Localized redox relays as a privileged mode of cytoplasmic hydrogen peroxide signaling.

Hydrogen peroxide (HO) is a key signaling agent. Its best characterized signaling actions in mammalian cells involve the early oxidation of thiols in cytoplasmic phosphatases, kinases and transcription factors. However, these redox targets are orders of magnitude less HO-reactive and abundant than cytoplasmic peroxiredoxins. How can they be oxidized in a signaling time frame? Here we investigate this question using computational reaction-diffusion models of HO signaling. The results show that at HO supply rates commensurate with mitogenic signaling a HO concentration gradient with a length scale of a few tenths of μm is established. Even near the supply sites HO concentrations are far too low to oxidize typical targets in an early mitogenic signaling time frame. Furthermore, any inhibition of the peroxiredoxin or increase in HO supply able to drastically increase the local HO concentration would collapse the concentration gradient and/or cause an extensive oxidation of the peroxiredoxins I and II, inconsistent with experimental observations. In turn, the local concentrations of peroxiredoxin sulfenate and disulfide forms exceed those of HO by several orders of magnitude. Redox targets reacting with these forms at rate constants much lower than that for, say, thioredoxin could be oxidized within seconds. Moreover, the spatial distribution of the concentrations of these peroxiredoxin forms allows them to reach targets within 1 μm from the HO sites while maintaining signaling localized. The recruitment of peroxiredoxins to specific sites such as caveolae can dramatically increase the local concentrations of the sulfenic and disulfide forms, thus further helping these species to outcompete HO for the oxidation of redox targets. Altogether, these results suggest that HO signaling is mediated by localized redox relays whereby peroxiredoxins are oxidized to sulfenate and disulfide forms at HO supply sites and these forms in turn oxidize the redox targets near these sites.