Glutaredoxin S12: Unique Properties for Redox Signaling

Aims: Cysteines (Cys) made acidic by the protein environment are generally sensitive to pro-oxidant molecules. Glutathionylation is a post-translational modification that can occur by spontaneous reaction of reduced glutathione (GSH) with oxidized Cys as sulfenic acids (-SOH). The reverse reaction (deglutathionylation) is strongly stimulated by glutaredoxins (Grx) and requires a reductant, often GSH. Results: Here, we show that chloroplast GrxS12 from poplar efficiently reacts with glutathionylated substrates in a GSH-dependent ping pong mechanism. The pK(a) of GrxS12 catalytic Cys is very low (3.9) and makes GrxS12 itself sensitive to oxidation by H2O2 and to direct glutathionylation by nitrosoglutathione. Glutathionylated-GrxS12 (GrxS12-SSG) is temporarily inactive until it is deglutathionylated by GSH. The equilibrium between GrxS12 and glutathione (Em(GrxS12-SSG) = -315 mV, pH 7.0) is characterized by K-ox values of 310 at pH 7.0, as in darkened chloroplasts, and 69 at pH 7.9, as in illuminated chloroplasts. Innovation: Based on thermodynamic data, GrxS12-SSG is predicted to accumulate in vivo under conditions of mild oxidation of the GSH pool that may occur under stress. Moreover, GrxS12-SSG is predicted to be more stable in chloroplasts in the dark than in the light. Conclusion: These peculiar catalytic and thermodynamic properties could allow GrxS12 to act as a stress-related redox sensor, thus allowing glutathione to play a signaling role through glutathionylation of GrxS12 target proteins. Antioxid. Redox Signal. 16, 17-32.