Redox signaling includes modification of amino acids via reactions with nitric oxide and oxidation of cystein residues (Hancock 2009). Such oxidative reactions depend on the redox status of the intracellular environment in which the protein resides and require the presence of a ferredoxin/thioredoxin system. Target enzymes of such a system have been identified in both chloroplast stroma and the thylakoid lumen, but it appears that a reduced and an oxidized form of enzymes are active, respectively, in the two compartments (Buchanan and Luan 2005). As a lumenal example, the case of FKBP13 has been described in Sect. 3.7.
C-type cytochrome thiol disulfide transporter CcdA is a thylakoid-specific membrane transfer system mediating disulfide bond reduction of proteins in the chloroplast thylakoid lumen (Motohashi and Hisabori 2010). More specifically, CcdA supplies reducing equivalents from the stroma to HCF164 (Sect. 3.6), considered to be a lumenal thioredoxin-like protein. Interestingly, 19 thioredoxin targets have been found using proteomics in the Arabidopsis thylakoid lumen (Hall et al. 2010). Among these targets are the two PsbOs and VDE. It has, therefore, been proposed that PsbO1 and PsbO2 may be degraded in a redox-dependent manner and that the VDE activity could be dependent on the thiol redox state of the enzyme.
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