Plants contain both cytosolic and chloroplastic GAPDHs (glyceraldehyde-3-phosphate dehydrogenases). In Arabidopsis thaliana, cytosolic GAPDH is involved in the glycolytic pathway and is represented by two differentially expressed isoforms (GapC1 and GapC2) that are 98% identical in amino acid sequence. In the present study we show that GapC1 is a phosphorylating NAD-specific GAPDH with enzymatic activity strictly dependent on Cys149. Catalytic Cys149 is the only solvent-exposed cysteine of the protein and its thiol is relatively acidic (pKa=5.7). This property makes GapC1 sensitive to oxidation by H2O2, which appears to inhibit enzyme activity by converting the thiolate of Cys149 (–S) into irreversible oxidized forms (–SO2 and –SO3) via a labile sulfenate intermediate (–SO). GSH (reduced glutathione) prevents this irreversible process by reacting with Cys149 sulfenates to give rise to a mixed disulfide (Cys149–SSG), as demonstrated by both MS and biotinylated GSH. Glutathionylated GapC1 can be fully reactivated either by cytosolic glutaredoxin, via a GSH-dependent monothiol mechanism, or, less efficiently, by cytosolic thioredoxins physiologically reduced by NADPH:thioredoxin reductase. The potential relevance of these findings is discussed in the light of the multiple functions of GAPDH in eukaryotic cells (e.g. glycolysis, control of gene expression and apoptosis) that appear to be influenced by the redox state of the catalytic Cys149.

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