The sulfinic acid switch in proteinsElectronic supplementary information (ESI) available: Frontispiece: The sulfinic acid switch in Prx responds to elevated hydrogen peroxide concentrations, turning antioxidant catalysis into a signal for cell death. The recently discovered Srx rescue pathway provides a cellular mechanism to flip the switch back to ?catalysis?. The centrepiece is an X-ray crystal structure of the decameric form of human (2-Cys) peroxiredoxin at 1.7 ? resolution.9 The figure of the protein was produced using BOBSCRIPT.24 See http://www.rsc.org/suppdata/ob/b4/b406180b/