Characterization of the glutathione-dependent reduction of the peroxiredoxin 5 homolog PfAOP from Plasmodium falciparum

Peroxiredoxins use a variety of thiols to rapidly reduce hydroperoxides and peroxynitrite. While the oxidation kinetics of peroxiredoxins have been studied in great detail, enzyme-specific differences regarding peroxiredoxin reduction and the overall rate-limiting step under physiological conditions often remain to be deciphered. The 1-Cys peroxiredoxin 5 homolog PfAOP from the malaria parasite Plasmodium falciparum is an established model enzyme for glutathione/glutaredoxin-dependent peroxiredoxins. Here, we reconstituted the catalytic cycle of PfAOP in vitro and analyzed the reaction between oxidized PfAOP and reduced glutathione (GSH) using molecular docking and stopped-flow measurements. Molecular docking revealed that oxidized PfAOP has to adopt a locally unfolded conformation to react with GSH. Furthermore, we determined a second-order rate constant of 6 x 10(5) M-1 s(-1) at 25 degrees C and thermodynamic activation parameters Delta H-double dagger, Delta S-double dagger, and Delta G(double dagger) of 39.8 kJ/mol, -0.8 J/mol, and 40.0 kJ/mol, respectively. The gain-of-function mutant PfAOP(L109M) had almost identical reaction parameters. Taking into account physiological hydroperoxide and GSH concentrations, we suggest (a) that the reaction between oxidized PfAOP and GSH might be even faster than the formation of the sulfenic acid in vivo, and (b) that conformational changes are likely rate limiting for PfAOP catalysis. In summary, we characterized and quantified the reaction between GSH and the model enzyme PfAOP, thus providing detailed insights regarding the reactivity of its sulfenic acid and the versatile chemistry of peroxiredoxins.