Hyperoxidation of peroxiredoxins 2 and 3: rate constants for the reactions of the sulfenic acid of the peroxidatic cysteine.

Typical 2-Cys peroxiredoxins (Prxs) react rapidly with H2O2 to form a sulfenic acid, which then condenses with the resolving cysteine of the adjacent Prx in the homodimer or reacts with another H2O2 to become hyperoxidized. Hyperoxidation inactivates the Prx and is implicated in cell signaling. Prxs vary in susceptibility to hyperoxidation. We determined rate constants for disulfide formation and hyperoxidation for human recombinant Prx2 and Prx3 by analyzing the relative proportions of hyperoxidized and dimeric products using mass spectrometry as a function of H2O2 concentration (in the absence of reductive cycling) and in competition with catalase at a fixed concentration of H2O2. This gave a second order rate constant for hyperoxidation of 12,000 M(-1) s(-1) and a rate constant for disulfide formation of 2 s(-1) for Prx2. A similar hyperoxidation rate constant for Prx3 was measured, but its rate of disulfide formation was ~10-fold higher, making it is more resistant than Prx2 to hyperoxidation. There are two active sites within the homodimer, and at low H2O2 concentrations one site was hyperoxidized and the other present as a disulfide. Prx with two hyperoxidized sites formed progressively at higher H2O2 concentrations. Although the sulfenic acid forms of Prx2 and Prx3 are ~1000-fold less reactive with H2O2 than their active site thiols, they react several orders of magnitude faster than most reduced thiol proteins. This observation has important implications for understanding the mechanism of peroxide sensing in cells.