Figure 1

The main and the alternative L-ascorbic acid biosynthetic pathways in higher plants. Reactions with question marks yet to be identified. 1, glucose-6-phosphate isomerase; 2, mannose-6-phosphate isomerase (PMI; EC 5.3.1.8); 3, phosphomannomutase (PMM; EC 5.4.2.8); 4, GDP-D-mannose pyrophosphorylase (VTC1 or GMP; EC 2.7.7.13); 5, GDP-D-mannose 3^′,5^′-epimerase (GME; EC 5.1.3.18); 6, GDP-L-galactose-phosphorylase (VTC2 or GGP; EC 2.7.7.69); 7, L-galactose-1-P phosphatase (VTC4 or GPP; EC 3.1.3.25); 8, L-galactose dehydrogenase (GalDH; EC 1.1.1.48); 9, L-galactono-1,4-lactone dehydrogenase (GLDH; EC 1.3.2.3); 10, nucleotide pyrophosphatase or sugar-1-P guanyltransferase; 11, sugar phosphatase; 12, sugar dehydrogenase; 13, L-gulono-1,4-lactone oxidase (EC 1.1.3.8); 14, D-galacturonate-1-phosphate uridyltransferase and D-galacturonate-1-phosphate phosphatase (possible); 15, D-galacturonate reductase (GalUR; EC 1.1.1.n9); 16, aldonolactonase; 17, myo-inositol oxygenase (MIOX; EC 1.13.99.1); 18, D-glucuronate reductase (EC 1.1.1.19); 19, L-gulonolactonase; 20, D-glucuronate-1-phosphate uridyltransferase; 21, D-glucurono-1-phosphate phosphatase.