L-Galactose pathway

Laing, W. A., Wright, M. A., Cooney, J., Bulley, S. M. (2007). The missing step of the L-galactose pathway of aseorbate biosynthesis in plants, an L-galactose guanyltransferase, increases leaf ascorbate content. Proceedings of the National Academy of Sciences of the United States of America, 104(22), 9534-9539. 

Laing, W.A. et al. (2007) The missing step of the L-galactose pathway of ascorbate biosynthesis in plants, an L-galactose guanyltrans-... 

Higher plants make large amounts of L-ascorbate, which in leaves may account for 10% of the soluble carbohydrate content.28 However, the pathway of synthesis differs from that in Fig. 20-2. Both D-mannose and L-galactose are efficient precursors. The pathway in Eq. 20-4, which starts with GDP-d-mannose and utilizes known enzymatic processes, has been suggested.28 29 The GDP-D-mannose-3, 5-epimerase is a well documented but poorly understood enzyme. Multistep mechanisms related to that of UDP-glucose 4-epimerase (Eqs. 20-1,15-14) can be envisioned. 

The metabolism of free L-fucose (6-deoxy-L-galac-tose), which is present in the diet and is also generated by degradation of glycoproteins, resembles the Entner-Doudoroff pathway of glucose metabolism (Eq. 17-18). Similar degradative pathways act on D-arabinose and L-galactose.60... 

As noted earlier, experiments leading to a bacterially expressed recombinant protein from chromosome 4 of Arabidopsis with the properties of MIOase have provided opportunity to test the lull potential for AsA biosynthesis through the inversion pathway (Lorence et al., 2004). It remains to be determined whether this route is ancillary to the D-mannose/L-galactose (alternatively, L-gulose) pathway or is a major source of AsA via MIOase in plants. 

Linster CL, Gomez TA, Christensen KC, Young BD, Brenner C. Arabidopsis VTC2 encodes a GDP-L-galactose phosphorylase, the last unknown enzyme in the Smirnoff-Wheeler pathway to ascorbic acid in plants. J. Biol. Chem. 2007 282 18879-18885. Davidson VL. Electron transfer in quinoproteins. Arch. Biochem. Biophys. 2004 428 32-40. 

D-mannose and L-galactose are efficient precursors for ascorbate synthesis, that proceeds through a pathway involving GDP-D-mannose, GDP-L-galactose, L-galactose, and L-galac-tono-1,4-lactone [223],... 

Similar to higher plants, unicellular microalgae (microphytes) are able to produce L-ascorbic acid from D-glucose via L-galactose. In this pathway the direct... 

In all plants and most animals, L-ascorbic acid is produced from D-glucose (4) and D-galactose (26). Ascorbic acid biosynthesis in animals starts with D-glucose (4). In plants, where the biosynthesis is more compHcated, there are two postulated biosynthetic pathways for the conversion of D-glucose or D-galactose to ascorbic acid. 

The galactose-l-phosphate uridyl transferase pathway is by far the most important in galactose metabolism it accounts for about 5/6 of the total galactose metabolized in mammals. It is less active in the newborn than in the adult, e.g., the liver of newborn rats has only 1/5 the adult amount of galactose-l-phosphate uridyl transferase per milligram of protein (13). 

Bissett, D. L. and Anderson, R. L. 1973. Lactose and D-galactose metabolism in Staphylococcus aureus Pathway of D-galactose 6-phosphate degradation. Biochem. Bio-phys. Res. Commun. 52, 641-645. 

Thomas, T. D., Turner, K. W. and Crow, V. L. 1980. Galactose fermentation by Streptococcus lactis and Streptococcus cremoris Pathways, products, and regulation. J. Bacteriol. 144, 672-682. 

Fig. 14. Salvage pathways of nucleotide sugars. L-fucose (11), (3-L-fucose-l-phosphate (12), GDP-p-L-fucose (13), A fucokinase (EC 2.7.1.52), B GDP-Fuc pyrophosphorylase (EC 2.7.7.30). D-galactose (14), a-D-galactose-l-phosphate (15), UDP-a-D-galactose (16), C galactokinase (EC 2.7.1.6), D UDP-Gal pyrophosphorylase. IV-acetyl-D-galactosamine (17), Al-acetyl-a-D-galactosamine-l-phosphate (18), UDP-lV-acetyl-a-D-galactosamine (19), E IV-acetyl-galactosamine kinase (EC 2.7.1. ), F UDP-GalNAc pyrophosphorylase (EC 2.7.7. )...

---