx. While PfAOP did not react with common thiol-disulfide oxidoreductases, the enzyme turned out to be promiscuous regarding the reduction by synthesized glutathione analogues and other low-molecular-weight thiols. Furthermore, we established a complete single turnover experiment for PfAOP with glutathione and the glutaredoxin PfGrx and identified the rapid H 2 O 2 -dependent hyperoxidation of PfAOP as the cause for the apparent preference of this Prx5-type enzyme for alkylhydroperoxides in vitro. Unlike promiscuous PfAOP, PfPrx6 was inactive with ascorbate, the physiological low-molecular-weight thiols glutathione, cysteine, cysteamine, coenzyme A, and dihydrolipoamide, as well as physiological protein thiols, including PfTrx1, PfGrx, and the resolving cysteine of the Prx1-type enzyme PfPrx1a in potential hetero-oligomers. Reduction of PfPrx6 was only observed with dithiothreitol and required the presence of a histidine residue, which protects the enzyme from hyperoxidation and is the major structural difference between the active sites of Prx5-and Prx6-type enzymes. We propose two alternative evolutionary adaptations of the 1-Cys Prx mechanism to hyperoxidation and the formation of alternative mixed disulfides that could explain the co-existence of promiscuous Prx5-and protected Prx6-type enzymes in a variety of organisms and subcellular compartments.

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