H₂S Is a Potential Universal Reducing Agent for Prx6-Type Peroxiredoxins

The absence of a universal reducing agent distinguishes the Prx6-type subfamily of peroxiredoxins from the structurally similar Prx1-type subfamily. A likely explanation for the lack of reactivity of Prx6-type enzymes with common reducing agents is that a histidyl residue at the bottom of the active-site pocket traps the oxidized enzyme in an inaccessible fully-folded protein conformation. Here, we analyzed the reduction of oxidized PfPrx6 from Plasmodium falciparum and human PrxVI by the hydrosulfide ion, HS^-, as the smallest possible sulfur-containing universal electron donor. We show that HS^- rapidly reacts with oxidized wild-type PfPrx6 or human PrxVI (but not the histidyl mutants PfPrx6^H39Y or hPrxVI^H39Y) with a second-order rate constant of > 10⁸ m^‒1s^‒1 at pH 7.4. The obtained protein-hydropersulfide species is neither reduced by thioredoxin nor glutaredoxin and glutathione, but further reacts with an excess of HS^- with a second-order rate constant around 10⁴ m^‒1s^‒1, yielding the reduced enzyme. In summary, we identified HS^- as a highly reactive, potential universal electron donor for Prx6-type enzymes. This study marks the starting point for the characterization of the complex reduction pathway of Prx6-type enzymes with implications for H₂S detoxification and redox signaling as well as iron-sulfur and persulfide metabolism.