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 Prx6-specific histidyl residue at the bottom of the active-site pocket traps the oxidized enzyme with a potentially hypervalent cysteinyl sulfur atom in an inaccessible fully-folded protein conformation. Here, we analyzed the reduction of oxidized PfPrx6 from the human malaria parasite Plasmodium falciparum and human PrxVI by the hydrosulfide ion, HS-, as the smallest possible sulfur-containing universal electron donor using stopped-flow kinetic measurements. We show that HS- rapidly reacts with oxidized wild-type PfPrx6 or human PrxVI (but not the histidyl mutant PfPrx6H39Y) with a second-order rate constant of >108 M-1s-1 at pH 7.4. The potential protein-persulfide species of PfPrx6 is neither reduced by thioredoxin nor glutaredoxin and glutathione, but further reacts with an excess of HS- with a second-order rate constant of 6.3x103 M-1s-1, yielding the reduced enzyme. In summary, we identified HS- as a highly reactive, potential universal electron donor for Prx6-type enzymes in eukaryotes, bacteria, and archaea. Our study marks the starting point for the characterization of the complex reduction pathway of Prx6-type enzymes with potential implications for H2S detoxification, redox signaling, and iron-sulfur metabolism.