The Peroxiredoxin/Thioredoxin/Thioredoxin-Reductase/Protein-Dithiol System (PTTRDS) is ubiquitous in Nature, but its functions remain unclear. Peroxiredoxins are very abundant proteins in many cells. They reduce hydrogen peroxide (H₂O₂) using reducing equivalents from NADPH, which are conveyed through thioredoxin and thioredoxin reductase. However, H₂O₂ can be more effectively eliminated through dismutation via catalase without spending metabolic reducing equivalents. This fact suggests that the abundance of Prx and ubiquity of the PTTRDS is driven by other functional requirements. Cells are occasionally exposed to high H₂O₂ concentrations, and these events often precede exposure to other electrophylic compounds. Both H₂O₂ and these compounds can irreversibly modify protein thiol groups, potentially causing deleterious loss of function and protein aggregation. The elimination of those reactive compounds is metabolically costly and requires a substantial investment in the protein defenses. Further, the transcriptional modulation of these defenses is too slow to avoid significant damage between the onset of stress and the full deployment of the defenses. We hypothesize that cells solve this conundrum by “blocking” the thiols through reversible covalent modification once H₂O₂ concentrations begin to increase.  We term this protective mechanism “anticipatory blocking” because it acts in anticipation of irreversible damage through detection of early signs of impending stress. The same process can simultaneously accomplish H₂O₂-dependent signaling. We further hypothesize that the PTTRDS drives anticipatory blocking of protein dithiols as disulfides. Here we examined the design requirements for such a system to effectively integrate H₂O₂ signaling and anticipatory blocking, and we compared these requirements to the actual design in human erythrocytes. To that effect, we developed a minimal model of the PTTRDS and defined a set of quantitative performance criteria that embody the requirements for (a) efficient scavenging capacity, (b) low NADPH consumption, (c) effective signal propagation, and (d) effective anticipatory blocking control. We then sought the design principles (relationships among rate constants and species concentrations) that warrant satisfaction of all the criteria. These were as follows, for human erythrocytes: (i) the equilibrium constant for thiol-disulfide exchange between thioredoxin and the protein dithiol [T(SH)₂ + PSS à TSS + P(SH)₂] should be in the range 0.01<K<20 to allow the protein to fully accumulate in the oxidized form as soon as the H₂O₂ concentration increases; (ii) the maximum flux of thioredoxin reduction must be lower than the maximum flux of peroxiredoxin disulfide reduction and formation. Additionally, we identified a trade-off between the robustness of signal propagation and the NADPH expenditure in the process. Human erythrocytes have a limited capacity for NADPH regeneration, and should thus sacrifice the former performance criteria to some extent in order to save NADPH. A comparison of experimental data to the theoretical predictions above indicates that the design of the PTTRDS in human erythrocytes accomplishes effective integration between anticipatory blocking, antioxidant protection and redox signaling.

We acknowledge fellowship SFRH/BPD/90065/2012 and grants PEst-C/SAU/LA0001/2013-2014, PEst-OE/QUI/UI0612/2013, FCOMP-01-0124-FEDER-020978 financed by FEDER through the “Programa Operacional Factores de Competitividade, COMPETE” and by national funds through “FCT, Fundação para a Ciência e a Tecnologia” (project PTDC/QUI-BIQ/119657/2010).