## 4. ascorbate and Glutathione
### Biosynthesis of ascorbate
This implies that the first specific step for L-Ascorbic acid biosynthesis in the Smirnoff-Wheeler pathway is the conversion of \begin{table}
\begin{tabular}{c c c} \hline \hline
**Genes** & **Enzyme and Its Alternative Names in Arabidopsis** & **Functions** \\ \hline _At4g24620_ & Phosphoglucose Isomerase, PGI, Glucose-6-phosphate isomerase & Transformation of d-glucose-6-phosphate into d-fructose-6-phosphate \\ \hline _At3g02570_ & Man-6-phosphate Isomerase, Phosphomannose & D-mannose-6-P formation from d-fructose-6-phosphate [264] \\ _At1g67070_ & isomerase, PMI & d-fructose-6-phosphate [264] \\ \hline _At2g45790_ & Phosphomannomutase, PMM & Transformation of D-mannose 6-phosphate into D-mannose 1-phosphate [265; 266] \\ \hline _At2g39770_ & GDP-D-mannose pyrophosphylase, GMP1, Vitamin C defective 1, VTC1, Cytokines Defective 10, CYT1, Embryo Defective 101, EMB101, Sensitive To Ozone 1, SOZ1, & Guanosine monophosphate transfer from GTP to GDP-D-Mannose [267; 254; 268] \\ \hline \hline \end{tabular}
\begin{tabular}{c c} \hline \hline  & The conversion of GDP-D-mannose to GDP-L-galactose. GME is also able to catalyze the 3\({}^{\prime}\) epimerization of GDP-mannose, giving GDP-L-glucose, which is the precursor of a possible side-branch biosynthetic pathway (the guidance pathway) for vitamin C synthesis [255; 269]. Plays a key role at the intersection of ascorbate and non-cellulosic cell-wall biosynthesis \\ \hline _At5g55120_ & VTC5 & Encodes a novel protein involved in ascorbate biosynthesis, which has been shown to catalyze the transfer of GMP from GDP-galactose to a variety of hexose-1-phosphate acceptors [270] \\ \hline _At4g26850_ & VTC2 & Phosphorylase, GGP \\ \hline _At3g02870_ & L-Galactose 1-P-phosphatase, GPP, VTC4 & Conversion of l-Galactose-1-phosphate into l-galactose [271; 272; 273] \\ \hline _At3g07130_ & Purple acid phosphatase with phytase activity, PAP15 & Encodes a novel protein involved in ascorbate biosynthesis, which has been shown to catalyze the transfer of GMP from GDP-galactose to a variety of hexose-1-phosphate acceptors [270] \\ \hline _At4g33670_ & L-Galactose Dehydrogenase, GDH & Conversion of l-galactose-1,4-lactone [254] \\ \hline _At3g47930_ & L-Galactono 1,4-lactone Dehydrogenase, GLDH & Oxidation of L-galactono-1,4-lactone to Asc [267; 274] \\ \hline _At3g05620_ & & Conversion of Methyl-D-Galacturonate into D-Galacturonate in the D-Galacturonate pathway [275] \\ _At5g04970_ & Methyl Esterases & D-Galacturonate in the D-Galacturonate pathway [275] \\ _At5g61680_ & & \\ \hline _At1g14520_ & Myo-Inositol Oxygenase, MOX1 & Convertion of Myo-inositol into L-Gulong-1,4-lactone Myo-inositol [257] \\ \hline _At1g65770_ & Ascorbic Acid Mannose Pathway Regulator 1, AMR1, ATFDA7, F-BOX/DUF295 ANCESTRAL 7 & Regulation of the mannose/L-galactose ascorbic acid biosynthetic pathway in response to developmental and environmental factors [276] \\ \hline _At3g23230_ & Ethylene Response Factor 98, ERF98, ATERF98, Transcriptional Regulator of Defense Response 1, TDR1, TTDR1 & Enhancement of the tolerance to salt through the transcriptional activation of ascorbic acid synthesis [277] \\ \hline \hline \end{tabular}
\end{table} 
Table 4: _Cont._GDP-L-galactose into L-Galactose-1-phosphate, a reaction catalyzed by GDP-L-galactose-phosphorylase (GGP) [280]. In Arabidopsis, this enzyme is encoded by two genes, _VTC2_ and _VTC5_ (Table 4) [281]. The plants with knockout of these genes had 20% and 80% of the vitamin C content of wild-type plants, respectively [270]. The double _vtc2_ and _vtc5_ mutants were unable to grow beyond the cotyledon expansion. Adding galactose or Asc to the seedlings compensated for the absence of both of these enzymes. These data suggest that, at least in Arabidopsis, the D-mannose/L-galactose pathway is the main source of vitamin C.  
_VTC4_ in Arabidopsis encodes galactose-1-phosphate phosphatase (GPP), which converts L-galactose-1-phosphate to L-galactose [282; 271].