Protein redox regulation in the thylakoid lumen: The importance of disulfide bonds for violaxanthin de-epoxidase

•Violaxanthin de-epoxidase has 12 conserved cysteines all fundamental for activity.•VDE is active in an oxidized state with six disulfide bonds.•VDE reduction may switch off enzyme activity in low oxygen conditions. When exposed to saturating light conditions photosynthetic eukaryotes activate the xanthophyll cycle where the carotenoid violaxanthin is converted into zeaxanthin by the enzyme violaxanthin de-epoxidase (VDE). VDE protein sequence includes 13 cysteine residues, 12 of which are strongly conserved in both land plants and algae. Site directed mutagenesis of Arabidopsis thaliana VDE showed that all these 12 conserved cysteines have a major role in protein function and their mutation leads to a strong reduction of activity. VDE is also shown to be active in its completely oxidized form presenting six disulfide bonds. Redox titration showed that VDE activity is sensitive to variation in redox potential, suggesting the possibility that dithiol/disulfide exchange reactions may represent a mechanism for VDE regulation.