Nature Biotechnology Abbreviations
Different groups of organisms synthesize different analogs of ascorbate and use different pathways for ascorbate synthesis. Ascorbates are synthesized by oxidizing the C2 of aldonolactones (aldose sugars with a carboxylic acid group at C1 in their aldonolactone form, shown in bold). This forms the enediol group that provides the antioxidant capacity of ascorbates. Aldonolactones are produced either by oxidizing aldose sugars at C1 using a dehydrogenase (green plants and fungi) or by reducing the aldehyde group of uronic acids (aldose sugars with a carboxylic acid group at C6) using an uronate reductase. The latter reaction inverts the numbering of the C skeleton (red font). Therefore the alternative routes to aldonolactone formation can be distinguished by feeding specifically labeled sugar precursors. The biosynthetic origin of the precursors in each group of organisms is shown in different colored boxes. Agius et al. have cloned a gene from strawberry that encodes a D-galacturonic acid reductase, which reduces D-galacturonic acid to L- galactonic acid (not shown), which is readily converted to L-galactonolactone. Therefore, the capacity for a Protist-type pathway may exist in plants in addition to the L-galactose pathway. Abbreviations: Ara/AraL, arabinose/arabinonolactone; Gal/GalA/GalL, galactose/galacturonic acid/galactonolactone; L-GalDH, galactose dehydrogenase; L-GalLDH, galactonolactone dehydrogenase; D-GalUR, galacturonic acid reductase; Glc/GlcA/GlcL, glucose/glucuronic acid/gluconolactone; GulL, gulonolactone; L-GulO, gulonolactone oxidase; GDP, guanosine diphosphate; Man, mannose; MeGalA, methyl D-galacturonic acid; NDP, nucleoside diphosphate; UDP, uridine diphosphate.